Non electro caps failing?

bunker · Joined: Mar 19, 2009 Posts: 58 Location: Manchester, UK

Hi all. A question that's been puzzling me and is related to a synth I own. Do film/polyester/polystyrene caps tend to fail over time? I know electro's can 'dry out'/short out etc. But this is due to the electrolyte inside them degrading isn't it?

The back story is this. I picked up a Crumar Bit 99 synth years ago as a non runner. I can't remember exactly what was wrong with it but to cut a long story short I got it 99% working. However there is a very obvious fault which sadly renders the instrument unusable as a six voice synth as one of the voices is displaying a rather odd problem. The voice doesn't articulate correctly and I assumed it was a duff electro cap in the EG circuit for that voice. Attack is fine but after holding the note you get a very sudden closing of the amp/filter as if there is a fault with the EG. Imagine holding a note while powering down..That's the envelope response I get for that one voice Rolling Eyes

I've not got the synth/schematics in front of me right now but from memory the EG's are software based and the voices are CEM3379 chips. I have swapped the chips around but the fault still remains.

I'm assuming that even though the envelopes are software based there will be at least one cap that charges/discharges at a rate determined by the EG? Or could I have a corruption in the software of the EG for that one voice? That I find hard to believe? Either way I'm pretty sure I replaced all the electro caps that looked to possibly be part of the circuit. There are other non polarised film/poly caps in the circuit and I was wondering, before I replace all those, if they can degrade/fall over like electro's can? As I say I don't have the schematic to hand and I'm pretty sure the other caps will be filter caps etc. and not directly related to the EG's.

I'm gonna possibly have another look tonight if I get time; the last time I had a play around wit it must have been 18 months ago so forgive me if I sound a little vague!

If any of you guys have had envelope issues with polysynths shout up! We might find some common ground!! I can't wait to get this fault nailed down as the synth sounds wonderful until you get the dreaded squelch/thud of the voice in question. I suppose it adds a bit 'movement' to a chord but its not something I want all the time!!

Thanks in advance.

elmegil · Posted: Thu May 16, 2013 5:17 am Post subject:

FWIW this is not a definitive data point, just an anecdote....

I picked up a non-working S-10. Initially it made nothing but a high pitched whine.

I replaced a handful of mylar caps, after which point it was working except for the data disk.

So I would say yes, non electro caps will also go bad over time.

bunker · Joined: Mar 19, 2009 Posts: 58 Location: Manchester, UK

Cheers elmegil, a complete re cap in the suspect area sounds like a plan. It's a keeper so its worth it to me to do it. Thanks for your response, I'll keep you informed of how I get on. It's so infuriating having 5 good voices and one duffer!! From what I've heard up to now the Bit can sound really nice. Designed my Mario Maggi of Elka Synthex I believe? Either way prices are rising so a replacement unit is looking less and less likely so whatever the fault is I'll get to the bottom of it!!

mph · Joined: Aug 25, 2007 Posts: 88 Location: France

elmegil · Posted: Tue Jun 04, 2013 1:24 pm Post subject:

What in the world is a tropical fish cap? Smile

mph · Joined: Aug 25, 2007 Posts: 88 Location: France

This stuff Wink

:

Posted Image, might have been reduced in size. Click Image to view fullscreen.

Some people claim they have a special MOJO (hopefully mostly in old fuzz pedals)... err I say they are crap.

elmegil · Posted: Tue Jun 04, 2013 2:20 pm Post subject:

Interesting. I've never seen those. Thanks for the info Smile

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

elmegil · Posted: Wed Jun 19, 2013 7:00 am Post subject:

Dragon's Lair · Posted: Wed Jun 19, 2013 8:09 pm Post subject:

Old tantalum's are prone to failure also.

Dave

bunker · Joined: Mar 19, 2009 Posts: 58 Location: Manchester, UK

Cheers guys! Work commitments have kept me away from the bench lately.

filterstein · Joined: Mar 17, 2007 Posts: 42 Location: The Netherlands

If the eg is software based i'd expect a cap to hold the voltage for the vca cv steady. This cap is usually somewhere between a multiplexer chip behind the Dac and a buffer opamp.
The cap should be large enough to keep the cv steady until the next software update cycle outputs a new value.
If that cap is bad it could cause what you experience.

analog_backlash · Posted: Fri Jun 28, 2013 3:10 pm Post subject:

bunker · Joined: Mar 19, 2009 Posts: 58 Location: Manchester, UK

nisios · Joined: Sep 02, 2006 Posts: 49 Location: Lisboa - Portugal

The gate on an Arp 2600 i'm repairing (3620 keyboard) was not working because of a dead ceramic disk ac coupling cap.

AlanP · Posted: Fri Oct 24, 2014 12:51 pm Post subject:

elmegil · Posted: Fri Oct 24, 2014 1:01 pm Post subject:

There's a difference between caps being leaky because of age etc, and claiming that there's a magic about certain types that changes the sound.