Sample & Hold not working

[saint gillis]

Hi I ve got a S&H module in this old modular synth (http://www.muffwiggler.com/forum/viewtopic.php?t=93641&start=0&sid=4f1809f3724fc7156302b9cf4eff3a98) that's not working properly,
  The synth has been restored by SynthRestore, and an additional oscillator has been added, the red part :

http://img197.imageshack.us/img197/810/n7jo.jpg

There was already an oscillator, the 2 PNPs at the top left, and the inputs marked "??" looks like negative voltage applied to vary the speed... I tried to remove the red part, to bias properly the oscillator (the 2 PNPs) and I found some kind of negative triggers at several places in the schematic, marked in blue.
  It still doesn't work properly, and my lack knowledge in matter of transistors etc preserve my from getting ideas to make this module work as it should..
  Any help would be appreciated, thank you forum!

 

[JohnRoberts]

Do you have an oscilloscope? With that just trace the signal path to see where it gets squirrely.

Without a scope, you can troubleshoot with a VOM measuring the DC voltages at various circuit nodes. Look for a voltage that is invalid for how the devices work.

I can't remember last time I saw a unijunction... I never used one in a design.

This looks like a good opportunity to learn about discrete design. THe fault is probably something simple like an open connection or bad component.

JR

 

[saint gillis]

Hi JR,
yes I have an oscilloscope that's how I can say that there are some triggers pulse at the 2 nodes indicated on the schematic..
  There have been some modifications in this module so it's not easy to guess if it can work properly as it is, I have replaced most transistors..

 

[JohnRoberts]

OK how is not working properly?

Of the output is wrong, work backwards from the output until it is working properly and just after that point is your problem.

JR

 

[saint gillis]

Actually it is not really sampling, the input signal goes through the fets, but maybe the trigger signal has not the correct voltage to sample, I hoped I could find somebody familiar with thoose sampling circuits to give me indications..
  I found this page :

http://webpages.charter.net/dawill/tmoranwms/Elec_Sample.html

I'm also trying to understand some things with those circuits :
http://experimentalistsanonymous.com/diy/Schematics/Full%20Synths%20Drum%20Synths%20and%20Misc%20Synth/AR-318%20Sample%20And%20Hold%20and%20Noise%20Generator.gif
http://yusynth.net/Modular/Commun/NOISE/NoiseSH-sch.jpg

I ll try to change the 100n capacitor between the 2 fets

 

[JohnRoberts]

When I was a young tech just starting out, I was able to troubleshoot and repair different types of equipment even with little understanding of the circuit design, just by measuring transistor and diode junction voltages to confirm they were good, and operating properly. Likewise with opamps, measure input voltages and confirm that output is valid for the measured input.

JR

 

[saint gillis]

Yes of course that's what I'm doing, but there have been modifications in this circuit, it comes from an old unique modular synth, it might be an interesting subject for synthetisers fans, for people who has worked on modules of this kind during the 70s, and at the same time helping for me...

 

[saint gillis]

I seems difficult due to all the modifications made on this module to find out how it was made at the origin..
So I v put instead on the stripboard a Yusynth S&H, it's great!