Amek BCII Possible PSU problem

Graham Langley founded Amek in 1973 with Nick Franks. Since then, AMEK and TAC recording consoles have been the choice of demanding audio professionals for over 25 years.

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Amek BCII Possible PSU problem

Postby mathsieve » Mon Jan 13, 2020 4:20 pm

I've had a BCII for about 10 years now, and it's been a real workhorse. Recently, it has started to malfunction intermittently. The LEDs will dim to about half brightness, some channels will go quiet or distorted, and a peculiar noise is emitted for 1-2 seconds - it's a descending square wave tone probably from about an octave below middle C. The desk did this again this evening, then went dead (no sound, no lights). I turned it off and on again a few minutes later and it was OK for a bit, but then the same again.
This seems like it might be some component failing. Certainly the noise sounds like something dying. It's too quick to catch whether it's a particular channel or the master bus, so I don't know which module to begin troubleshooting.
I am also guessing that it may be the PSU because the lights dim, and the sound drops. I haven't tested to PSU because I am not sure what to test.
I know this is a bit vague... but I don't know where to start. Any information is gladly received!
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Re: Amek BCII Possible PSU problem

Postby jensenmann » Tue Jan 14, 2020 10:18 am

Which PSU does your desk have? There are a few spots which are more likely to be responsible for the problem. Fuseholders sometimes go bad, sockets of voltage regulators and their power transistors sometimes make trouble, occasionally caps go bad, esp on older 2000 Series PSUs with tantalum caps inside. No matter if the PSU actually is responsible I´d have it recapped as this is like renewing oil in a car.
Nevertheless repair work on a PSU should only be done by a qualified tech.
Jens
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Re: Amek BCII Possible PSU problem

Postby jensenmann » Tue Jan 14, 2020 10:24 am

When the PSU works properly and the problem still is there then remove all channels from the frame and power the empty frame up. Then reinstall one channel after another and power it up with each new channel to see which of them causes the problem.
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Re: Amek BCII Possible PSU problem

Postby mathsieve » Tue Jan 14, 2020 11:12 am

I have the Amek TAC Matchless PSU which I think was common for the later desks. It's exactly this one, except it is 240V:
https://www.ebay.com/itm/AMEK-TAC-Match ... 2784684627

I'd prefer not to get the PSU recapped unless absolutely necessarily, simply because the reliable techs I know have fairly long waiting lists / turn around time and that might mean not being able to work for a month or more.... I've thought about removing all the modules as a test. This seems like it might be the way to go.

Besides identifying a dodgy module is there anything I should be looking for in the chassis?

Some more info: I have the 24 module chassis. I noticed last night when the problem struck that all sound on the first 12 modules (all mono channels) was completely lost. On the second set of 12 modules (4 stereo channels, 4 groups, the master, aux and 2 head phone channels), sound appeared present, if quiet and distorted, but the LEDs were out.
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Re: Amek BCII Possible PSU problem

Postby mathsieve » Tue Jan 14, 2020 1:45 pm

I've just noticed that the +V LED on the PSU also dims when the desk suffers from the problems described. It's always been slightly dimmer than the -V LED, but I assumed that was nothing to worry about. I guess this does point to an issue with the positive power rail.
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Re: Amek BCII Possible PSU problem

Postby Jim Williams » Sat Jan 18, 2020 12:44 pm

Unlike their competitors, AMEK elected to use a 1N4002 rectifier diode in place of the usual power supply fusing resistors you find in other designs.

When there is a problem on a module, the power will continue to flow to it, even if there is a direct short. Other consoles will get those fuse resistors burned through, creating an open circuit to protect the rest of the module/console/power supply.

If you have a defective module with a fried opamp, current will continue to flow possibly pulling down the power rails. The only way to find that error is to install them one at a time, a PITA.

That's a good time/reason to remove those diodes and install 10 ohm fusing resistors. If you want to avoid resistor burn effects, use the flame proof versions and mount them a few mm above the pcb surface to prevent that from getting burned.
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Re: Amek BCII Possible PSU problem

Postby mathsieve » Tue Feb 18, 2020 9:20 am

My BCII has been working OK for the last month or so, with this fault occurring intermittently. I've either given it a rest or found that unplugging the PSU, letting it run on its own for a bit, then replugging it seems to sort the issue. I've tested the 4 pin XLR power lead and it seems fine though. The voltages from the cable read: +V: 17.79; -V: -18.19; Phantom: +47.92.
Today unfortunately, the desk has become completely unresponsive and the red LED on the +V light of the PSU is unlit when plugged into the desk. I've pulled the modules one by one and there's no particular culprit. The desk will run with the master channel and any one module connected - but connect a second module and no sound passes, and the +V LED is again unlit.
I presume this definitely indicates there's a problem with the PSU? I've tried reseating the fuseholders, and have tested them obvs.
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