Sample & Hold Malfunction

[Samuxx]

Hi!

I built myself a quite nice Modular Synth - Yusynth layouts. But this very last module is finishing me off! This is already my second pcb try. - Noise works well...

The sample rate works, as well as the led. But when i give input to the s&h (for example white noise) - nothing. At the middle position of the slew rate pot I get some very quiet sound (about 8000hz) beeping in the rythm of the sample rate.

BC547C as Q1
BF 256B as Q5
TL 074

Thank you very much for any feedback - Sam

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[Samuxx]

I changed:

Q5 back to a F245C
TL074 with a new one
the C4 cap from 1n to 4,7n
the C5 cap 100 n with a new

No result. There is still the same effect.

[PHOBoS]

hard to tell what it is without having it in front of me (and even then it can be difficult).
But, if you haven't already, try to measure the (input) voltage before and after the FET.
Best to use somethng with a steady (DC) voltage on the input, although an LFO could also work.

oh and

[yusynth]

Do you have an oscilloscope ?

[Samuxx]

Thank you for the welcome. Yes, I do have an oscilloscope, but just a simple digital one...

[yusynth]

First I suggest that you check with the oscilloscope the voltages at pin 14 of U2, then between D3 and Q5. With a noise signal at the input (max range +/-4V) and the jumper on LOG check the signal at pin 1 of U2

[Samuxx]

So I tried to measure 14 of U2.. Nothing. I found out that I get signal (a clean square) at the 'left' side of c4 but nothing on the other. I tried 1n and 10n now for c4.

[yusynth]

On the other side of C4 you must see a brief positive spike (1 to 3 ms). If this is not the case check there is no short cut to the ground at the junction of C4 D2 and R11. Check that D2 is not in short-cut.

[Samuxx]

Finaly found some time to do some measurements. Unfortunately my oszi is very bad. Plase note that D3-Q5 is only 1 pike - not 2 like in the picture.

I think the pictures are quite ok, except pin5 - am I right? 1 changed almost every part twice and cheched all the connections, but I can not find any wrong part.

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[yusynth]

All your scope snapshots look OK to me. What bothers me is the amplitude of the noise signal you input in the circuit, merely 1Vpp, I would expect something bigger like 5 to 8Vpp.

[Samuxx]

I checked everything again, and for some reason it finally works. I have no idea why.
Thank you very much for your support. It seems my first rack, filled with Yusynth modules, is finished. It took me one year.

[yusynth]

If so (not working then suddenly working) check for a bad solder or a loose-contact...