[R-390] Cap Question

[Roy Morgan]

At 10:24 AM 3/16/2005, Barry wrote:
>Looking at the caps in my "new" R390A RF deck, I'm considering replacing the
>three brown beauties in it.... in the heater thermostat circuit of the 
>crystal oscillator can. ... they are the big, yellow ones in the upper 
>right corner:

The yellow caps in the picture are not brown beauties. They are yellow. 
That means that they are metal film caps, not paper-foil caps. Leave them be.

The fact that they are in the 6.3 volt heater circuit means that any 
leakage in a cap would not cause the trouble we avoid when we replace paper 
caps.

In a nutshell:

  - The yellow caps are almost certainly metal film and not paper dielectric.
  - *Paper* caps are the ones to worry about.
  - Leakage in caps in such places as screen bypass spots, and especially 
plate to grid coupling and most especially in AVC circuits causes trouble.

The heater voltage is 6.3VAC and I'm wondering why the designers used 400V
>or better caps here.  Is this necessary because the noise spikes are
>considerably larger than 6.3V

Yes, switching spikes can be of much higher voltage than the normal circuit 
voltage, especially when there are inductances involved (which is not the 
case in an oven heater.)

>or is it possible that these were just handy
>since there were so many other 0.1mfd @ 400V caps used in other places in
>the radio.

Yes, very likely.

>Any comments?

Leave any yellow caps in place unless you have determined that they are in 
fact giving trouble.

Roy

- Roy Morgan, K1LKY since 1959 - Keep 'em Glowing!
7130 Panorama Drive, Derwood MD 20855
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