Vintage Synth Repair

On Thursday 08 May 2008 16:28, Malte Rogacki wrote:
> At 16:07 Uhr -0400 08.05.2008, Roy J. Tellason wrote:
> > I *strongly* disagree with a few statements that he makes there, one of
> > them being how tantalum capacitors degrade with age -- that's not the
> > problem in the Omni, running them too darn close to their rated voltage
> > is, and replacing them with 35V units (where the originals were 25V)
> > takes care of that. I also strongly disagree with his assertion that
> > 4000-series CMOS parts degrade with age -- there's no basis for that at
> > all, as far as I can see. He also doesn't impress me as being much of a
> > technician if he smoked the power supply somehow (twice!) while
> > troubleshooting some other problem...
>
> Given the number of smoked tantalum caps I've seen in various synths I'm
> simply not sure if the caps produced about 30 years ago might not have a
> more generic problem.
>
> The german wikipedia states that older tantalum caps are sensitive to
> low-ohm switching and could easily fail under such circumstances. Don't
> know if this could be a reason.

I don't know,  though I do see many references to them being a problem in 
older equipment,  Tektronix scopes being one of them.  Most of the time it's 
their sensitivity to overvoltage that's described as being the problem.

> As far as the 4000 series CMOS goes: I've already quoted another URL (also
> found on Peter's site) with a description that I found pretty convincing:
>
> http://www.oldcrows.net/~oldcrow/synth/tips.txt

I did look at that page as well and I'm not convinced,  though he at least 
gives me something to look for in terms of search terms.

> In the last Omni I'm currently fixing about 5 of the 4000 series chips were
> partly dead. Replacing them took care of all "dead" voices at once.

How did this manifest itself,  and what sorts of things were you observing in 
terms of the chips?

-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin