[R-390] 0.5-8.0 Low Sensitivity (Long)

[dmartin]

Have a 390A here with the low sensitivity problem below 8 megs. It isn't the first 
390A I've dealt with having this problem. Problem on this one is characterized by a 
1st variable IF alignment that won't "hold", sensitivity that is good at first but can 
go numb after several hours of operation, and generally odd alignment results. Threads 
for slugs on Z213-2 and -3 cans may be all the way out during one alignment effort and 
then have to be screwed all the way in flush with the rack during a subsequent 
alignment hours later. With my IF gain set for -7.0 VDC with 150 uv of 455 kHz into J-
513, "good" sensitivity on lower bands is defined as a carrier level of 40-60 db with 
the calibrator signal and -7.0 VDC at the diode load requiring but 1-2 uv out of my HP 
sig gen. Hours later this may be only ~20 db for the calibrator signal and 30-40 uv 
for -7.0 VDC! When the rig drops to the "low" sensitivity values, realigning the 1st 
IF, with vastly different final placement of slug positions and accompanying changes 
in the trimmer caps usually brings it back.

The 1st oscillator, V207, is sound and puts out a robust peak-to-peak 17 meg signal on 
my scope. Resistances and voltages on V207 and V202, the first IF 6C4 mixer, are 
typical. I've subbed NOS tubes for both the 1st mixer and oscillator without change. 
Problem seems to be lots of loss, but intermittently so, in the Z213 network and I 
suspect caps at fault, either the trimmers and/or the ceramic caps associated with 
Z213 under the rf deck. 

If I keep the power off, pull the 1st mixer and oscillator tubes and all three Z213 
cans, that leaves me with but the passive, discrete Z213 components underneath the rf 
deck to test. If I next pump in a sig gen signal of 24.3 megs (7.3 received) on pin 1 
(plate) of the mixer of 4.5 volts peak-to-peak on my scope I can only recover ~0.5 
volts peak-to-peak on jack 3 of Z213-2 and Z213-3. This suggests very little signal is 
getting through C281 and C282, the low value 1.5 pf coupling caps between Z213-2 and -
3. 

Is such a loss typical? Short of a shotgun approach that replaces all three trimmer 
caps and all five mica caps associated with Z213 under the rf deck (ugh!), anyone have 
ideas?

Dan
WB4GRA

--
Open WebMail Project (http://openwebmail.org)