Hallicrafters S-77A receiver

Single conversion general coverage receiver. The S-77A is the AC-DC Marine version of the S-40B and is rarer than the S-40B itself. Its predecessor, the S-52, is the AC-DC version of the S-40. Like the others in the series, this set covers 4 bands from AM broadcast to 44 MHz. It features electrical bandspread, BFO, an RF amp and two stages of IF amplification. Its external appearance is nearly identical and its circuitry is similar to the S-40B with a 25Z5 rectifier and 25L6 audio output substituting for the 5Y3GT and the 6K6GT in the S-40B. The radio's 9 tubes include a plug-in ballast that was selected for either 120 volt or for 240 volt operation. The B- in this set (one side of the power line) is "floating". While safer than directly connecting one side of the line to the chassis, as found in Hallicrafters models such as the S-38 and S-41, an isolation transformer or GFCI outlet (ground-fault circuit interrupter) is recommended for safety. For this radio, I opted to retrofit with an ALCI plug (Appliance Leakage Circuit Interrupter) and cord. The original price was $89.95 in 1952.

Repairs
Both the Hallicrafters manual and the Sams Photofact for this set are in BAMA in djvu format. I recommend the Hallicrafters schematic since the Sams schematic has lots of parts identification mistakes. (For example, numerous resistors of different values as well as a few capacitors, are all identified as R15 and several caps are labeled C12).

After a thorough cleaning that included using some "Goo-Gone" to a few old dried paint spatters, I removed the chassis and front panel. The chassis top was cleaned with white waterless hand cleaner. The cracked tuning dial window was replaced with a reproduction. Several critical capacitors were replaced including one across the power line, from the B- to chassis, and the input cap to the final audio amp (25L6). The 1000 ohm dropping resistor in the B+ line had increased to 1400 ohms and was replaced. The electrolytic had been replaced at some earlier time. I cleaned up the poor job of wiring (in-the-air splices) for that electrolytic. Several other caps in various locations that allowed some B- leakage to the chassis were replaced. Controls and switches were cleaned.

A slow power-up with an isolated variac followed. The set played but was less sensitive than expected. I checked all the tube voltages to be sure they were adequate. The set apparently needed some alignment. Alignment of the RF and the converter stages made a significant difference in sensitivity. After alignment tweaking, sensitivity was excellent on broadcast and very good on the lower shortwave bands.

Despite replacing capacitors, there is still enough inherent leakage from B- to the chassis in this set, because of the 60 Hz reactance from the capacitors, to trip a GFCI protected outlet when a ground is attached directly to the metal cabinet or chassis. That means that more than 4 or 5 mA leakage is possible. While a properly fused 3-wire plug could be used, plugging the 3-wire cord into a GFCI-protected outlet will trip that GFCI . A ground connection can be made to the S-77A via its ground terminal but that connection is through a capacitor and not directly to the chassis.

An isolation transformer or permanent ALCI is strongly recommended for use with AC-DC sets.

Adding an ALCI for safety.
As noted above, I opted to replace the entire plug and cord with a small polarized ALCI plug and cord removed from a hair appliance. I look for older hair appliances with a two button ALCI for re-use with AC-DC radios. That is a relatively non-invasive change to make the radio a bit safer to use. The little cord-mounted ALCI will trip and shut off power in the event of a shock hazard. Note that the ALCI has both a test button and reset button.