[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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