Last updated: January 22, 1999
Disclaimer: No warrantee provided, your mileage may vary
Unless you are using a 28 volt military vehicle electrical system, you'll have some kind of external power supply for the R-392. The nominal supply is 28 volts at 3 amps. Of this, 2.5 amps is for tube filaments, 0.5 amp for plates. While the manual reports full sensitivity at supply voltages of 25 and above, many people report improved RF performance by running the plate supply at 28 to 35 volts while keeping the filaments at 28 volts. It has been my experience that the receiver, when properly aligned, meets its sensitivity specifications at 26 to 28 volts on a combined filament/plate supply.
Al Marshall reports that it is possible with some minor modifications to run the filament string on 28 VAC, reducing the regulated 28 VDC power requirement to 500 ma. The mod consists of lifting one end of the antenna relay coil. DO NOT USE THIS MOD IF YOU HAVE THE SOLID STATE AUDIO MODULE -- It runs on the filament, not plate circuit.
There are some military and commercial power supplies, with beefy outputs suitable for running a room full of R-392s (and prices to match), that are available from Fair Radio and other sources. Examples include Lambda (28V, 19A, 62 lbs, $195), PP-2309C (0-36V, 15A, 75lbs, $145), and PP-4763/GRC (28V, 50A, 145lbs, $195). A variety of less expensive alternatives include the PP-3700/PRC-41 (24-28V, 4.5A, 15 lbs, $35-$50), copier switching and linear power supplies in the 25-30V range, as well as homebrew designs.
Some problems with sensitivity (and unresponsive S-meter) can be traced to weak tubes, leaky caps, or out-of-spec resistors, particulary those in the S-meter bridge circuit (V502, R509, R622, R621, R512, R624, C714, C624). A common problem is a leaky C509 in the cathode circuit of V502, the 2nd IF. Tubes in the RF/IF chain should be checked by substitution; tubes that test OK on a tube tester can still have dramatically different in-circuit performance. Place "hotter" 26A6 tubes in the 1st & 2nd RF Amp, and first few IF stages. The 26FZ6 can be substituted for the 26A6 in the RF/IF amps for an increase in sensitivity. (Paul Bigelow and George Rybicki)
While I don't experience problems with intermod, some folks do. Step one in reducing these problems is keeping VHF/UHF energy out of the front end with a regular amateur or CB low pass filter. These filters pass RF energy below 32-35 MHz to the receiver and keep all the FM and TV stuff out. Proper alignment and putting "select" 26A6s as the RF amps is another step. And double check your AGC chain. The first RF amplifier has its own AGC circuit; the other AGC circuit controls the 2nd RF, following IF stages and metering circuit.
Always check suspected intermod with another receiver. It is possible for the intermod to occur OUTSIDE the receiver -- bad electrical connections in antennas, feedlines, power and telephone wiring, even fences and aluminum gutters.
Vacuum tube purists will probably want to skip this section
The final AF audio output amplifier tube is a 26A7GT. This tube consumes 16 watts of filament power (26.5V, 600ma) to produce only 200 milliwatts of audio output. Helps keep the radio nice and warm, too. A number of plug-in solid state audio replacements have been made over the years. They all provide greater audio output and consume less power than the 26A7GT. The greater output is obtained without overloading the B+ circuit by using the beefier (in amperage) filament circuit. As previously noted, don't try the AC filament mod if you have a solid state audio module.
If push comes to shove, and you really can't round up the requisite number of 26A6 or other tubes, a 40763 MOSFET can be more or less directly substituted in the signal and B+ path. Or so it has been said. Hey, I *told* you that you might want to skip this section. For the record, my R-392's are completely tubed at the present time......
Barry Hauser has a R-392 which has been completely converted to solid-state active devices and to 12 VDC vice 24-28 VDC. Don't blame him either, it was that way when he bought it. The front panel has 3 of the fuse holders (plate and 2 spares) removed (in addition to all panel markings being painted over). The IF/AF/PTO/Calibrator modules clearly show tubes replaced by either single transistors or IC modules. Barry is currently collecting some basic performance data to see how this radio performs vis-a-vis the original R-392 specs.
