[R-390] No receive below 8 Mc

[Jim M.]

The caps in question are across the primaries of coils that feed B+ to the plate of previous stage.  They are subject to full B+ especially before the tubes warm up. I only assumed that these would be failure or drift prone in all stages that feed plates via primary coils.  

I did replace all bypass caps earlier which also helped, but replacement of the caps across primaries improved performance as well. No I didn't measure sensitivity or noise floor before and after. Comparison was subjective based on the S-meter using the calibrator and on air.  Of course the radio had to be realigned after cap changes. 

Noise floor measurements seem to me to be of uncertain value in an RF environment where atmospheric noise in "S-6" and above, and any receiver induced noise is well below.  

Jim N4BE

Sent from my iPad

On Oct 19, 2016, at 12:38 PM, Barry <n4buq at knology.net> wrote:

>> ... I recently did a shotgun replacement of all primary caps in my SW 390a
>>> and noticed an enormous improvement on all bands.
>>> 
>> I wonder if by "primary caps, where the primary is in the plate circuit of
>> a stage" you mean bypass caps from the B+ side to ground, or if you mean
>> the cap meant to tune the coil, across the primary.  And, I wonder how many
>> caps were found INside the coil cans.
> 
> I took that to mean the caps in the transformers.  If that was the case, did you check before and after performance for each replacement?  It would be interesting to know if improved performance was due to more than one replacement.
> 
> 73,
> Barry - N4BUQ