[sdiy] Debugging problem with 4-track

Pete Hartman pete.hartman at gmail.com
Sat May 26 15:34:34 CEST 2018

On Sat, May 26, 2018 at 5:25 AM, Corey K <coreyker at gmail.com> wrote:

> That's interesting, because I've read in several different places that
> electrolytic capacitors dry out after 20 to 30 years, and often need
> replacing. Is that false?

"It Depends"

It is true that electros do dry up, but there is no point in replacing
every single capacitor in a piece of gear "just because".  If you have
evidence of a malfunction, as you do, you trace down the cause and then
take appropriate, targeted action.  Bulk changes trying to troubleshoot
with a shotgun, as Gordon said, are likely to cause more problems.

Observe the state of the caps.  Are the electros bulging?  Leaking?  Then
replace them (and clean the board if there's leakage).  Are other types of
caps discolored, like they were getting too much voltage?  Then replace the
ones that look burnt.

But more importantly: is the PSU putting out the rated voltages?  What's
different about the voltage output when you use 7.5V vs 12V input?  Does
the voltage change when you attempt to press the record button?  How about
with batteries, how does it behave with them?

Answers to those questions will focus you in on components that actually
need changing.  And perhaps they are the electros or other caps in the PSU
section.  But blind guessing doesn't usually fix much.

The other caveat here is that there are some synth techs who swear by
re-capping.  So note that all this is my opinion as a troubleshooter, but I
don't have the same experience.  However, that experience of those other
techs will usually give them some intuitive sense of how likely a given
synth is to have those kinds of faults, so a re-capping by someone who's
been doing this for 10-20 years is perhaps more justified than one by
someone with little or no experience who's just trying something because
they heard it's what you do.

"Oh yeah, I remember that other oberheim, the caps they used were crap, we
better just go ahead and replace these" does have some basis in likely
faults.  "I better replace all the caps in this gear because someone once
told me that's how they fix things" does not.


PS: the bulk of my experience is troubleshooting and repairing medium to
large computer equipment, mostly by Sun Microsystems.  I really wish I
could get back all the hours lost when a customer said "there was a single
correctible memory error, you must replace ALL THE DIMMS" and then the
situation cascaded into additional faults because the machine was touched
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