[sdiy] Power supply caps

[David Cornutt]

Not strictly a synth problem, but it has to do with my monitor amp...

The TEAC receiver that I use as a monitor amp developed a nasty 60 Hz  
buzz.
It was audible in both the speakers and the headphone output, and it was
insensitive to the volume knob or the other controls, so I figured it  
had to
be in the output supply.  Interestingly, this amp has a piece-built  
supply,
with discrete diodes instead of a packaged rectifier.  I scoped both  
sides
of the output supply and saw that the positive side had a ripple.

Unsoldered the diodes from that side, and they checked out OK
(using an old analog meter).  I then unsoldered both of the filter caps.
On my digital hand-held, which only measures up to 20 nF capacitance,
one of the caps read infinite, which I expected.  (I put the meter on a
resistance setting, and I could see the resistance going up as the
meter charged the cap.)  But the other one only measured about
700 pF capacitance, and on the resistance measurement it read
at around 2K ohms, and I did not observe any charging behavior.
The suspect one also has visible bulging at the ends, so that pretty
much confirms it.

Now the problem.  I don't know where these caps came from and
I can't find a replacement.  They are marked with the brand name
"devcon".  They are marked as 8200 uF and 105C, 80V.  Physically,
they are about 3" tall and 1" high.  That made me wonder since
it seems like an awful lot of farad to cram into a package this small,
but hey, I'm vaguely aware that there have been advances in
large-value caps in the last few years.  I don't usually mess with
caps this big, so what do I know?

Well, I got out the Mouser catalog and started searching.  According
to my scope, the (unregulated) supply runs at about 60V, so I
figured maybe it would be better if I replaced these with 100V
types (I'm going to replace both of them while I'm at it).
Problem: I've only found two 100V caps at near that value in
the catalog, a Mallory part and a Vishay-Sprague part.  And
they are both larger than the part that they are replacing, plus
they have screw terminals which means they can't be soldered
directly into the board the way the old ones were.  Finding space
inside the enclosure to mount them is going to be a problem.
Plus, the darn things run about $25 a pop.

So the questions are: (1) Has anyone ever heard of this "devcon"
company?  The only thing Google turns up is a company that
makes epoxies.  (2) Could these things really be 8200 uF in
that size package?  Or did TEAC get shafted by some counterfeit
parts?  (3) Does an 80W/channel stereo amp really need caps
that large?  Looking in the Mouser catalog, I can get some
snap-in types (which would solder right in where the old ones were)
at around 2200 uF, 100V, and spec'ed for high ripple current,
for less than half of what the higher-value ones cost.