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

[Cecil Acuff]

I don't think you have trimmer problems but I do think you have mica caps 
drifting all over the place with temp changes.  There is a great article in 
this months Electric Radio covering just this subject...

It's a must read....

Cecil...
----- Original Message ----- 
From: "dmartin" <dmartin at visuallink.com>
To: <r-390 at mailman.qth.net>
Sent: Wednesday, August 16, 2006 5:28 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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