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

[Jim M.]

Assuming it is not mechanical...
I have had several failures of the resonating capacitors *inside* variable 
IF and RF cans that caused sensitivity drops after warmup and other odd 
behaviors.  These are mica caps inside the cans, usually the ones that are 
across the side of the coils that feed a plate circuit (HV breakdowns with 
age).  It's fairly easy to remove the can cover and replace the cap with a 
new modern mica cap (must be mica/RF caps, original specs., no ceramics, 
glass or others).  Alternately, you may be able to buy replacement cans from 
Fair radio or someone on the list.  The best diagnosis is to replace the 
suspect caps or can and see what happens.  Shotgun approaches wok well too!
Jim


----- Original Message ----- 
From: "dmartin" <dmartin at visuallink.com>
To: <r-390 at mailman.qth.net>
Sent: Wednesday, August 16, 2006 6:28 AM
Subject: [R-390] 0.5-8.0 Low Sensitivity (Long)


> 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
>
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