[sdiy] When good caps go bad!

John Henson synthnerd at eircom.net
Wed Jun 11 02:27:29 CEST 2014

I guess everyone's experiences of bad Caps depends on their own 
experiences, and what they have to repair, if a Tech.
I definitely do not hold with the "re-cap everything" mania that seems 
to have taken over the synth world over the last few years, and don't do 
it myself with the exception of as Gordon pointed out, cheap SMPSU's 
where they do get a very hard time and are poorly spec'd for the long 
haul, and a lot of the time do need replacement.
But there are exceptions, the Tantalum bead capacitors across the power 
rails in mainly American products are prone to going short, I have 
personally seen this dozens of times, and if the client agrees, I will 
replace them all in a synth with higher voltage rated low ESR 
electrolytics, there are usually less than 20 of these in an American 
synth. That being said, where Tantalum's are used for timing purposes or 
in the audio chain I have never seen a problem, and in fact in a few 
Korg Polysix's where the LFO and PWM timing calibrations couldn't be 
achieved with their Electrolytics, I replaced them with Tant's which 
sorted that one right out, the lowest speeds of course!
Over the years I have had surprisingly few problems with disc ceramics, 
sorry Gordon, but perhaps we fix different things, although I have a 
Jupiter 8 CPU board which keeps draining it's backup battery, and at 
this stage it's either a faulty Ceramic Disc cap or a faulty memory 
chip, and my instinct leads towards those caps at this time.
I bought a secondhand Tek TDS420 about a year ago which passed it's 
diagnostics when I bought it but within a week was failing it's boot 
diagnostics every time on one or other of the Analog interface systems. 
When I got round to opening it up, it showed Electrolytics leaking all 
over both boards, with the same symptoms as battery acid damage, 
including bad smells, eaten through tracks and dead chips. This Scope is 
all surface mount stuff, so perhaps SMT Electrolytics are something to 
worry about for the future. I replaced all the caps,45 of them, repaired 
the damaged tracks and replaced the dead chips and the scope has been 
good since.
Random Electrolytic failures really are that, the Bi-Polar caps in 
Roland synths have cropped up many times, something to watch for but 
nothing I would instinctively replace.



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