[sdiy] Octave Cat modulation weirdness

[Michael E Caloroso]

> In nearly 30 years, I've replaced maybe about five or six electrolytics that
> weren't in a switched-mode PSU.
>
> What I have done is recover the aftermath of people "re-capping" things, to
> the point that when someone brings me a piece of gear that they say has been
> "re-capped" I just tell them to take it somewhere else, like the municipal
> tip.
>
> Go over it carefully and check your soldering, really carefully.  Clean off
> every bit of flux.
>
> Once you've done that, dig out the circuit diagram and try and work out
> where the signals are going.  Don't just blindly rip out and replace
> components and hope that this will fix it - more damage is done by the "HERP
> DERP NO WORKY MUST REPLACE CAPACITATOROSZZZ" brigade.

Good gravy is this ever true.

I quickly discovered that re-capping electrolytics is not a universal
catch-all.  When I recapped my OB-X I took before/after pictures of
the scope display of each power rail.  After the re-cap, there was no
difference.  However the "tropical fish" mylar caps got replaced
because they crack apart with age.

The driving factor is heat, especially in tube guitar amps or power
amps.  Electrolytics do not like heat.  I have a few guitar amps, and
the one that didn't need new caps was an old 1963 british Selmer whose
power amp is in a separate chassis from the preamp and allows heat to
escape naturally - passive rising.  Fenders and Marshalls are too
enclosed and their designs trap heat.

And novices should NOT be attempt their own repairs.  It only makes it
worse and when they take it to a tech to fix their f*ck-ups the repair
bill will only be higher.  It's a LOT cheaper to let the experts fix
it in the first place.

MC