[R-390] R390A 2nd osc. hot R406, R407

[Charles Steinmetz]

Nelson wrote:

>Any thoughts on what might have cooked those two resistors?

Leaky capacitor(s) attached to S402 (C417 through 431E).  More likely 
one or more of the fixed capacitors (C417 through C428), but the 
variable trimmers are suspect, too.

A note about troubleshooting procedure:  Resistors are cooked by 
higher than normal current flow.  In the case of R406 and R407, the 
current flows from the B+ supply, through these resistors, to the 
oscillator circuit.  So, we are looking for where B+ current could 
possibly go, *downstream* of R406 and R407.  That means C411, T401 
(primary could have a short or leakage path to ground), V401 
(extremely unlikely), or leakage through one or more of the 
capacitors mentioned above.

Also note -- there was absolutely no reason to replace R404 -- it was 
barely out of spec (assuming it was a 10% resistor -- if it was a 20% 
resistor, it was well within spec).  Every bag of 120k, 10% resistors 
used by the manufacturer on the day the radio was made contained some 
resistors with values higher than this.

Very few resistors in a 390 or 390A (or, for that matter, any tube 
radio) are critical as to value.  In most cases, you would never 
notice any difference with resistor values +100% or -50% from nominal 
value, and in many cases you would never notice any difference with 
resistor values +200% or -75% from nominal value.  There is simply no 
reason to go through a boatanchor with a DVM looking for resistors to 
replace.  Replace the ones that cause symptoms (i.e., troubleshoot 
the radio when it is not working properly, and otherwise leave it 
alone).  The exceptions are the well-known troublesome capacitors and 
a few resistors that run too hot by design.

Best regards,

Charles