[fixed] No sound and filter issue

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rv0
 
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[fixed] No sound and filter issue

Post by rv0 »

I'm trying to fix a friends "first build". I could solve most of the problems so far but there's 2 problems I can't seem to fix:

- no sound on outputs
Seems like the VCA is not working.. I can inject sound on IC15B pin 7&8, but not on pin 2&3.
Voltages:
1: 5,32
2: 5,32
3: 0
4: 0
5: 0
6: 5,04
7: 5,04
8: 5,36
9: 11,97

- very strange filter behaviour
When I listen to the filter at some settings it starts to warble severely, like an LFO on the signal. Funny is that this happens with a short delay. I made a short recording to demonstrate:
https://soundcloud.com/rvooh/x0xc14-2/s-iCDNy
A cool sound, but completely wrong ;)

Suggestions are welcome.. I've tried a few things already, double checked most of the parts and reflowed some points, but no progress any more.
Last edited by rv0 on Sat Jan 03, 2015 10:01 pm, edited 1 time in total.

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rv0
 
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Re: No sound and filter issue

Post by rv0 »

One week later, but I haven't made any progress so far. Suggestions are welcome.

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mmm
 
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Re: No sound and filter issue

Post by mmm »

As far as the filter behaviour goes: I've experienced similar sounds when the filter ladder was not in balance. Usually Q22 is to blame here. You could try another Q22 or solder a "makeshift" Q22 out of two transistors.

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rv0
 
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Re: No sound and filter issue

Post by rv0 »

@MMM thanks for the reply

The filter issue is fixed..Q22 was alright but my measurement method was flawed due to not using a resistor (this combined with reversed 2&3 on J4 caused the warble)

J5 was also twisted, causing the headphone out not to work. Could this have damaged anything?

When I turn it on now, I hear a note from the output, If I'm quick I can play a pattern but it fades out quickly after a few seconds.

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mmm
 
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Re: No sound and filter issue

Post by mmm »

darffader wrote: J5 was also twisted, causing the headphone out not to work. Could this have damaged anything?

When I turn it on now, I hear a note from the output, If I'm quick I can play a pattern but it fades out quickly after a few seconds.
While I wouldn't rule out that twisting J5 could have damaged something I don't think this would extend to the regular output.

Let's go back to your measurements at IC15B. Perhaps there's something wrong there. While the values look so far ok it would be important to measure again what happens at pin 4 when playing a note. Without it it should be 0V (like you wrote) however when playing a note this should rise to about 1V. If it rises to about 12V it indicates that IC15B has failed in one way (there may be other failure types though).
Pin 4 is the control input for IC15B. It might be that the control signal (the volume envelope) doesn't make it to IC15B.

While you're at it you can measure if you're getting a proper Gate signal at the Gate output.

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rv0
 
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Re: No sound and filter issue

Post by rv0 »

@MMM
Thanks so much for your help. It got me thinking in the right direction and It's fixed now!

There were 4 more issues:
- C62 had the wrong (non-tantalum, non-polarized) cap in it
- C72 & C55 values were swapped
- the cpu board i was testing with was broken. (I was using an old textile cpu mod proto). This caused the gate voltage to be constantly on. Replaced with a x0xlarge and all is well now.

Anyhow, glad this got fixed, thanks again. (and thanks to antto for the support on #x0xb0x IRC channel)

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