[sdiy] When good caps go bad!

Adam Inglis 21pointy at tpg.com.au
Wed Jun 11 03:19:04 CEST 2014


John
perusing your blog, it struck me that it should be recommended reading  
for anyone about to purchase a vintage synth!
Some real horror stories there...

Adam
On 11/06/2014, at 10:27 AM, John Henson wrote:

> 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.
>
> John
>
> www.retroactivesynth.com
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