[sdiy] Reasonable cap replacement policy

Michael E Caloroso mec.forumreader at gmail.com
Thu Feb 6 21:28:31 CET 2020


I do not subscribe to the convention of blanket recapping of old gear.
The prevailing "recap" convention is a carryover from guitar amps.

The significant contributor to cap degradation is heat.  Guitar amps
and class A/B power amps generate a lot of heat and this dries out the
caps, so a recap is warranted.  Not the case with most pro audio and
synth gear as they don't generate that much heat.  There are some
exceptions some pro audio gear and synth do generate high heat.  If
your rack gear are tightly packed this can create an environment where
high heat can exist.

Heat aside, caps do fail due to age or faulty design and should be
replaced period.  Tantalum caps from the 1970s/80s are notorious for
failing with age with a short circuit (not cool on power rails).  The
cases on Mullard "tropical fish" caps crack with age.  Some brands
have a bad reputation - RIFA caps are notorious for exploding.  About
fifteen years ago there was a run of electrolytic caps from Asia that
were built from a stolen design, and there were massive failures in
the field.  These caps were used in consumer products including
computers, and you can see the visible bulge in the tops of the cases
as they fail.  We had a rash of failed computers at work.

Other than that, if it ain't broke then don't fix it.  I took
before/after pics of the power rails on the 'scope during a recap of
an OBX and found no improvement.

Caps can drift from their original value and render a tuned circuit to
malfunction, a cap meter or impedance bridge are good tools to test
the caps.  On the other hand, if I see polyester caps in a tuned
circuit I have substituted better dielectric caps and gained better
performance.  Processors like phasers can sound better with the proper
dielectric.

Teflon or glass are the most reliable dielectrics but you will never
see them in pro audio or synth gear because of their cost.  Caps will
fail for one reason or another regardless of dielectric.

Electrolytics can last a long time.  I have a British tube guitar
amplifier from 1963, I expected to need new caps with an amp that old.
When my guitar tech inspected it he said the original caps were fine.
Why?  The amp design allowed heat to rise in open air to prevent
accumulated heat, thus it prevented the premature decay of the caps.
That's over 50 years on the original caps!

Bad design will ruin caps.  There are products using caps with 16V
maximum voltage rating on 15V power rails!  That's asking for trouble
with such a close margin.  Failing to secure large caps from movement
will eventually crack the solder joint(s) and/or the bond(s) to the
leads.

MC

On 2/6/20, MTG <grant at musictechnologiesgroup.com> wrote:
> I'm starting to see a lot of failures on Yamaha units with switching
> power supplies. Basically all the hits from the 80's. On the items I've
> fixed, I replaced the electrolytics on the PS, but not the various
> "main" boards. YMMV.
>
> GB
>
> On 2/6/2020 10:14 AM, John Speth via Synth-diy wrote:
>> Hi folks-
>>
>> Today I called a shop about replacing the belts on my 30 year old
>> cassette tape deck (Fostex X-26). The guy quickly recommended also
>> replacing the caps (I assume just the electrolytics). I'm guarded
>> against folks who do repairs on stuff that don't show signs of need,
>> which my tape deck doesn't.
>>
>> What are the prevailing opinions on replacing old electrolytics in
>> equipment from the 1980's?
>>
>> I hope I didn't start an opinion war.
>>
>> Thanks - JJS
>>
>>
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