[R-390] Tracking down low sensitivity 0 to 8 mhz problem

[Jordan Arndt]

After a long break from working on 390As, I'm back into a 390A I bought at 
least 25 years ago.

It's the most mongrel 390A I've ever seen, with a modules and assemblies 
from S-W, Teledyne, Capehart in an early single fuse chassis of unknown 
mfgr. The front panel is a re-paint and re-letter and the ID tag is a 
generic after-market type.

The RF deck front plate and gear assembly is Teledyne with the deck chassis 
being a S-W and with a Capehart xtal oscillator module refurbed at 
Tobyhanna(sp?).

I've gotten a good deal of the required rejuvenation work done, but I found 
an issue where the 1st xtal osc. voltage is rather low when measured as 
rectified at E-209.
I cannot get more than 2v DC measured at E-209, while T207 peaks smoothly.

What I've done so far regarding this problem:

Replaced the 17Mc xtal, and then the entire oven ass'y including xtals from 
a working radio to no avail.

I then swapped out the 6AK5 osc tube with several good 6AK5/5654 tubes, then 
did the same with the 6C4 1st mixer tube. Again, all to no effect.

Since it has been 25+ years since I originally worked on this radio, I 
suspected component problems in the RF deck, so I pulled the deck.

I pulled C275 and C268 as the originals had been replaced with disk ceramics 
(what I had on-hand at the time) but both test just fine at 99 and 105pF 
respectively. I'll replace them with S.M. caps.

I then checked every resistor in the RF deck and aside from R203, which had 
drifted up to 90K from 82K, they all are within spec and show no signs of 
overheating.

Aside from checking feedback caps C324 and C325 to see if one or both have 
issues, is there anything else I should look at...?

Any suggestions or advice would be much appreciated...!

73...Jordan VE6ZT