COTA troubleshooting - was working but now silent

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bloke_zero
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COTA troubleshooting - was working but now silent

Post by bloke_zero » Mon Sep 21, 2020 4:19 am

Hi,
I just took my COTA module out of storage and put it in a new rack and it's no longer working. I'm pretty sure I didn't plug it in wrong though I can't be 100% sure.

The symptoms are - when plugged in to power it does not pass any audio except bandpass which seems to almost work but not with resonance. Both LP outputs are silent. Removing power sound passes through.

My question is, is it worth trying replacing the chips? It seems unlikely to be mechanical/solder related as the module worked perfectly 2 weeks ago and a visual inspection shows no visible solder errors (it wasn't built by me - I think it may have been professionally built by Krisp1 - it's certainly at a much higher standard than my builds!).

Any input welcomed!
Alex

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Re: COTA troubleshooting - was working but now silent

Post by Synthbuilder » Mon Sep 21, 2020 4:58 am

The first thing to do in these cases is check the power supply on the board. Make sure that +15V and -15V are getting to the board and that both pins 2 and 3 are both connected to 0V of the power supply. The best way to do this is with a multimeter on the resistance setting with the power off but the module plugged into the distribution board. Check the resistance in each case is lower than 2 ohms from the relevant point on the distribution board to the relevant place on the module. The best place to measure at the module end are the solder pads of the power connector on the underside of the board.

Tony

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Re: COTA troubleshooting - was working but now silent

Post by bloke_zero » Mon Sep 21, 2020 5:50 am

Hi Tony,

Thanks for those suggestions!

* I get very low resistance measuring from the distribution board to the module in the manner you suggest.
* I've tried known good cables and plugging into a whole different power supply

Same results.

I just tried swapping U2 & U4 which made the -12db LP output start working *but* with no resonance - 24db still no audio.

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Re: COTA troubleshooting - was working but now silent

Post by Synthbuilder » Mon Sep 21, 2020 6:24 am

bloke_zero wrote:
Mon Sep 21, 2020 5:50 am
I just tried swapping U2 & U4 which made the -12db LP output start working *but* with no resonance - 24db still no audio.
That sounds like U4 could be dead.

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bloke_zero
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Re: COTA troubleshooting - was working but now silent

Post by bloke_zero » Mon Sep 21, 2020 8:17 am

Cool - thanks - I've ordered a couple of replacments.
Cheers!

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Re: COTA troubleshooting - was working but now silent

Post by bloke_zero » Thu Sep 24, 2020 4:51 am

That was the problem - replacing U4 solved it. Many thanks for taking me through the troubleshooting Tony!

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Re: COTA troubleshooting - was working but now silent

Post by Synthbuilder » Thu Sep 24, 2020 7:52 am

bloke_zero wrote:
Thu Sep 24, 2020 4:51 am
... replacing U4 solved it...
Excellent news.

I wonder what happened to it. Those things are usually pretty resilient. Although a short to ground on pin 1 and/or pin 16 with the power applied will kill them.

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Re: COTA troubleshooting - was working but now silent

Post by bloke_zero » Fri Sep 25, 2020 4:17 am

I was auditioning modules in a case and they weren't fully screwed in so a short isn't impossible. It was a shoot out between your SVT an old MOTM GX-1 filter clone and the COTA and I guess the COTA actually died! I really like the COTA but currently can't help but use the SVT all the time - something amazing about the tone and how the resonance doesn't kill the bottom end. Though I always return to The Super Ladder 3 I built first.

Thanks Tony for the help and all the great filters!

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