Did I fry my R*S serge panel??

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sdhains
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Did I fry my R*S serge panel??

Post by sdhains » Sat Jun 16, 2018 5:55 pm

Greetings forums,

Was removing a board and managed to plug some MTA cables in the wrong way. I noticed that when I plugged the PSU back into the panel, PSU lights turned off.

After this I noticed that my dual slopes were acting weird. The left side was producing voltages but it was struggling to get into the audio range and turning down the FREQ did not bring the slope to a stop, just to a strange chugging LFO.

The right side didn't seem to be getting triggered at all. I kept testing, turning the unit on and off. Eventually the module stopped working all together - with both sides lights permanently turned on - no sound, no CVs ... flatline.

I also noticed the Variable Q filter (VQ VCF) was also not working properly. Sound was not coming through. The list of 'symptoms' for the VQ VCF are a little more complicated. They are:
1. Notch doesnt work at all, on any input setting.
2. 'IN' input seems to have no effect on the output.
2. 'LOW', 'BAND' and 'HIGH' produce an audio output with 'ACG IN' but the Q is locked to a high setting.
3. With no input - 'LOW', 'BAND' and 'HIGH' produce a very quiet, high pitched, resonant noise when you move the 'FREQ' knob. This noise is the same across settings. Perhaps this is just a result of the high Q setting that the filter is locked onto.
4. 'TRIG IN' not working.

:sadbanana:

I'm really not sure what to do about this. I checked the PSU and it seems to be outputting 12V of DC, so I don't think thats the problem. I'm considering just buying a new Dual Slope/VQVCF from R*S and rebuilding them. Or maybe just buying the parts and doing a big desolder 'n swap. I dnno.

If anyone has any advice about how to proceed with debugging this unit, or possible solutions please reach out! Wanting to get back up and running ASAP!

Thanks forum :hail:
Last edited by sdhains on Sun Jun 17, 2018 2:25 pm, edited 1 time in total.

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jersupereq
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Post by jersupereq » Sat Jun 16, 2018 8:04 pm

I would post some pictures of the boards so we can see if anything looks burnt

sdhains
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Post by sdhains » Sun Jun 17, 2018 10:28 am

Hey, yeah good idea. Here they are:

VCFQ:

Image

DSG bottom:
Image

DSG top 1:
Image

DSG top 2:
Image

sdhains
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Post by sdhains » Mon Jun 18, 2018 7:21 am

So after much anguish and research I am planning to fix my panel in the following way.

Replacing the IC's and the capacitors on the VCFQ (through hole).

Purchasing a new DSG pcb.

I looked into repairing the DSG, which would likely involve replacing the SMT component op amps and capacitors. However, it seems that some of the op amps on the board have been 'scrubbed' by R*S. They are protecting a trade secret :hmm: . I also was having difficulties trying to identify the capacitors on the board.

Markat
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Post by Markat » Tue Jun 19, 2018 3:17 am

If you're talking about the 8 pin ICs I'm pretty sure they're just standard op amps.

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Pelsea
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Post by Pelsea » Tue Jun 19, 2018 12:32 pm

Generally, when you misspower a board, one of the components gives its life for the greater good, shorting out so the PSU shuts down. The trick is finding the corpse. First, get an ohm reading across the power pins. It's probably a dead short. Check this as you replace components-- when the reading jumps into the kohm range you have likely found the bad one.

Transistors are usually the first to go, so you can pretty much plan on replacing all of them. The exception is those that turn on LEDs-- the LED may have protected them.

ICs will be next in line. With through hole components, you can sometimes isolate the power pin and check if that particular chip is shorted without removing the whole thing. Some chips die open, so an OL reading on the power line also indicates RIP. You can do this by cutting traces, but that will leave your board a mess.

While many caps are polarized, they can usually survive a brief power reversal. Also, they go messily, so absent visible damage or sludge on the board, you can put them off till last.

"Scrubbing" chips does little to thwart reverse engineering, so I doubt they bother to do that. (You can tell if a chip is an opamp just by looking at the connections.) When you buy chips in bulk, the labels are often badly printed.

However, if it were my module, I'd just buy a new board. This will take days.
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fuzzbass
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Post by fuzzbass » Wed Jun 20, 2018 8:41 am

Pelsea wrote:However, if it were my module, I'd just buy a new board. This will take days.
Yes. Unless you know what is bad, you are possibly in for a lot of trial and error SMD rework. Im guessing because you ask the question, you don't have an SMD rework station. Using an iron, you likely will end up lifting pads, swearing and then ordering a new board. Just skip the first two steps.
Wired for weird

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cygmu
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Post by cygmu » Wed Jun 20, 2018 9:32 am

fuzzbass wrote:
Pelsea wrote:However, if it were my module, I'd just buy a new board. This will take days.
Yes. Unless you know what is bad, you are possibly in for a lot of trial and error SMD rework. Im guessing because you ask the question, you don't have an SMD rework station. Using an iron, you likely will end up lifting pads, swearing and then ordering a new board. Just skip the first two steps.
Does the advice change if you know that the boards cost $200?

The ICs are definitely scrubbed, by the way.

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wackelpeter
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Post by wackelpeter » Wed Jun 20, 2018 10:51 am

What IC's are on that board? The 14pin should/could be the LM3900 the 2 8 pin below 2xTL072/ LF353 or equivalent, the 2 6 pin 2 matched trannies and the other one on the low end is what? or did RS used 2 single OP amps were CGS for example used a dual-Opamp…

The schematics should be Overall almost the same and will be a good advice if you want to probe around wiht your scope, figuring out the damaged parts…

That's a reason why i would avoid to buy in SMD apart from that i didn'T have the Tools all the components are a bit too Tiny for me you can't just swap the IC's in and out...

Positive side is you need less space and many through hole IC's that are obsolete in that Format are still or again available in SMD... but for an average DIYler like myself TH still has more advantages…

Edit: in my previous lines i just referred to the DUSG where as it seems to cycle probably the Op amp processing rise and fall times or the comparator are faulty…

what resistance can you measure between gnd to + and gnd to - with power turned off?

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