[sdiy] Reasonable cap replacement policy

Pete Hartman pete.hartman at gmail.com
Thu Feb 6 22:01:28 CET 2020


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> 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> 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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