HELP! TAC Console PSU Eating Regulators

[lookn4tone]

Man, I'm begining to think my luck has taken a turn for the worse.

I have a TAC Matchless, and out of the blue, the PSU (which had been completely recapped and updated 6 months ago, with no problems since) has just started eating the + side voltage regulators.

I'm assuming there's a short somewhere in the console, but I can't figure out where (and why it would happen without so randomly). If I turn on the PSU without connecting it to the console, there's no problem.

I took out all of the channel strips and there still seems to be a significant current draw (the LEDs to the + and -17V go dark as soon as I turn off the power; phantom goes out slowly as normal). If I leave the PSU on (attached to the console), I blow a voltage regulator within minutes (again, plus-side only). Any thoughts?

And how would I go about testing? Could it be a cap on the meter bridge or mother board? I've looked for glaring shorts, but nothing seems obvious. And testing is very difficult, since I blow the regulators quickly.

This just sucks.

As an aside: Should I add fuses to the PSU BEFORE the regulators, so I blow fuses instead of costly regulators? Is this a design flaw from TAC? The fuses (in the circuit currently) don't blow, just the regulators (of course).

Thanks in advance.

Steve

 

[jensenmann]

Steve, did you fix that issue? What was the fault?

 

[BYacey]

If the channel strips are on plugs, unplug them all, monitor the B+ rail with a current meter and start plugging them in one by one until you notice a sharp rise in current draw. If it's in the master section look for signs of heating and touch the components looking for heating. Sometimes it helps to run at a reduced voltage on a variac preventing things from burning up right away and blowing fuses. Whatever the problem, you have to isolate it to a specific section first.

 

[pstamler]

What kind of regulators?

Do they include a diode from output to input (cathode at input) to protect the regulator against getting zapped during turn-off?

Are there any tantalum decoupling capacitors in the board?

Peace,
Paul

 

[lookn4tone]

The short answer is yes and no (with regard to whether I fixed the problem). I took out all of the channels, and it still zapped the regulator. Then I changed all the diodes and caps on the mother board, and still had the problem. Then I disconnected the meter bridge (which I should have done first), and the problem was (seemingly) gone. I put each channel back in. No problem. I reconnected the meter bridge, one section at a time, and seemed to isolate the problem in the PFL meter section. I reconnected that PFL section to double check, and everything worked fine. Though (big caveat here), I have not run the desk for hours and hours at a time. In fact, I've run it very infrequently because I've been recapping compressors and doing diy incessantly instead of recording music.

If you own this desk, I strongly recommend that you contact Colin at AML. I recapped the desk and upgraded all the pre to the Langley's from parts he provided. The results have been impressive (a lot of work, but well worth it).

Here's a tip: When recapping, be very careful around the big axial cap in the preamp section (can remember the value, but it's the only axial cap on the strip). I've lifted several of the traces, as one side is a terminal and lifts easily. Apart from that, it was a fairly straight foward recap. Oh, yeah, some of the caps near the faders on the stereo channels are not installed according to the orientation printed on the board itself (and at least one is not marked at all). Make sure you have a schematic before you begin.

Regards,

Steve

PS--Colin also has some mods for the PSU on his website. They eliminate the regulator plugs, requiring the regulators to be soldered directly to the PCB. Don't know if it is worth the trouble, but seemed like a good idea.

 

[jensenmann]

thanks for your info :thumb:
:guinness: :guinness: :guinness: