[R-390] R-390 IF Deck

[John Vendely]

Howdy,

I'll offer an opinion on your question about the glass/metal 
hermetically sealed paper capacitors.  THese capacitors (e.g., Sprague 
Vitamin Q and others) resulted from a major industry research program 
analyzing paper capacitor failures, which had always been a chronic 
problem.  Failure rates of these capacitors are very low, and in over 50 
years of working on a large quantity of military radio gear, I've had to 
replace perhaps four or five. I've also measured capacitance and leakage 
current at rated voltage on quite number of them disconnected from the 
circuit as a purely precautionary measure, and these all measured 
in-spec for new parts.  I have a coffee can full of unused ones, and 
measured dozens of these as well, and found none out of spec.  So 
there's really no reason for indiscriminate, "shotgun" replacement of 
these capacitors.  Just do the usual troubleshooting, and replace only 
those which actually measure bad.  You'll rarely find one.

Yes, there's the "death capacitor" said to destroy 2kc mechanical 
filters in the R-390A.  I suppose it's cheap insurance, so replace it if 
it gives you peace of mind.  However, in receivers which have the 
glass/metal hermetic type at this location, there's no problem leaving 
them in place.

73,

John K9WT

On 1/12/2025 9:24 AM, Byron Tatum wrote:
> Hello-    I picked up a nice condition clean R-390 IF deck, checked it out and got selected tubes in it, and tested it in an R-391 receiver I have here. Allowed it to run about an hour. It is working well as best I can tell but I do have one issue that bothers me. Based on the carrier meter reading the 8 khz BW position is about 12 dB down from the 2,4 and 16 khz positions (these 3 positions are all fairly close in reading).    I have studied the theory of this IF deck and how the 8 khz BW is done. I have carefully gone through the IF alignment procedure as follows -        1. Fed in 455.000 khz from generator through receiver test point on RF deck
>          2. Placed BW in 0.1 Khz position and tuned gen for max reading (I assume this is done to align IF for the particular freq of crystal in crystal filter Z-501). Leave gen set here.        3. Placed BW in 2 khz position and adjusted T-506,505, 504, 503, 502 for max reading on VTVM connected to diode load terminal on rear panel.        4. Placed BW in 8 khz position and adjust T-501 for max.Have done this several times but get same results.
> Another question I have is in regard to the glass metal sealed capacitors throughout this deck. Friends have told me they have never seen a bad one, some of them have many years and much gear using them.       I did replace the pair of 1.0 ufd at 400 vdc oil caps with high quality film, and dotted silver mica caps (100, 180 pF) going to ground off of pins 8,9 of V-507 limiter and C-543 (220 pF) going from Z-503 to AGC rectifier V-510. Have not replaced any of these same type caps in RF circuits.Thanks,Byron W5FH
>
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