[sdiy] speaking of tantalums...

Michael E Caloroso mec.forumreader at gmail.com
Mon Nov 26 06:39:58 CET 2018


Tantalum caps on the power rails in any synth from the 1970s and
early-mid 1980s are ticking time bombs.

Their maximum voltage tends to degrade with age.  I had the hood open
on my OBX and the power applied when one of them started to smoke
right in front of me - and these are 35V rated caps with 18V
differential on the leads.  That magic smoke STINKS.  I immediately
turned off the power and went to work replacing ALL tantalums on the
power rails.

The exception are tantalums in the audio path as they are not subject
to large constant voltage differentials like power rails.  They're in
the audio path of the voicecards of my OBX and I left them intact to
preserve that classic Oberheim "dirt".

MC

On 11/25/18, Adam Inglis <21pointy at tpg.com.au> wrote:
> I’m getting a Prophet 5 rev 3.3. (no midi) ready for sale. I’m wondering if
> I should be replacing the tantalums in it. They seem to be considered a
> ticking time bomb. One site I came across suggested replacing the decoupling
> tants with low ESR electrolytics, I assume you just use identical farad
> values:
>
> "Our rule of thumb is that if one side of a Tant has a raw power rail from
> the power supply on one terminal and Ground at the other, then replace it
> with a low ESR electrolytic. Simples!
> There are 19 of those in a Prophet 5 that we replace, 8 0n the CPU board, 7
> in the power supply, 3 on the Voice card, and 1 on the left side of the
> front panel pcb, a mixture of 2u2 and 10 uFarad.
>
> One other side of the Tant is that it has very low leakage, it doesn’t
> dribble its charge away with the gusto of most Electrolytics, there are a
> couple of Tants involved in the Battery backup system for the presets, they
> have never failed and have never been replaced, but if they were replaced by
> Electrolytics I would suspect that the lifespan of the new battery would be
> reduced considerably due to unnecessary capacitor leakage."
>
> Anyone experienced with these machines care to comment?
>
> Adam




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