Hallicrafters S-120 Refurb
A Daily Use Radio Gets a Refresh.
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 longwire plus a solid ground.
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 recap.
The knobs are plastic, the chassis is separated from the outer cabinet by plastic 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.
"Light Refurb" Items - December 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-
- Replace the Selenium Rectifier - or at least leave it in place and remove it from the circuit. Need to determine the DC voltage across C31A to determine the necessary dropping resistor to use with a 1N4007 diode.