Help!!! My Sample & Hold stop working.

Sebo · Joined: Apr 27, 2007 Posts: 564 Location: Argentina

Hi Yves:
My Sample & Hold was working right. I have set a patch from a week or so, because I will use the Modular in a gig. Today when I was setting up the system for a rehearsal the SH module didn't work right.
The module samples the input, but the output slowly decay to 0.
I will try to change C5 to see if the cap failed.
Any ideas will be welcome.
I need to repair it in next 3 days because the gig is next friday.
Thanks in advance.

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

If your ICs are mounted on sockets, try first to replace the TL074 (U2). If this does not solve the problem, then change the cap C5, and you may also change the JFET Q5.

Sebo · Joined: Apr 27, 2007 Posts: 564 Location: Argentina

Hi Yves:
Thanks for the response.
I've changed the TL074, nothing happens, the I replaced C5, again nothing, then I replaced the JFET, originally I had a BF245A, I changed it for a BF245C, now the things get weird:
With noise (from the same module) at the input, at the output I get noise burst at the clock pulses.
With the Random output (from the same module) connected to the input I get the random only for the time the pulse is on.
With any other source (I've tryed various LFOs and the noise of my MS-10) the SH works perfect.
The noise circuit seems to be working right as I get white & pink noise and random voltages without any problem.
I'm lost...

Sebo · Joined: Apr 27, 2007 Posts: 564 Location: Argentina

After some more experiments I discovered that if the noise is attenuated a bit, the SH works right.
So I think the problem started with the BF245C.
At this point I don't know what is best, to change the JFET to a BF245A again, or to attenuate the noise output.
It's seems that with the BF245C works very well with any source except with noise (high level). May be my noise is too high.

yusynth · Joined: Nov 24, 2005 Posts: 1314 Location: France

Attenuate the noise input, it should not exceed +5V/-5V

Sebo · Joined: Apr 27, 2007 Posts: 564 Location: Argentina

Hi Yves:
I lowered R34, R27 and R31 in order to have lower levels of white noise, pink noise and random voltage. The levels where way to high
Now the SH is working OK.
I have to note that the SH is very sensitive to input level, with a little overdrive, the SH start to fail to hold the voltage.

Anyway, now I'm ready for the gig.
Here is a video presentation of it, featuring the modular:
http://vimeo.com/30870986

COSAQUITOS EN GLOBO from BigMole on Vimeo.

Thank you very much for the help.