Eurodesk PSU

[Enzo]

yes^^^

You will never trace the board, there are a million little pathways.

Isolate the issue to one board.

Let it sit a minute running and see if any ICs get hot.


Sorry bud, but a common cause of something like this is a shorted bypass cap. Each IC has a small cap to either power rail, and one of those somewhere croaks. We look through a lens for a tiny black mark or something on a tiny blue cap.

[BalancedLineOut]

Remove the jumpers between the larger boards, one-by-one first, each time checking to see if the voltage comes up.

Yes, OK. THere are several groups of black wires, which I am guessing are the audio lines. I've left them alone for now. On the Main board section...where the power connects to the first PCB... there are 3 different sets of jumpers. The topmost is smaller than the other two. When I disconnect the topmost one the voltage reading rises from .7DCV to .9 DCV. removing the other two doesn't cause any further change, nor is there a change when the first one in left in place.

[BalancedLineOut]

In addition to the rainbow wire jumper groups, and the "audio' bundles of wire, there was a flat ribbon connector that ran between the main PCB and the output section. Removing the ribbon wire had no affect. However when the bundles of black wires that ran between the main PCB and a smaller PCB that looks to be just a bunch of I/O connectors - aux sends, returns that stuff. The voltages +48 and -17 started to drop. Just enough to notice. connection the black groups back, the +48 and -17 stabilized at their normal levels. +17 never came up beyond the .9VDC mentioned.

I was thinking about those bypass caps... as long as they are not SMT's I should be able to replace them. I've had a lot of solder and de-solder training, it's just been years (ok, you got me...decades) since I've had to do anything like this. My electronics theory isn't top drawer anymore. Solder skills are still good though. Better equipement than most, too. So I'm up for it if I have to. But, lemme figure out what the problem is first. Mucho question I'm sure to follow.

Steve

[BalancedLineOut]

I've pulled the master section PCB out. This in the one the power connects to. I am figuring since the drop doesn't go away when the other board are disconnected that the problem at least has to start in this main section. Might not be limited to here, but I guess this is the place to start.

This is an analog board, should I be OK to power up only the master PCB and start tracing the +17? Initial inspection doesn't show any caps that look failed, but I know caps can dry out and look perfectly fine. Nothing looks blackened our otherwise smoked.

I've gone this far...

[BalancedLineOut]

Well I managed to wrestle the main board out of the mixer frame. Where to start? A quick look at the electrolytic caps and there "might be one that is different than the others. Hard to tell wihtout a magnifying glass. I was hoping to take a picture of it, but the camera doesn't pick up the subtleties.


Anyways....can I power up this PCB with the PSU and start tracing?

This is a cap I thought looks suspect:



What is the value listed? 100/10? Would tha be 100uf/10V? Scratch this, my son was able to read it...

As far as replacement CAPS... KEMET, RUBYCON, or NICHICON

If I unsolder a suspicious cap, should I see voltage come back up if that was the culprit?

[Jazz P Bass]

C171 capacitor is a 100uf/ 10 V part.
It's function is a DC blocking cap.
It keeps any Vdc from the opamp out of the next circuit.

I do not see how that could pull down the power supply.
Lifting it will simply stop the signal flow.
As would the mute switch.

[BalancedLineOut]

OK well then I guess it's gonna be something in depth. C171 just looked a little "Different" thatn the rest of them. BUt without a magnifier I can't be sure.

Suggestions as to next steps?

[Jazz P Bass]

Measure the resistance to ground of any opamp power pins.
Pin 4 is the -17 rail & pin 8 is the +17V rail.

[BalancedLineOut]

If I power this board up on the bench without being in the frame, is that ungrounded?

OK, here's a new, "fun fact"... PC Board is still unpowered. I'm considering the metal frame of the pots as a ground point. Measuring between any of them gives 0.000Ω. The power input connect has 3 pins, 48, -17 and +17.

Using ground point as mentioned above, 48(vdc) pin gives infinite Ω's (O/L). Same with the -17 pin - infinite Ω's (O/L). When I do the same test on the +17 pin I get 0.000Ω.

Checked and re-checked.

[g1]

Yes, your +17V line is shorted to ground. Unfortunately, there are hundreds of things connected to it and any one of them could be causing the short. The part responsible will be heating up

Let it sit a minute running and see if any ICs get hot.
Sorry bud, but a common cause of something like this is a shorted bypass cap. Each IC has a small cap to either power rail, and one of those somewhere croaks. We look through a lens for a tiny black mark or something on a tiny blue cap.

It's not the can type electrolytic caps you are looking for, but the tiny blue ones.

[Jazz P Bass]

Well then I guess the pots are not connected to power supply ground.

There is a 5 pin header from the power supply.
It's labeled X7 on the schematic.
Pin #4 is a common ground for the power supply.
Find where the header goes to the main board.

Pin #4 will be your ground reference.

[BalancedLineOut]

X7 on my board is a 10 pin connector. If you look at the first picture I posted with the rainbow connectors, X7 is the connector with the rainbow wires at the bottom of the image, has a red test lead running right above it. X7 on the MAIN PCB is on the left.

X4 on the otherhand is where I was measuring voltages for =48, -17 and +17. It's 3 pins at the top of the board, no ground. I was referencing ground to the metallic case of the DIN power plug.

I can see the tiny blue caps, not that it's been drawn to my attention for the second time. FOrgive m, I'm a bit in over my head, but I'm this far in now...

Now that I take another look - after what you just said JazzP, the common ground is "common" to everything. That would include the LAMP socket. When I disassembled and removed the PCB I had to disconnect the LAMP socket from the frame. There was a brown wire going to the center termnial and a black wire which attached to a lock washer to provide the return path. This connects directly to the PSU connector jack on the rear of the frame. I'm guessing everything is common grounded to the frame, so that's the connection back to the PSU for all voltages. Did that make sense? IF so where does that put the ground point, or at least a valid one for test purposes?

It took me a while to find X4 but here it is on PG 32...

[diydidi]

Sometimes they keep analog and digital grounds separate. One is marked A the other D.

[g1]

You are measuring 0 ohms from the +17V line to the pot casings. This means the +17V line is shorted to ground. This is what was expected because you had isolated the problem to this board and were looking for a short or low resistance from the +17V line to ground.
No point trying to chase down where ground is.
You need to find out which component gets hot.

[bkahuna]

The part responsible will be heating up

Not necessarily. I've seen some shorted components not get warm at all, especially when a bunch of stuff is powered from one of these 3 pin regs.

That's where the trick from my post earlier in the thread comes in handy.

Failing that, you can try measuring all the bypass caps with an ohm meter. They will all measure a short, but assuming you're making good contact with each of them, the one that measures the lowest is likely to be the culprit.

Could also be a shorted op amp, as others have noted.

If you feel like you're going to replace all the caps anyway, you could start snipping them out with the unit powered on until it comes up, then just replace the ones you snipped. That could be a large number, though.