Cgs synth

Hi group

I have had some fun trying to troubleshoot
a CGS22 master divider (old version) that I bought a couple of years ago from a kind person in this group. I ask these questions to the group as a whole, because that may increase the chances of reaching somebody who has had a similar experience... so here goes:

Main problem was that upon measuring the resistance on the power input I found that there was a short between +VE and ground. I know I'm a noob in this business but there should be some measurable load between the power inputs to the module! Shouldn't there?

I scrutinized the board and solder joints and could not see any bridge.
After a while I started snipping off the el. caps and found that the tantalum cap was the culprit. With that one out, I had no short anymore.
Replaced the tantalum cap and it still measured fine.

When I hooked it up, it worked nominally for a couple of secs and then the tantalum cap blew up in firy fury. Green flashlight all over my flat. Spectacular :)

I wonder... how can this be??
NOTE 1: I have links instead of leds at some of the outputs. No output is hooked up by now, I just wanted to see if it worked and to watch the leds flash.
NOTE2: 3904s is used instead of bnc547s. The 3904 pins are soldered in reverse order, as they should be.... I think.
NOTE3: I have powered it up on 15V and 18V but this should be ok. All components should handle the extra volts. Both times something fatal happened. The 15V fried the filtering resistor, and the 18V just went along and blew the tantalum to kingdom come. Clearly, there is still a short.
NOTE4: As far as I can tell, the necessary mods have been done as explained on the web.
NOTE5: All the logic chips work, not shure about the lm358, but can't see how that should matter in blowing resistors and caps!

Anybody got some wise words?


Tor Olav Heldal,
Norway

>When I hooked it up, it worked nominally for a couple of secs and then the

tantalum cap blew up in firy fury. Green flashlight all over my flat.
Spectacular :)

1) Maybe you put the Tant in backwards.
2) Maybe you did not have one with a high enough rated voltage.
3) Tants do that for no apparent reason from time to time. I refuse to use
the nasty things myself. Use regular Electros with a minimum rated voltage
of 25V.

>NOTE3: I have powered it up on 15V and 18V but this should be ok. All

components should handle the extra volts.

Do NOT power CMOS on 18V - that is beyond their maximum rating and usually
results in the chips going the same way as the Tant. Yes, I mean explosive
releasing of smoke.

>Both times something fatal happened. The 15V fried the filtering resistor,

and the 18V just went along and blew the tantalum to kingdom come. Clearly,
there is still a short.

Sounds that way - or you have your power connected backwards to the whole
module. Or the Tant was backwards and was acting as a short at the lower
voltage, and blew at the higher one.

>NOTE5: All the logic chips work, not shure about the lm358, but can't see

how that should matter in blowing resistors and caps!

I'd replace that with a TL072, but only to make things work better.

If the chips are socketed, remove them from the sockets before you go short
hunting again.

Ken
_______________________________________________________________________
Ken Stone sasami@... otherunicorn@...
Modular Synth PCBs for sale <http://www.cgs.synth.net/>
Australian Miniature Horses & Ponies <http://www.blaze.net.au/~sasami/>

Thanks for the input!

You where right at assuming this: