[sdiy] Reasonable cap replacement policy

Jimmy Moore jamoore84 at gmail.com
Thu Feb 6 21:49:36 CET 2020


>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!

No shade intended toward your guitar tech, but how did he test them? simple
capacitance check can read in-spec, but the ESR can make it very leaky.
I've wanted to build a capacitor checker as a side-project, it seems like
it would be a useful device:

https://www.youtube.com/watch?v=LhovRIM5xAo



On Thu, Feb 6, 2020 at 1:32 PM Michael E Caloroso <mec.forumreader at gmail.com>
wrote:

> 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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