R-392 suggested modifications
This page last revised on March 24, 1999.
OK, you've gotten this far; apparently you are not totally adverse to modifying this radio.
So far, the only revision which I've made to my own R-392 has been to replace the tube shields with IERC-type shields. This should serve to dissipate the tube heat a bit better, and extend tube life somewhat.
Here's a list of additional suggested mods. I have not yet performed any of these mods on my own R-392, but they are all "in the works" (I can hear you saying "Yeah, right..."). As I complete each mod I will modify this page to so indicate (and to discuss the results). Note that the main thrust of these mods is to reduce the internal heat load (about 64 watts for filaments alone, with a 26.5 volt supply!)
- Replacing crystal calibrator with solid state unit:
This mod should be pretty simple. Recent editions of the ARRL handbook, for example, provide details of a digital-IC-based crystal calibrator (perhaps using a 4 Mhz clock crystal). Should be relatively inexpensive; would reduce internal heat load (by eliminating V701, V702, and V703) by about 5.8 watts.
- Replacing audio stages with solid state line-level driver: This would require that the audio be driven to
"listen-able" levels by an external audio amp. For me, this is a simple task; I've got a set of "computer" speakers with an internal audio power amplifier. These speakers accept line level input (about a half volt RMS across 2 Kohms) and drive the speakers to a few watts output. To implement this mod, I will have to provide a solid-state driver circuit within the R-392 to drive the external amplifier (input Z of a few K ohms). Assuming that the power requirements of this R-392 internal "line level" driver stage(s) would be minimal, the elimination of V606, V607, and V608 would reduce internal heat load by about 18 watts. (If you have a solid state audio output module, this may not be as much of an issue. But the "stock" 26A7 vacuum tube output stage [V608] puts out only 200 milliwatts of audio while consuming 16 watts of filament power, so it's something of a pig.) Note that the filament circuit to V601 would require modification (unless V601 were also removed; this is discussed below).
- Remove V601, V602, and V603; replace the diode sections with solid state diodes (perhaps 1N914 units?). This would result in loss of the 455 khz cathode follower stage V601A; perhaps this could also be replaced with an equivalent solid state circuit. Filament wiring to V606 and V607 would require rewiring (unless this mod was done in conjunction with the elimination of audio stages V606/V607 as described above.) Heat loss reduction = 5.7 watts. Note that this mod might be easy to try in "baby steps": the 1N914 diodes could be soldered to short lengths of #18 AWG solid wire, which would then be inserted into the V602 and V603 tube socket contacts to test suitability.
If all these mods are implemented, they should serve to reduce the internal heat load by almost half. This would probably increase the lifespan of many of the internal components.
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