[sdiy] Reasonable cap replacement policy

synthdiy at adambaby.com synthdiy at adambaby.com
Fri Feb 7 03:03:23 CET 2020



> On 7 Feb 2020, at 7:01 am, Pete Hartman <pete.hartman at gmail.com> 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


Pete, I bought one of these a few years ago

https://evbesrmeter.pt/index.htm

which is designed to be used in circuit (powered down). It is essentially a “low ohms” meter, using a pulsed signal I believe.
I can say that is has reliably identified faulty electrolytics in a variety of vintage gear dating back to the 60s, so no more guessing and shot-gunning the repairs.

Adam






















> 
> 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 <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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