CGS114 DUSG strange behavior

artilect99 · Joined: Oct 01, 2018 Posts: 49 Location: USA

Just finished building the CGS114. One side works perfectly. The other side doesn't work at all. All checked all power rail voltages to the chips and even swapped the chips between the working and non-working sides to verify all chips were OK. Working side still works and non-working side still doesn't.

I've been feeding in signals into C (sig in) and A (trig in) in order to trace them to the problem. Feeding a signal into C, it is present at pin 6 (-) of LF353 and pin 7 (output) but nothing is heard on pin 5 (it is heard at pin 5 on the working side).

Its goes out pin 7, makes it past the transitor pairs, and enters pin 2 of the 353, but doesn't exit out at 1 (the output) as it does on working side. It seems to stop at the 22n cap between pin 2 and 1. On the working side, it is present at pin 2, both ends of the cap, and outputted at pin 1. The non working side sees the signal going into one end of the cap, but not coming out the other side. Yet I can find no discontinuity there. I don't have a capacitance test mode on my DMM but both caps read the same on resistance (not sure that means anything). Should I try replacing this cap?

The obvious answer would seem to be yes, but the A -> M on this side doesn't work either. A gate sent into A is very faint at pin 13 (+ in) of the LM3900 and is not present at pin 9. With a gate going into A, the diode leading from A to pin 4 of the LM3900 (near end out) shows a very different voltage coming out of it than the one on the working side does (>10.8v on the working side vs. 0.12 or so on the non-working).

Is there some complex malfunction here that I do not understand? Is it possible for the diode to read normal on a DMM and yet not pass signal, or is there some more complex interaction going on that is jamming up the circuit?

Crying or Very sad

Any help from someone more conversant with this circuit than myself would be vastly appreciated and I will bestow infinite hossanahs upon thee.

Thanks!

EDIT: The non-working side is also the side where the pin 5 pad of the LM3900 lifted. Looking at the schematic, it appears as though pin 5 is left unconnected, and indeed this is precisely why the pad lifted -- because it is not connected to any traces on the PCB. So pin 5 on this LM3900 was just left floating. This is OK, I assume? There's nothing to jumper it to, right?

artilect99 · Joined: Oct 01, 2018 Posts: 49 Location: USA

Psynth · Joined: Jul 18, 2018 Posts: 34 Location: UK Audio files: 8

I feel for you! I've built two USG (ie one half of a DUSG) on stripboard. One works fine, the other doesn't. Can I find the problem....

The circuit is very complex, you have to remember that the LM3900 is a *current* based device - voltages around it are not necessarily relevant.

Have you looked at Tim Stinchcombe's page:
http://www.timstinchcombe.co.uk/forums.html?pge=vcs

There is also a long post in the CGS subforum about a year ago where Tim is helping someone else troubleshoot a VCS, which is essentially the same circuit:
http://electro-music.com/forum/topic-56076-0.html&postdays=0&postorder=asc&highlight=vcs

Good luck!

Peter

artilect99 · Joined: Oct 01, 2018 Posts: 49 Location: USA

Thanks for that thread Psynth... Yeah, I've been trying to wrap my head around Tim Stinchcombe's analysis of the circuit but it's not yielding much in the way of insights. All I know is that this thing is not behaving as it should and I have no idea why. Can't find any simple mistakes, which probably means I'll be unable to solve without test equipment etc. which I don't have.

I'm willing to just count my blessings and use the single working side, but I still another slope gen and don't want to roll the dice on another CGS114 pcb. What a nightmare.

I'd love to see some pics of your stripboarded DUSG, that sounds like an heroic undertaking!