[R-390] Fwd: R-392, IF stage gain

[David Olean]

Hello Larry and Mr. Becattini,

I do enjoy playing with the R-392 receivers and have brought four of 
them back from the dead.  That being said, I always have taken the 
alignment instructions with a grain of salt.  I usually skip trying to 
couple in with a small coupling capacitor if I can avoid it. What I can 
do is to try to duplicate the coupling method used by Mr. Becattini and 
record my results. The IF tubes running on 28 volts can show some 
variability and bear no resemblance to what a tube tester might say. The 
best way to test them is in a unit and monitor relative stage gain as 
you change tubes.  I sorted all my tubes and color coded them with dabs 
of paint.  Now all I have to do is not lose my color code!!

The important thing to do is have the IF gain peak right at 455 kHz. An 
accurate 455 kHz signal is important.  Do not rely on a dial setting. I 
do the same thing with the PTO. I take it out of the receiver and 
install it in a test fixture with an accurate dial so I can check the 
end points and the linearity using a good frequency counter. After that, 
I have never worried much about stage gain and concentrated on whether 
each stage seemed to be working properly.  The IF stages all have silver 
mica capacitors resonating the IF windings and I find that these can 
cause trouble in the following way. At low signal levels, all looks 
good, but, when you increase the RF signal level, strange things start 
to happen. You will hear a crackling noise and your AGC voltage will not 
increase properly as the levels are increased.  You might also find that 
your AGC voltage will drop over time with a constant rf signal applied, 
as components get warm. I spent a number of hours removing Vitamin Q 
capacitors in the AGC circuits  and testing them to find that they are 
perfect after all the many years since they were manufactured.   I did 
find that a few silver mica postage stamp capacitors inside the IF cans 
would start to leak or actually arc over at very low RF voltages. I have 
two capacitor checkers, a Military ZM30 bridge (which is very cool) 
along with a General Radio 1657 RLC bridge. They both give similar and 
very accurate readings, but the crackling does not show up when testing 
the capacitors as there is no significant RF voltage across the 
capacitors during the tests!!.

My solution is to use a regulated adjustable HV supply and connect the 
positive lead to one side of the capacitor, and then read the DC voltage 
with a very high impedance voltmeter on the other lead of the capacitor. 
I use a Fluke DVM and reference it all to the negative lead on the power 
supply. Raise the voltage slowly and you can watch for leakage on the 
voltmeter. I found that some of the capacitors were fine up to about 8 
to 12 volts! Above that, and they started arcing and making the 
crackling sound. A new capacitor fixes everything.  This method of 
supplying a high DC voltage across the cap is an accurate and effective 
way to test for leakage. It will expose problems not seen with RLC 
bridges even if they can measure dissipation.  There are SM capacitors 
in the RF coils as well, and, if you pay attention to which bands seem 
to have the noise, you can localize it to a particular set of coils used 
for those bands. A high frequency scope will allow you to find the right 
stage. It can be difficult especially in that chain of IF stages in the 
R-392, to determine where the noise is coming from.  A bad cap in the 
plate circuit will also show a little noise back into the grid stage of 
the same tube, so always keep that in mind.

Once I had all my IF and RF stages working properly, I never bothered 
with checking the stage gains. I figure that, if the receiver will not 
meet its' performance levels, then I would check into it further.  These 
radios have no problem meeting their spec, and you can run into a point 
of diminishing returns very fast.  If you note that a stage does not 
tune up properly, than that is cause for worry.

I will play around with a nicely working R-392 and get back to you all 
with what I find as compared to paragraph 104.

Dave K1WHS

On 1/7/2022 10:50 PM, Larry H wrote:
> Hi  Dave,  It looks like from your posts that you have some R-392 
> experience.  Would you mind looking at the attached and see if you can 
> help  him out.  Nobody has responded to his request.  Thanks.
>
> Regards, Larry
>
> ---------- Forwarded message ---------
> From: *GIOVANNI BECATTINI via R-390* <r-390 at mailman.qth.net>
> Date: Wed, Jan 5, 2022 at 6:15 AM
> Subject: [R-390] R-392, IF stage gain
> To: <R-390 at mailman.qth.net>
>
>
> Hi,
>
> I am sure a little bit pedantic and tried to check if my R-392 (1951, 
> Collins) fulfills the IF stage gain chart (par. 104 of TM 11-858). I 
> started from this point a long sequel of tests and component 
> substitutions, with some obscure behavior.
>
> First of all: how do you do the tests? The manual is not precise about 
> that. They say to apply the siggen output thru a 50nF capacitor. But 
> in this way the generator output impedance is not matched, so, when 
> you think you apply n-microvolts, you are really applying the twice 
> and the value of the table are widely exceeded. So I simply multiplied 
> by two the nominal output values of the signal generator.
>
> In this way, I am “rather" aligned with the requirements (4KC 
> BANDWIDTH) for the last 3 tubes:
>
> V506: 46 effettive mV (already doubled), it should be 50..70mV
> V505: 23 effettive mV (already doubled), it should be 22..32mV
> V504: 32 effettive mV (already doubled), it should be 25..40mV
>
> But to achieve that, I had to reduce to minimum the gain of V505, 
> acting on R532 (the potentiometer in series with V505 cathode).
>
> Also the first two tubes V501 and V502 have a right gain. The problem 
> is V503. I am rather sure that every component is OK but, if I want to 
> keep on the table, I had to add a 3K3 resistor in parallel with the 
> 6K8 to have the about 1 correct gain factor.
>
> To be short: V503 stage is gaining too little, V505 is gaining too 
> much and I am not able to explain why. Most the passive components are 
> new. The tubes are 110% in the tube tester and also swapping them the 
> results are not so much different.
>
> There is furthermore a mistake, I believe, in the manual: in par. 115, 
> letter (i), they say to adjust R532 so that you have -2.5V with 300uV 
> on J510. That is not compatible with the stage gain chart, that 
> requires 130…180uV instead of 300.
>
> By the way, I can state that the overall sensitivity is not very much 
> affected by the IF gain: getting more gain does not imply a better S/N 
> and so no better sensitivity. That is true particularly for R532.
>
> So, you could ask: if anyway the IF chain has the right gain and the 
> gain it is not very important, why are you losing your time on that? 
> That’s the more difficult question…. :)
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