The S-120 resided in my office at work for almost a year until it developed an annoying 60-cycle hum that finally wouldn’t go away once it warmed up. Time to replace the electrolytics.
The Hallicrafters S-120 is essentially the same circuit as the S-38 series: an All-American 5 tube, minus the rectifier — replaced with a selenium rectifier. I bought this swell little radio from a fellow who thought it was dead, except for a few AM Broadcast stations. I brought it home and after a quick spray of Deoxid on the bandswitch and 3-in-1 oil on the tuning shafts (they were almost frozen) within 10-minutes it was working on all bands. Total cost: $10.
Antenna is a ferrite rod, built-in. The rear panel has clips to hold an extending rod antenna — mine is missing. Variations of this model include the SW-500 (same, but a sort-of slate-blue metal cabinet), and the WR-600 (same, but a wood cabinet). It’s a swell consumer-level general coverage receiver, and works great when provided a proper 75-foot long-wire plus a solid ground.
The knobs are plastic, the chassis is separated from the outer cabinet by p
lastic spacers and screw attachment points — this is a hot-chassis radio. The ‘ground’ in the schematic is actually B-; chassis is separated from B- by C29 and R18 (in parallel). Polarizing the power cord forces B- to always be the ‘neutral’.
C31 – A, B, C, and D – the 4-in-1 electrolytic capacitor sits above the chassis, the same side as the tubes. Unfortunately, there’s no good place to put a terminal strip; plus the leads to the 4-in-1 would have to be spliced to extend above the chassis. So, I relented and put the 4 individual caps near their + connection point; B- is available at several points, which makes this a convenient way to go. An alternative would be to measure this 4-in-1 electrolytic and contact Hayseed Hamfest for a modern replacement. At the time of this writing, they have an exact replacement for the ‘can’ in the S-120.
Updates as of 2008:
- Replaced the 4 Electrolytics, leaving the old 4-in-1 in place, but not electrically connected.
- Replaced all the tubulars, basically any by-pass capacitors.
- Reception is extremely strong on all four bands. Didn’t do an alignment.
- Polarized the power plug.
- Added a safety cap from ‘after the power switch’ to B-
After rotating the radio home, and an extended stay playing there, the selenium rectifier gave up it’s ghost, with the accompanying stink. The selenium rectifier was replaced with a modern 1N4007 diode and a series 200 ohm, wire-wound resistor. The modern diode has quite a bit less voltage drop than the old selenium, and the resistor compensates to keep all the internal voltages ‘pretty close’ to what’s on the schematic.