[R-390] Caps

[ToddRoberts2001 at aol.com]

I once ran across an intermittent B+ short in  an  R-390A that was traced to
a Brown Beauty bypass cap C545 in the IF  deck, part  of the plate
circuit of V508.

The symptoms were the  receiver would  sometimes blow the 3 amp line fuse
and other times would  seem to run OK, but  the overall gain seemed low.
This was the single  line-fuse version  R-390A
on the rear panel. Also the 26Z5 tubes were  very low on 
emission  probably due to a constant overload.

The  cap must have been very leaky at  times also putting a long-term 
strain  on the rectifier tubes. When the cap  sometimes shorted it also 
put a  direct short to ground across resistor R542 -  2200 ohms 1/2 watt. 
Over  time the resistor went much lower in value, down to  10-15 ohms 
which  placed almost a full short-circuit on the B+ line.
This is  probably  what took out the 26Z5 tubes. If the receiver had the 
3-fuse rear   panel the B+ fuse probably would have blown protecting the 
26Z5's. The  owner  of  the radio replaced the 3 amp fuse with a 9 amp 
fuse  which was probably the worst thing someone could do.  Electronics  101
- never replace a blown  fuse with a
higher value! The  original fuse blew for a reason!

The  resistor fell apart when I  unsoldered it. Apparently it was soldered 
in so  tightly that it was  forced to hold together otherwise it would have 
acted as  a fuse also.  Before I found the intermittent Brown Beauty cap
it had ruined 2   sets of  26Z5 rectifier tubes.

All the other Brown Beauty caps  seemed fine  but if ever there was a 
case for shotgunning caps I would  definitely replace  all those brown 
beauty 
caps in  that IF deck  if I had it to work on  again.

73 - Todd WD4NGG