Mixing Consoles

Mixing consoles are the backbone of any recording studio or console PA system. They have a multitude of controls and buttons and are prone to service issues just due to the fact that there are statistically so many controls, pots, buttons, etc.

Dead Spots in pots.
The biggest issue with mixing consoles are the pots developing the typical scratchy sound when rotated or pushbuttons having a pop when depressing them. What usually leads to this is the lack of use on these controls once and a while on mixing consoles that stay stationary with the same band. The dead spot in a pot is caused by the corrosion of the pot's contact wiper and the carbon track with which it makes contact. If it's left to stay in one position for several months or years, the contact between the two will naturally corrode and usually eat up part of the carbon track leaving an empty or dead spot. The cure is simple, don't let it happen. I always recommend to clients that they "exercise" the pots on channels once and a while. Just simply rotate each and every pot and toggle the pushbuttons a few times on the console when time permits. For a weekly band gig or church performance, the sound tech can choose two or three channels and rotate the pots and toggle the pushbuttons. The following week, do the procedure to a couple different channels, and so on and so forth. After a month or two, every pot and pushbutton has been moved and the pot's and pushbutton's life spans will be dramatically increased.

Cleaning Faders
One of the most asked questions I receive is regarding the proper way to clean faders and whether or not it's good to spray tuner cleaner into a fader. Spraying a "cleaning" or "lubricating" chemical into a fader can be a great help in a hasty situation, but can significantly decrease the life of the fader in the long term. If it's an oily type of lubricant, then it's assisting in a sort of wet-sanding that occurs when dust is attracted to the tacky lubricant on the carbon track.
There are some decent fast drying, non-lubricating solvents that can be used to break up the scratchy sound that a fader may have developed. And, in a pinch where there is no time or alternative to clean it correctly, then spraying a little into a fader to clean it is a quick fix. However, when time permits, the time should be taken to clean the fader internally. To clean a good fader such as a long throw Alps or Penny & Giles, one must simply remove the channel strip, access the fader, disassemble the shell and clean the carbon tracks with a cotton swab and de-natured alcohol. The track itself does not need any lubricating solution applied to it and once the swabbing with alcohol is done, the track should be dry and free of any dust or debris. The only items that require a small amount of lubricant within a fader are the metal rods onto which the fader wiper slides.

What a killer piece of engineering! Digitally controlled analog signal path and engineered with a grace and excellence not found often. Incredibly reliable and extremely easy to diagnose and service. I've been in constant contact with a studio that has had one for well over a decade now. And considering the shear volume of circuitry between the desk, the tower, a cube and the patch bays, I've been amazed that the number of problems, usually minor, averages out to be about one a year or so. These things are built exceptionally well and built to last.

Another piece of digitally controlled analog signal path gear that compliments the CS2000/3000 series of boards. Incredible attention to detail with the design, easy to troubleshoot and service. I've noticed the multi-turn trimpots used to adjust the VCA DC offsets once and a while drift. The fix is simply to rotate the pot back and forth and re-adjust the offsets back to null. From time to time, the cooling fans on a few of these units had used up their life span and this led to some extensive heating inside the chassis of the units. Sometimes, the power supply caps become leaky and this leads to hum in the audio signal. It's always a good idea to replace caps like these with higher temperature caps to help extend life spans a little moreso.

The bar/pub band console workhorse. These boards have a legacy of performing in some of the most horrific of circumstances for electronic circuitry. I've seen these boards in such a state of used, their silkscreen labeling on every control and pushbutton was gone and yet, the boards still fully operational and intact. Most repairs with them deal with swapping out the pushbutton switches once they start making pops and clicks during actuation. Repairing these consoles taught me about the results of different beverages being spilled inside onto the circuitry. Which one do you think is more evil and causes more destruction, water, beer or Coke?

An expanded upon design from the 200, the delta series are another bar band workhorse. Very easy to repair with a very forgiving preamp that will take a massive beating signalwise and still sound acceptable.

The Venue expands on the Delta series with some additional features to make it bridge the gap between a bar console to a larger format concert hall style board. Essentially the same circuitry on the inside as the Delta series.

The Vienna takes the leap into the large format world yet still borrowing the features and core circuit designs from the Venue. I've dealt with some minor repairs on these consoles themselves, some pushbuttons and a fader. But I've had three different clients each with a similar problem with the power supplies. There was several loose connections with the power transistors in the regulation section of the bipolar audio supply and one side would fail. Once that happened, the other side blew out the rectifiers. I suspect that the failure shifted the load to the remaining side of the supply and stressed it to the point of causing the additional failure and subsequent burnout.

I always had the impression that either DDA or Soundcraft or both were going through the other's trash cans looking for schematics as these boards shared alot of similarities to each other. Same issues as with the Soundcrafts with the noisy pushbuttons.

The quintessential large format workhorses that got the touring world through the late 80s and early 90s. These consoles are fairly easy to service and most of the time, failures weren't circuit malfunctions, but simply pots wearing out... especially on the 3Ks.

Some really basic feature consoles ideal for small churches, bar bands and playing around. The 2404 was a lightweight console that had analog meters and an aux section with 4 auxes, arranged in a square instead of the typical up and down line. Lots of space on this console between knobs, and relatively easy to access for repairs.

Incredibly tough console to physically troubleshoot as the internal circuit boards are spanning multiple levels and interconnected to one another with ribbon cables that are fairly short. This means that one has to sit the console on its side and work at making scope/probe connections through a very small gap so as not to disturb the ribbon cables. Failure of the A/D converters is fairly obvious when the input metering will indicate signal with no input and noise is heard on the corresponding channel. The A/D IC is a rather pricey Burr-Brown PCM1760U which has been discontinued but still can be found with some searching.

This one is a little easier to physically get at internally. In fact, the top cover flips up somewhat like the hood of a car. The cover hinges at the front of board and flips up from the rear outward toward the front edge of the console. Don't forget the four screws under the front edge. One unit I was working on once had a blown output D/A converter, a Burr-Brown PCM1702.

Fairly heavy boards, and tight on the knob and button arrangement. Modular construction and not too tough to deal with when problems arise. Had a couple of these boards come through the shop a few years ago needing just some minor preamp issues fixed up.

A fairly obscure modular construction board with very similar features as the Soundcraft Venue. Large knobs and plenty of real estate, not to mention a rather robust board which is great for those rowdy bars where the drunkards once and a while turn and charge on the sound technician.

Another typical robust and well built console and nearly an industry standard there for a while prior to the digital console boom. Had one come into the shop that had a drink or two of a nice Pilsner it seemed, well maybe just half a pitcher of Bud. Anyway, fairly routine, couple of modules needing cleanup and check out.

A fairly serviceable modular console that found its home with a number of touring companies in the early 90s. Typical failures that I found were fried balanced output amplifiers on the aux, matrix and groups. These IC amps are capacitively coupled and I suspect that a lot of the burnouts I came across, were caused from hot patching these outs into another console input with the phantom power turned on.

Nice basic recording console. Had an issue with one that had some corroded connections between the modules and the main backplane interconnecting the modules. Some cleaning and tightly securing the modules cleared up the issues.

A fairly tough console for the most part. There are some header connectors used for the interconnecting of the modules. After a pretty long ride on a rough road in the back of a truck, I found one of these consoles with several broken and compromised solder joints. I ended up repairing the joints with a harder silver/tin solder to keep the joints a little tough in case of future off road adventures.

A tough board to service as the channel strips are not on a flexible ribbon cable, but instead on a header card strip running the length of the board. However, the channel strips were fairly well protected with fuses on each channel strip for the incoming power supplies. Most of the failures I had seen, were simply just fuses blown for unknown reasons. Once in a great while, I'd have a situation with one of the monolithic epoxy sealed mic-preamp circuits, but rare. Ramsa/Panasonic deserves some credit here for applying the idea of using LEDs for illumination. They had come up with a small PCB strip onto which they soldered four amber T-1 LEDs and used this inside the analog meters for illuminating the scale. While it wasn't incredibly bright, they hardly ever burned out.

This board shares a lot of similarities with the 840. I've typically found output coupling caps blown and even circuitry ahead of the caps burned up. Most likely, these kinds of failures are due to hot patching the console output into a phantom engaged input of another console. This is OK to do, but it's best to make the connection with the phantom off as when making the connection, there could be a tremendous DC offset created in the small time period when the pins of the XLR make contact and the output circuit reaches an equilibrium to the spike.

M320
M520
M3500
M208
M216

Tascam has a distinct styling about everything that they build. Their mixing consoles always have had bright primary color schemes, analog metering, and plenty of physical spacing and real estate with which to work. Even though most of their boards aren't modular and accessible from the face, they're not all that difficult to work on usually by removing the bottom cover and troubleshooting with the board on its side. I've rarely had an instance where I spent an inordinate amount of time inside one of these consoles. Usually, the problems were simple and easily diagnosed and fixed.

Pretty solid console, not modular but stands up pretty well to a liquid spill. Gotta get to the electronics on this one from the bottom panel. Removal of a channel strip requires removal of all the pots knobs and securing nuts for the particular channel in question. Plenty of spacing and room to fit plump fingers between the pots.

The first larger format console I ever completely disassembled and cleaned. I remember spending a couple of days removing the thick layer of bar crud that had accumulated on this board including the disassembly and cleaning of all the faders. When the client came to pick it up, they refused to believe that the clean and shiny specimen they got back, was actually theirs. I think the sound tech had fun that first night getting the board re-introduced to the bar filth again.

I'm only mentioning Peavey consoles here as they deserve a mention because they really helped to fuel the bar bands through the late 70s and early 80s with some simple and reliable little consoles which I really think gave birth to the compact analog mixers we still see today. I ran sound for a band for a few years using a MD 16 X 2 that never gave me a lick of trouble even after being subjected to the grime of the typical band trailer and some outdoor events with rainstorms.

About 5 years ago, after physically wrestling with an SR40 just to troubleshoot a minor issue, I made a rule at the shop to not allow any Mackie or Behringer boards in through the door. These boards, while attractive pricewise and aesthetically, do not have modular channels and consequently aren't very friendly when it comes to service. Their circuitry is arranged in groups of 8 channels on large PC boards with the electronic circuitry parts facing the inside of the board, i.e., not accessible just by removing the bottom panel of the console. In order to change circuitry parts including pots and connector jacks, it's necessary to remove all the knobs and jack hardware in the 8 channel section where the problem is occurring. I have suggested that if ever someone has a problem with one of these consoles, that they should wait until they have multiple problems and handle them all at once. Just the process of opening up a Mackie or a Behringer to assess the problem is quite a chore and commitment in time in and of itself.

See comment the above about Mackies as it applies to the these mixers as well. Inexpensive, attractive looking, but a bear to repair the most simple of issues.

Recorders - Analog

I had to deal with a odd problem that I couldn't quite solidify the exact chain of events that lead up to the scenario. A client had called and told me that one of the tension arms had physically snapped off his ATR80. I arrived on site, and sure enough, the thick cast piece had completely snapped in half. Being that the machine was in a separate room from his control room, no one actually saw what caused it. We suspect it came from a fast cue with a mass of tape on one reel and somehow a quick reverse of direction produced a loop in the tape that them drew up its slack quickly and sorta snared the tension arm WITHOUT the tape breaking. I ended up installing a new tension arm and some dasher pots to help dampen some of the vibration, thus leading to a little additional tighter control of the tape. I have a suspicion that the original tension arm may of already had a stress fracture and it snapped simply due to use.

Sometimes, tape machine problems can be attributed directly to the quality of tape being used and not the machine itself. This was the case of an ATR60 I once serviced. The call came in and the user described that the machine was making incredibly loud squeaks and creeks and he thought that the bearings in some of the rollers were worn. When I came out to his studio, I discovered that he had a rather "loud" reel of 456 that had developed just the right balance of stickyness that it was "singing" all by itself whenever it would be played or fast wound. He had told me he had been using that tape for years and never had a problem. Just to clarify what he said, I asked him just how many years had he been using THAT tape? Magnetic tape does have a limited lifetime of usability. There is no particular rule of thumb on just how long this is as it's very dependent on the conditions the tape is exposed to, humidity, temperature, physical use, etc. Sometimes, even the manufacturers have had bad runs of the binding agents and such... really, it's true!

This unit was a nice basic 1/4" four track that can handle 10" reels with NAB hubs and like a lot of 70s gear, has real wood side panels. Nice big analog meters and lots of great controls for stand alone operation with just a microphone and a pair of headphones. I have one in my storage for the odd tape transfer every now and then. When set up properly, these units and the A3440 series are nice little workhorses for basic stuff.

These were great "boat anchor" heavy workhorses for the basement radio spot and jingle studios prior to DAWs. I know a few guys who still use them for odd and ends. Massive motors and power transformers and I've rarely seen one fail on the power side of things. Usually, it was break tension issues and general alignment and calibration stuff that they ended up needing.

A really nice replacement series for the 40-4 and 80-8. Tascam applied their characteristic aesthetic styling and put some very clean and nice user controls and big meters on them. I always enjoyed going through these units and getting them back into alignment and sonic usability. I never encountered a circuitry problem with any of these units, it was always just adjustment and calibration issues. I had heard somewhere, Dave Stewart/Eurythmics cut the hit "Sweet Dreams" on a 38.

The 40 series was essentially a step up from the 30 series of recorders. Sonically, the differences were minor, however, mechanically, the 40 and 50 series had some differences in the reel motor and capstan department that made for more accurate and solid tape control. I never saw a malfunction with circuitry on any of these units. The worst that I had seen aside from general cleaning, alignment and calibration issues was a 48 that was damaged in shipping. It was only cosmetic damage to the chassis. Believe it or not, the head and tape path alignment was rock solid and didn't need changing.

The 50 series has a symmetrical tape path arrangement. See the Otari MTR90 for a brief description of this methodology. Sonically, these units are very much the same as the 40 series.

Turn the analog meters on a 38 into LED and take away a few of the adjustment parameters on the channels, add "noise reduction" and what you have is a TSR8. Saw a couple of these for calibrations, but I always felt the 38 was a superior machine.

Take a TSR-8 1/2" 8 track machine, double the tracks and you have an MSR16. At the time, prior to the digital revolution, this deck was a nice budget alternative for someone starting a project studio. Sound quality was OK, and in the hands of someone who knew what they were doing, it could result in a reasonable end result.

This was a really interesting piece of gear that kinda lead the way in the small project studio world of the 80s. It consisted of a 1/4" 8 track open reel tape deck built into the console of an 8 channel mixer. I dealt with a couple of these back in the late 80s and they were quite fun to play with. I never encountered any problems other than routine cleaning and alignment issues.

A little similar to the size, color and layout (metering) of Tascam ATR-80, 2" - 24 track analog. Although, this unit has a symmetrical feed and take-up tape path(sometimes referred to as an "Omega" path) meaning, there are the same tension and neoprene feed rollers on either side of the head stack with the tape making a symmetrical path on either side of the head stack and there is no capstan/pinch roller combination which some feel is more gentle with the tape. It's a great machine that mechanically, is quite robust and exceptionally trustworthy under fire. The ATR-80, like all multitrack machines, takes time to scrutinize and setup. But once completed, with care and attention to detail, the unit will shine and perform reliably for years on end.

When I branched out and started my own business, this was the first multitrack that came through "new business" door. Another great little 8 track that was great in the context of the project/basement studio prior to the birth of the DAW. This unit didn't have any problems other than it being filthy and needing some TLC to get it back into calibration and sounding good. Quite often with multitracks that I've seen come through the shop, most of the problems with them were simply filth. The owner's didn't know the exact procedures and timing of cleanings and such, and always thought the machines were to blame rather than to point fingers at a tape that might of been shedding so bad and waxing over the heads as if it were a crayon drawing on the heads. Quite often, in a darkened studio, users can't really see the residue caked on their heads and many times, that residue is even "polished" onto the head and looks like it should be there. Of course, fidelity goes down and then the user compensates with EQ somewhere along the line and then can't figure out why everything sounds so terrible when they take the tape to a friend's studio and play it back on another machine. This was the case with this MX-5050. I had to really "dig deep" to get this head cleaned. Once cleaned, aligned and calibrated, this machine performed drastically better.

This unit is a basic 2" 24 track format analog deck. I dealt with one that had been sitting in storage, unused for several years that had some substantial tape transport control logic problems. Upon initial power up by the user, the transport controls quit working and the machine was non functional. I got the call and came out to investigate and found that the DC power supply malfunctioned. I had repaired that section and then, subsequently, the take up side reel motor drive quit. I had no real obvious indicators as to why the system wide failure had occurred. Once the machine was back into operation, I went through a complete calibration and the unit came to life quite nicely.

Nice small footprint, 2" 24 track analog deck. These things are really small and work great for studios with limited space. I dealt with one that had a supply reel motor drive issue. The driver circuit must of had some issues prior as the power transistors in the circuit weren't the original specified devices. This occasionally occurs and it's always a good idea to make sure to replace any burned or defective devices with devices that are the exact replacement or devices that exceed the original current and voltage specs. Many times, in studios, technicians are in a bind and need to get a machine up and running under the gun. In this scenario, they may use a part they happen to have on hand which solves the problem temporarily. Then, after the pressure is off, they forget to go back and actually repair the unit to spec with the proper part. This unit was purchased second hand by the client and I suspect prior to his ownership a hasty repair was done with this 760.

These units are a real nice pro-level cassette deck for use in the studio, broadcast or otherwise. The MR-2 is more of a consumer piece with unbalanced inputs and outputs, however the two share similar qualities otherwise. The typical problem that I see with these is the idler tires not grabbing well and slippage on fast wind modes. This leads to the deck eating tapes if the idler tire isn't making a good friction contact with the reel tables. Other than that, I haven't seen any other failures.

Well, what can one say about one of the most intricately engineered pieces of audio equipment ever built. I had to actually do some circuitry work on one that had several transistors in the transport control non-functional. Transistors typically burn and short when they fail, but on this occasion, I found not just one, but several that had not shorted, but instead opened up. I wasn't sure really what had caused this condition to occur and the repair was fairly straight forward and easy to accomplish due to the ease at which the circuitry is accessible on these decks.

Another one of my very first clients when I opened up the shop is a gentleman who has a cassette tape duplication house. He had a mastering studio to create a master 1/2" - 4 track tape, two tracks running one direction, and two tracks running the other direction. This tape was strung up on a continuous loop bin which played at 80 times the 1-7/8 cassette speed. The unit was attached to several slave units which had pancakes of 1/8" blank tape onto which the audio was recorded at 80 times speed. The pancakes were then unloaded and put on a series of machines that pulled a length of tape that comprised the entire finished cassette program into a blank cassette shell. It's a fascinating operation to watch. I would get a call from time to time and either the master loop bin or the slaves would have some issue. Typically, it seemed to always boil down to a motor drive problem. The Lyrecs used the typical 2N3055 power transistor, but for some reason the original SGS Thompsen devices they had in there kept burning up. I replaced them with Motorola devices and never saw another transistor burn up. I'm guessing either the ST transistors were counterfeits or they were misprinted devices with lower current ratings. The Lyrec units were a Danish built system and consequently, these units had some very meticulous wiring harnesses, all bound together with the waxed string as well as some very colorful custom wire colors involving easily discernable colorful stripes. The Lyrec loop bin and slaves are very easy to service with the documentation, ease of accessibility and the colorful wiring harness.

Recorders - Digital

A really solid DAT machine that Tascam introduced as a budget entry for musicians to master with. The original model has some nice aluminum rotary knobs for input and output levels. The SCMS(Serial Copyright Management System) can be disabled on this unit by simply removing jumper W402 on the left front of the digital PCB.

The 60 is a pro level DAT machine. However, I seemed to have always had more maintenance issues with these 60s than the 30s. It seems the pinch roller is the culprit 99% of the time. The pinch rollers on these decks didn't last very long. They'll get hard and not grab the tape smoothly which leads to damage on tapes and a possible nice tape meal for the machine.

A stripped down and budget version of the DA-88. Basically, it's missing the time code capability. The 38 along with the 88 seem to be the most reliable and forgiving machines in Tascam's lineup of DTRS machines. On both the 38 and the 88, I've never had any circuitry issues and it's always been about mechanical cleaning, lubrication and alignment.

The DA-88 is a solid and reliable digital 8 track that opened up the doors of digital multitrack recording for the home/project studio. The transport on these units is very serviceable. It takes a few hours of time to completely disassemble one, clean and lubricate it. When done properly and routinely, the machine will give very reliable long term service. The heads will typically last well over a 1000 hours before the error rates start to increase and it's time to consider replacing the drum. I've typically seen the CMOS battery dead before the head is changed, so be a little wary when buying a used one, checking drum time and seeing 000 on the display. The key sequence to get the drum time is to power off the machine, hold the stop and play keys, power on the machine and release the keys before 2 seconds. The key sequence to turn the first two LED meters into error rate displays for the A and B heads is to power off the machine. While holding the fast forward, stop and play buttons power on the machine, then, press stop before two seconds, and finally pressing the remote button.

While this deck can record 24 bit, it's a much more finicky machine than the 38 and the 88. It seems that Fuji digital tape is the only tape that will somewhat work in these for a reasonable amount of time before head cleaning is necessary.

Basically this is a DA-88 on the inside. The differences are the balanced XLR in and out cards and an AES digital card as opposed to TDIF. Oh, and most importantly the play and stop keys are switched in position from how a DA-88 has them. The transport is the same however. Although, there were some early PCM-800s that had a flaw with the logic cam and the sector gear in the guide mechanism and it's necessary to replace the two. The original logic cam has a small pin that isn't the proper height and after some use, the sector gear will start to prematurely wear and when the pin comes out of its slot, the transport will seize up, give an S-err 11 and perhaps eat a good and yummy master tape.

This was Sony's flagship DAT machine when the DAT format was introduced. If the DAT recorder is combined with a PCM-2500B digital interface, the pair are quite a robust, well built and fairly heavy. The transport on the 2500 is fairly solid and reliable. The entire inside of the chassis is copper plated. The power supply is run from a good old transformer, a big and heavy one at that.

There's a belt drive on this unit's loading mechanism. I've seen more than one of these units try and eat a tape because of a simple broken belt.

Sony's flagship digi-beta machine. While this isn't an audio machine so to speak, it still is a tape machine and I don't discriminate when it comes to fixing gadgetry in the professional studio environment, that is unless there's a broken Mackie or a TV on the fritz there. I dealt with one that had a significant amount of copper dust causing some contact issues between the brushes and slip ring contacts on the head stack.

The budget competitor to the Tascam DA-88, with its format being S-VHS tapes instead of 8mm. A lot of small basement studios got a hold of these and used them quite extensively for tracking purposes before DAWs were fully capable of multitracking within the budgets of the typical basement studio. I encountered a few of these and mostly, they just needed cleaning. The idler tires are a very common item to need replacing as they'll become hard and not have enough friction to grab, leading to potential tape spaghetti and meatballs with no doubt a critical master tape.

While technically, this isn't a tape recorder, I thought I'd stick it in this area as it deserves a mention. I had one come through the shop that was locking up after a short time of being powered on. I was intrigued to find that the main clock crystal was behaving erratically and not oscillating with consistency. After swapping out the crystal, the unit performed flawlessly. The very same unit came back about a year later and it needed the hard drive replaced. I was able to access the drive long enough to clone the contents onto another drive. Once the drive was swapped, the unit appeared and behaved like nothing happened.

This is a rather rare 32 track digital open reel machine. It used 1 inch tape and recorded with a stationary head. These along with the Sony PCM3348 were the flagship digital recorders back in the mid 80s. I had to deal with the alignment and calibration on a DTR-900 that a client needed for pulling some tracks from some old masters. It's a fairly robust machine, but quite finicky if the alignment isn't dead on. Ticks and pops are fairly common if everything isn't happy.

These DAT machines were Panasonic's answer to Tascam's DA-30 and Sony's PCM-2500. Just picking one up, it's immediately obvious that there isn't the same mechanical stuff under the hood as these things are very lightweight. With most of the ones I've seen, problems usually revolved around the transport. Occasionally, the tension arm guides will sorta bind due to the fact that one of the arms nudges the other one in position. If ever there had been a hangup or any kind of physical jarring to the transport, it's possible that the arms will warp and lead to this problem. When repairing this transport and re-assembling it, I disengage the loading motor gears and manually move these arms until I'm satisfied there isn't any warping and they freely move with little effort. Be conservative and accurate with any lubrication applied to mechanical parts on a DAT transport. Just a little grease in the right spots is the key.

Power Amplifiers

The introduction of the PM-1.5 in the professional audio arena around 1985 was quite a profound achievement at the time. Audio power amplifiers capable of pushing large amounts of current and delivering a good dose of power to a load were, up to this point, handled by the good old fashioned traditional design with huge and heavy power transformers and capacitor banks. The PM-1.5 weighs 21 lbs in a two-rack space chassis and can deliver 600 watts/channel into a 4 ohm load so it was quite a huge leap in the way of physically lightening up the traditional power amplifier. I really cut my teeth learning about audio power amplification with the repairing of these amps. While the circuitry itself takes a little bit of extra effort to grasp, the amplifier is surprisingly easy to physically work on. In fact, compared to current amplifier designs today, this amp is still much easier to work on than most out there. Output devices are usually the number one failure. Changing them however, is a speedy and easy task. It was as if Bob actually designed this amp to be serviceable... hmmm, what a novel idea! The first thing to remember when attempting a repair on these is to disconnect the protection circuitry by de-soldering the protection LED from the motherboard card... it's the white wire with the yellow stripe. The protection circuits are very handy, but it's nearly impossible to do anything in terms of troubleshooting, with the protection engaged.

The first time I opened one of these beasts up, it became clear to me that who ever designed the chassis on this had some aerospace and military electronic design experience. These amps weigh 11 lbs and can deliver 625 watts/channel into a 4 ohm load. This amp has pretty much the same audio circuit section as the PM-1.5, but differs immensely in the power supply department. It's pretty typical to lose the main power FET switches in a pretty catastrophic sorta way. Getting them out can be a bit of a puzzle, but the rest of the amp is fairly easy to deal with. The output devices however are plastic packaged TO-3P and of course need to be de-soldered to remove them, unlike the PM-1.5 which were metal can TO-3 and could be removed simply be pulling the bottom cover plate and unscrewing the mounting screws.

If there is any one family of amps that carries the torch for the bar band scene, it's the QSC MX series. They're heavy, they're reliable, they can take a tremendous amount of abuse and keep functioning and they're fairly inexpensive. The core fundamental circuit trick that QSC really used as the cornerstone of all their amps was the idea of taking a typical class AB circuit, swapping the position of the PNP and NPN devices and coupling the speaker output through the power supply filter capacitors. This achieved a pretty interesting result. It allows the output devices to be physically mounted to the chassis without mica insulation, hence making heat transfer a little better. But, also, since the cases of the output devices are typically the transistor's collector, it keeps the cases at zero potential, thus isolating the high voltage rails a little bit moreso from the bumbling electronic service technician. A typical failure on these that I've seen involves the low voltage rails for the preamp circuitry. They're derived from the high voltage rails through some high wattage power resistors and a set of zener diodes for regulation. Occasionally, I've seen either the resistors or the zeners fail. Also, if there is any fault on the low voltage rails, the amp's output will slew DC to the direction from which the failure is occurring as the amp is fully DC coupled from the input attentuator to almost all the way to the output, prior to those power supply filter caps. Sources of the faults can be difficult to find, but it usually boils down to making certain those low voltage rails are in good shape. The "a series" of these amps contain some SIP circuits which control the power supply commutation. If early or asymmetrical clipping is witnessed, then it's possible that these SIPs are the failure.

An EX4000 is nearly identical circuitwise as an MX3000a.

Crown has such a legacy created in the pro audio world. Their amps are seen usually in herds rather than just one or two in a rack. Like QSC, Crown has a core fundamental to their power amplifier circuitry. Most conventional class AB designs use a bridge rectifier with a pair of caps to derive a positive and negative DC rail as well as a ground reference with which to reference everything. Crown uses only one huge filter capacitor and the rails "swing" so to speak. The negative speaker terminal is connected to the ground reference as well as the "low-side" series of output drivers which act to servo the rails to the direction required to drive the positive output terminal appropriately. Typical failures I've seen have been output devices, and consequently, usually the emitter resistors are compromised and have opened up. So, when changing output transistors, check the emitter power resistors to make sure they're still good.

The 5000VZ expanded on the idea from above, but added a 2nd pair of transformer windings in the power supply which switch in either series or parallel based on the current requirement of the load connected. High Z load = windings in series, low Z load = windings in parallel. Watch yourself picking these up and moving them around on the bench as they're 70+ lbs, so watch your fingers.

QSC, Crown and Crest sorta make up the "BIG 3" in the way of audio power amplification in the US playing field. Their amps always struck me as having a reasonably decent cooling system. Nothing special really than that which most other amps have... a central cooling channel with forced air over the surface area of the heatsinks. But Crest seems to dedicate much more real estate to the heat sink, of course making the amps heavier, but they seem to stay somewhat out of the hot zone, that is if they're racked up with access to fresh air flow.

What garage band DIDN'T have one or more of these. Simple, powerful, reliable and fairly easy to repair. The output sections are built upon two large aluminum heatsinks under the top cover of the amplifier. The circuitry can be accessed and assessed by simply turning the entire heatsink over being careful to slide the cabling off to the side and flip it in the direction with the slack.

I have one of these sitting above the test bench with the back of the amp facing outwards. It's has the XLR and 1/4" inputs, input attentuators and the input switches as well as binding posts all nicely laid out and accessible for the purposes of testing speakers and such on the bench.

Hafler made a name for themselves with this MOSFET output amplifier that was offered as a kit for the owner to assemble ala "Heathkit" style. It was a nice amp that still is found in basement studios running monitors. There are still companies out there that have kit forms of there products usually less costly and it's a nice feeling to have had a hand in actually putting together something, using it and having that satisfaction of powering it up the first time.

Effects

If your effects processor is experiencing issues such as the display is showing funny characters, there are patches that sound noisy or multiple patches sound like the same noise, it's possible that the memory where the unit holds the programs is corrupted. Nearly all effects processors have the ability, through some key strokes, to clear this memory and reset the unit to the factory program or "new" state. The keystrokes are different for each piece, but almost always involve having the unit powered off, holding some keys down and then powering on the unit. Many times, this is all it takes to clear up a processor problem that appears catastrophic in nature. Be wary though as any user edited programs that you've created and stored will most likely be wiped out. The key sequences are usually listed in the owner's manual or just do a search on the net for the model name of your piece and something like "memory reset" or "factory reset."

These half rack space digital effects boxes were introduced by Lexicon back in the 80s to give people a chance to taste some decent sounding "Lexicon" verb in their home studios without having to invest heavily. These units were and still are quite useful. They have a simple user interface consisting of a of rotary switch to dial in the program the user wishes and then a couple of "trim" pots to adjust slighting some parameters. The units are fully programmable with the MRC remote control box via MIDI. Typical failures on both of these units are the rotary encoders and the pots. They simply wear out and in the case of the encoder wearing out, the unit becomes somewhat disabled. They're easy to repair as the chassis is an extruded case and the main PC board slides in on some slots on sides.

This unit is basically the effects of an LXP-1 and an LXP-5 together in a full single rack space piece with an LCD screen to assist in editing the parameters. Like the LXP-1 and LXP-5, the pots on these will wear out. I had a unit in once that had blown a filter cap in the power supply effectively shorting the negative DC rail resulting in some very interesting sounding "effects."

One of the more flagship model processors that Lexicon is known for, this piece is a two rack space bit of reverb firepower that has some of the personality of the 960. I had one in once that had a burned up power transformer. Some typical problems with these is the wonderful vacuum fluorescent display that tends to dim over time and eventually will burn out. These are still available from Lexicon, however, they're not cheap.

The SPX-90 or 900 are pieces of effects gear that are found in nearly every bar band house rack. They were great, inexpensive general purpose reverb units that didn't sound atrocious. I've seen a few of the SPX-90s with the very same symptom of changing patches or going into bypass when physical bumped. This problem seems to be caused by a bad connection with the patch select and bypass jack's sleeve with the chassis. It seems that there is some kind of spike or glitch that is caused when the unit is bumped and the sleeves on these jacks make an intermittent contact with the chassis. This fix is simply making sure the chassis screws are tight and the internal chassis panel the jacks are mounted to is firmly attached. I've even placed some thin star lock washers under the screws to better help keep things tight and making good contact.

An upgraded true stereo version of the SPX-900. I've seen the rotary encoders on these units go bad. The symptoms are sporadic adjustment when rotating the dial and inability to dial in a precise number.

Had one of these once which had its input and output coupling caps pretty much shot. Fairly simple repair. When dealing with something like this, always remember to have a look at the input and output 1/4" jacks that are usually soldered directly to the main PC board with no other strain relief. Sometimes, the solder pads can be fractured. It only takes a few seconds to touch them up if the soldering iron is hot and the chassis is open.

I had an M5000 come in to the shop once and was told the unit exhibited some distorted outputs. I thoroughly tested the unit for proper functioning and found it to be operating properly. I did however find that there might be some confusion as to how the metering and signal level headroom are displayed. And this leads to an example of a really critical component of understanding about gain staging and digital gear. The M5000 meter on the front panel is labeled simply "dB." The LEDs are displaying a dB measurement, but the reference level is neither dBV(1Vrms) or dBu(.775Vrms), it's rather a VU meter referenced upon the M5000's own internal maximum headroom capability and weighted to be a sort of slow responding "average" meter without much indication for peak transients. Hence, 0dB on this meter is the "average" brick wall, beyond which the unit isn't capable of passing anything without severe clipping. The -12dB LED on this meter represents roughly 0dBV or 1Vrms of the signal level of a steady state sine wave. Therefore, +12dB of signal above that or the red 0dB on the meter would be the dynamic limit of the unit's signal path circuitry and this would be somewhere in the neighborhood of 4Vrms, 12dBV or 14.2dBu. I suspected that some of the distortion problems the client complained of might have been due to clipping either pre converter conditioning circuitry or the A/D and D/A converters. Some of the programs, especially a reverb with the PRISM shape selected will add significant gain to the signal and potentially clip the output section. It's important to note that when the meters are selected to display INPUT, the meter will show the signal PRE attenuation and when the meters are selected to display OUTPUT, the meter will show POST gain control, essentially signals present at the input and output jacks. Now, using this unit with a typical professional console, the VU meter on the console will most likely be referenced to +4dBu or 1.23Vrms. So, it becomes apparent quite quickly how easily it could be to mis-align the gain stages if one doesn't keep in mind not just reference level but also the available headroom a piece of equipment actually has available.

Had one of these units come in with a problem of intermittent power. It turned out it was the Molex connecter that mates to the Main PCB from the power supply. There were just a few corroded pins that kept the +5VDC current from getting to the motherboard reliably.

I had one of these come in that looked like it had been dropped on its corner and face. There were several of the meter LEDs sheared off, the rotary encoder had bent, the display glass was cracked and the chassis face was severly damaged. D. Rooney at Eventide customer support took care of finding me what they called a "factory reject" faceplate at a greatly reduced price. She sent me pictures of it and I couldn't find any faults with it. She sent me the LEDs and remaining parts and I can't stress enough how having good customer support makes a company shine brightly. Way to go Eventide!

Dynamics Processing

For the most part, all of the analog signal processing that BSS has made over the years has been pretty robust and stands up very well to the rigors of the pro audio touring world. The main problems that I've seen with these units has been simple things like pushbutton switches wearing out or pots being broken off from being physically hit. Every once and a while, there would be an op-amp in its socket that would be slightly intermittent due to some moderate corrosion from the IC socket. Simply removing the chip and inserting it a few times usually clears the problem. A permanent solution is to remove the socket and just solder the IC in place. This makes it the unit a little more time consuming to repair, but the IC socket corrosion problem is solved permanently.

A rather odd issue that I had with a couple of 960s, was the toroid power transformer. Occasionally, a slight hum will develop in the audio channel whose majority of circuitry is located closest to where the power transformer is mounted. By loosening the center mounting screw for the transformer, and ever so slightly and carefully rotating the transformer while listening to audio through the troublesome channel, it's possible to reduce the hum to a near null state. After the position is found, tighten the transformer mounting bolt back up and listen again to make sure the hum hasn't come back.

During the mid 80s, there was so much analog Rane processing gear out in the field, e.g. eqs, compressors, crossovers, etc. The EQs were famous for pushing idea of constant Q on the filters. They worked, they sounded reasonably good, and they weren't too expensive. About the only electronic problem that I've seen with some of the rack mount Rane gear is the internally switching jacks they used. With the crossovers especially, the jacks would become different outputs depending on the hookup scenario. Sometimes, the normals inside the jacks wouldn't switch or would be intermittent. The solution is to change out the jack or really exercise it if in a tight pinch.

Some of the older versions of these models had some output FETs that acted as muting switches. Every once and a while, I found these FETs non-functional and keeping the corresponding output channel from passing signal. The really cool thing about Rane was that there was a schematic diagram of the electrical circuit in the owners manual. This is quite rare amongst manufacturers. But, with this philosophy Rane became a VERY tech friendly piece of gear to repair. To this day, they have all of their products schematics online, nicely organized and easily downloaded, no passwords, etc. One particular event that sealed my fondness for Rane's customer service was a situation where a customer had a parametric EQ that was vandalized in-so-much as all the control knobs were stolen. I told customer service at Rane about the issue and they sent a full set of new knobs to us at no expense. How cool is that?

Had one of these come in the shop recently with one channel non-functional. It turned out there was a manufacturing flaw and a few parts on the left side channel were marginally soldered in place, not cold solder joints, just not completely soldered. It would work for a bit, then cut out.

Had one of these units come through the shop in a nearly dead shape. It had suffered extensive road damage. Several of the pots were sheared off the PC board along with several broken leads. There was even a tube that was shattered to pieces. But, after all was re-soldered and tube replaced, the unit fired up and went back to work.

Had one of these come through the shop with some odd thumping that would manifest itself after the unit was turned on for a while. Turned out it was the calibration pot for the +48VDC rail. I loaded down both channels with phantom mics and then determined the proper value of resistance necessary to maintain regulation. Then, just changed out the pot with a 1% fixed resistor. It seemed the pot was fluctuating its value when the unit would heat up.

Fairly robustly built piece of gear. The only problems I've seen is simply tubes burning out and needing replaced.

I had an interesting noise issue come up in a very low noise studio. The client had an array of these EQs in line with their surround sound control room monitors. From channel to channel, there were some low level hums that seemed more prominent on one side of the CP-10. In a stereo scenario, the hum is negligible. But in a surround arrangement, the hum of each channel can add up and be very perceptible. As it turns out, there is a very small amount of residual hum that is caused by the internal power transformer magnetically coupling with adjacent circuitry. The solution was to not use the internal power supply. I built an external +/- 15VDC power supply to supply the array of CP-10s their required rail power. The external supply was located on the bottom of the same rack as the EQs and there was a separate power jack on each CP-10 that then plugged into the external power supply. Internally to the CP-10, the external supply parallels with the internal supply with some diode protection. So, if ever the studio needs to use the CP-10s without the external supply, the internal is still operational. By using the external supply, the hum that was originating from the CP-10s is virtually gone.

Converters (AD/DA)

A local studio in the area picked up seven of these units to expand their ProTools rig. At the time, 1998ish or so, this was prior to the HD rigs and these units were considered the top gun. The studio received some of the very first units off the production line. The following months, Apogee released several engineering changes including many SMT part changes to improve performance as well as a more robust power supply. I went through each of these units and performed all the changes and the ran them through testing and diagnostics before sending them back to into action. To this day, these units are still in operation.

The Lynx series of converters started out with the LynxONE PCI card and followed with the LynxTWO. They were PCI cards made to be installed INSIDE a PC case. While logic would have most people thinking this wouldn't be a wise idea for low noise, these cards are perhaps the most lowest noise PCI sounds cards available even still today. I use a LynxTWO with some diagnostic software to make various audio measurements, noise, distortion and otherwise. The LynxTWO uses Burr-Brown INA134 input receivers as its front-end preamplifier. On one occasion, I had made an erroneous hookup to a power amplifier for testing and managed to burn up the INA134. However, this part is easily accessible and after replacing it, the card was returned to perfect operation. Throughput residual noise was exactly as the adjacent channel.

In the process of upgrading their existing PTools rig, a client of ours picked up set 192 I/Os along with some new storage and the PTools HD software upgrade. In the process of bringing up some older projects, they started to discover some inconsistencies with the levels. I went in and had a look at the new I/O boxes and found that the new converters were outputting a slightly hotter signal level and then compensating with a slightly attenuated input gain setting to result in a net 0dB throughput. The 192 I/O interfaces have an A and a B set of trim pots to accommodate two different calibration settings. We left the A side at the factory setting and set the B side by playing back a calibrated 0dBv tone in a Pro Tools session and then adjusting the B trim pot to produce a corresponding signal voltage level at the output jack of the interface while it was plugged into a load. We then routed a 0dBv signal to each input channel, monitored the output and then adjusted the input B trim pot for a 0dBv input, repeating the procedure for every channel on each unit. It was interesting to discover that there were differences in the levels from channel to channel on the average of 1 - 2.5dB. I also discovered one of the output card's trim setting relay switching was not functioning when engaging from the workstation. This experience just reinforces the fact that converters still require attention in the calibration much like a multi-track recorder. If you're critical of coherent and consistent levels, set some time aside to check your converters from channel to channel to insure you're getting consistent results from project to project. Over time, circuitry does drift in performance due to heating and other factors beyond the user's control. While most converters don't have provisions for adjustments, one can still make some measurements and document the results and the compensate during mixing in the workstation accordingly if need be in the case of some extreme inconsistencies from channel to channel.

Speakers

I had a 1031A in the shop that had apparently vibrated off its stand and fell on the floor. Upon inspection, I found some of the circuitry in the power amp had actually fallen off the PC board. This was due to the tin plating on the PC board flaking off the copper traces. The parts then just simply fell off while still being soldered to the tin plating. The solution was to simply clean the copper traces, re-tin and resolder. I have not seen before or since any circuit that has had this rather odd symptom.

I've had a few of these monitors come across the bench. A couple of them needed new drivers due to extreme use. BTW, the indicator LED on the bottom front face of these is normally green when things are good. When it turns red, that's bad. I had one in that had a blown pre-driver op-amp, BB OPA445B, which is an odd beast in a 8 pin round metal can package and needless to say, the red LED was stuck ON until everything was cool if you know what I mean.

Microphones

This is a rather obscure stereo condenser mic. Two 1 inch diaphragms stacked atop one another with each of them able to be adjusted in different directions from each other. The unit came with it's own remote control and powering box that has a multipin connector used for connecting the microphone with an umbilical. I had one in the shop that was not functioning due to the multipin connector being twisted around and shearing off the internal connections. It was necessary to make new connections internally with jumper wiring and then key the multipin so that it wouldn't spin.

So, here's probably the most commonly used dynamic microphone on the planet. I just wanted to include it here to discuss a little about the common problem of windscreen getting knocked off by the hit of a drumstick. since this mic is frequently used on a snare drum, these mics would always come in with the windscreen knocked off. This is not really a big deal. The plastic cap style windscreen is held on with a small retaining ring. To re-install the windscreen, it's a bit of a puzzle, but not all that difficult. First, slip the retaining ring inside the windscreen into the slot that it will normally ride, but leave just the ends of the ring exposed and not seated. Then, simply tilt the windscreen partly to one side while it's placed over the capsule. The retaining ring is still hanging somewhat out of the edge. Now, it's all a matter of compressing and squeezing the retaining ring ends together with needlenose pliers while pushing it all together. It's a trick, but once you've done it once, it'll all make sense.

Synths & Keyboards

This keyboard ended up becoming a workhorse of the Mexican music scene. It's almost impossible to see one of these styles of bands without one of these. The main problem with them is the key contacts. They are a rubber membrane with carbon contacts that comes in contact with the PC board contacts to actuate the sound. What happens is that the contacts become sorta of dirty and clouded over with a residue that prevents a good contact. Repairing this can be done by replacing the membranes, or, better, just to clean them with a chamois and some denatured alcohol. Be careful not to scrub too much, and also when disassembling the keys and springs, take note that the white keys and the black keys have different length and tension springs, so sort them so that they can be replaced correctly.

I took care of a few of these for a local touring band. We performed some customizations on them, including adding some electronic balancing circuits and some permanent jacks to plug lite-lights in on the top of them and the usually hard drive and memory expansion kits. The keys were subject to the same contact issue as the D-50 and required cleaning from time to time. As well, I swapped out a few floppy drives. One of the main weak spots on them is the backlight for the display. The backlight is a high voltage electro-luminescent sheet that goes dim over time. It's possible to replace this backlight and there are several online retailers that sell backlight sheets custom trimmed to fit the K2000 display.

Miscellaneous