[R-390] R-390 C101 and C103 Caps

[Drew P.]

On Jan 3, 2009, at 11:06 AM, Perry W. Remaklus wrote:

> I am working on a R-390 and C101 (10uf 600VDC B+ filter) and C103  
> (10uf 300VDC audio decoupler) are both are leaky --- both are paper  
> in metal cans.

Perry,

I don't see any problem using an electrolytic for the audio decoupling
capacitor.  

An electrolytic might cause problems if used for the main B+ filter,
however.  The R-390/URR (non-A) has just that one capacitor for B+ filtering
before the electronic voltage regulator circuit.  That cap sees a high ripple
(not to be confused with Chuck) current and an electrolytic as small as 10uF
might heat up excessively.  I've also wondered about the outcome of using an
electrolytic here. You might try one and see if it gets hot and report to us
your findings.

Years ago I worked on improvements to a machine control system which used
small solenoids powered by full wave rectified 120 VAC and small electrolytic
filter caps of about 5uF.  These caps exploded quite regularly from ripple
current induced heating.  IIRC, the solenoids drew about 400 mA each.  
Methinks the B+ drain of a non-A to be considerably less but still reasonably
sizeable.

If the electrolytic warms up, then replacement with a cap having a lower loss
(than electrolytic) would be in order.  Distributors such as Mouser sell poly
dielectric caps that would work - you might need to parallel a few to get the
value you need.  Alternatively, you could use a motor run capacitor. These in 
small applications are often poly dielectric these days.  Ceiling fans use 
caps of this sort in about the value you would need.  Check your local Home
Despot for these or salvage one from a junk unit.  These are AC rated - the DC
rating would be 1.4 times the AC voltage rating or possibly somewhat more.

Another possibility would be to use an electrolytic of greater than 10uF.  The
ripple current handling ability would be higher.  You would get lower ripple,
higher average voltage into the voltage regulator circuit, higher peak rectifier
and power transformer currents which would cause all to run hotter. In
reasonable proportions, the added heat might not be too objectionable (but
don't forget to put a cooling fan on the regulator's 6082 pass tubes - these run
hot as it is).

Drew