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 <jacque...@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@mailman.qth.net > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html ______________________________________________________________ R-390 mailing list Home: http://mailman.qth.net/mailman/listinfo/r-390 Help: http://mailman.qth.net/mmfaq.htm Post: mailto:R-390@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
