R-392 receiver mods

The R-392 receiver is hot, stable, and provides full coverage between 0.5 and 30 MHz. However, the compromises involved in modifying it for 28 Vde on filaments and plates need shaping up. Here are some suggestions.

First bring out the plate and filament leads separately. The receiver is happier with somewhat more than 24 volts on the plates, somewhere between 30 and 35 volts. Build a supply that will provide over 30V from a bridge rectifier. Filter the supply with 3000 uF or more (the receiver tolerates 2 to 4 V of hum). Power the entire receiver this way first. The tubes in my set withstood 37 V for a few hours. Next add a dropping resistor (2 ohms, 20 watts) to the filament lead. Leave the plate lead on the higher voltage.

The audio output tube has to go. This tube pulls a total of 1.3 watts, plate and filament. The designers must have known it had to go: the manual shows a plug-in transistor substitute. However, their substitute uses 15 components including four obsolete transistors and two custom transformers. A better substitute is shown in fig. 4.

Use a single Darlington power transistor with no extra heatsink; connect the collector to existing pin 8, emitter through 22 ohms to existing pin 2 and ground, and base to existing pin 1. Bias the Darlington with 27k to pin 2 and ground and 470k to pin 5 (+28 V, formerly the screen supply). Build this assembly on an octal plug. Adjust the 470k resistor to obtain 0.2 to 0.7 V across the 22-ohm resistor.

An improved circuit is shown in fig. 5.

It is reasonable that an identical circuit could be used for the other half of the 26A7 to have push-pull audio through the output transformer. With enough negative feedback those darlingtons could operate in class B push-pull.

A LED makes an excellent 1.5 V zener - much better than a zener at low current. | used 10k to a LED to obtain 1.5 V and 47k from the LED to the Darlington base. That's a total of five components. The dropping resistor in the power supply can now be increased to 4 ohms. You have removed 16 watts from the filament string.

Next we operate on the detectors, V602, V603 are 12AU7s with filaments in series. They are used as four diodes for detectors and agc. Pull out V603 and insert two germanium diodes into the socket holes, one with anode to pin 1 and cathode to pin 3 (see fig. 6), and the other with anode to pin 6 and cathode to pin 8.

The set should work, even with V602 dead. Insert two more diodes in the same places on V602 socket to get the squelch rectifier and agc working again.

You can now increase the power supply dropping resistor to about 8 ohms, and the filament string will have decreased from 3 to just over 2 amps. The radio has now become practical, and you haven't had to take the chassis apart. The next step should involve fet substitutes, a very practical possibility with only 30 volts on the plate bus.