[R-390] R-390 IF Deck

[Barry Scott]

The schematic for the 4th IF Amplifier shows R519 (and others) to be
selected from a range of 560 to 1100 ohms.  Anyone know the criteria
for how that was selected?

Thanks,
Barry - N4BUQ

On Sun, Jan 12, 2025 at 9:55 AM Jacques Fortin <jacques.f at videotron.ca> wrote:
>
> Hi Byron,
>
> As there is only the 8kHz BW that develops less gain than the other positions, my prime suspect for the problem you experience will be R519.
> The cathode bias for the 3rd and 4th IF stages depend on different resistor values switched by S504 to "balance" the IF gain VS the desired BW.
> R519 is responsible to set the global gain of the IF for the 8kHz position.
> If ever the actual R519 value is way higher than it is supposed to be, the gain in the 8kHz position will be lower than it should.
>
> The second resistor values to check are all the 2.7 ohms resistors (R507, R514, R524, R534) because they are used only in the 8kHz position again.
> Normally, these resistors are an early form of carbon-film construction and should not have failed, but who knows ?
> Also check the corresponding IF transformer windings for continuity: a failure in one of those cannot be excluded.
>
> For the metal-glass sealed capacitors (Sprague Vitamin Q): they are the longest lived paper-in-oil types that were ever made.
> If there is no sign of oil leakage around the wires exiting from both sides, they are surely still good.
> You can perform an electrical leakage test on some to be sure.
> But if there is signs of oil that leaked from those, that means that the oxygen also gets in and turned the internal oil in something else (the PIO disease) that result in value change, leakage and ESR rise.
>
> 73, Jacques, VE2JFE in Montreal
>
> 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
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html