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

[GIOVANNI BECATTINI]

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…. :)