[sdiy] Reasonable cap replacement policy

Tom Bugs admin at bugbrand.co.uk
Fri Feb 7 09:22:31 CET 2020


Have a hunt for the Peak Atlas testers - eg. ESR60
Very good for such specific tasks (electro caps - presume tants too) and 
not too expensive.

On 06/02/2020 21:01, Pete Hartman wrote:
> Do such things actually work reliably in circuit?
>
> e.g. my cap meter or the cap meter setting on my multimeter absolutely 
> do not work with caps soldered into a circuit.
>
> pete
>
> On Thu, Feb 6, 2020 at 2:51 PM Jimmy Moore <jamoore84 at gmail.com 
> <mailto:jamoore84 at gmail.com>> wrote:
>
>     >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 <mailto: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
>         <mailto: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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>         >>
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