Hallicrafters S-120, Refreshed

Hallicrafters S-120 Gets a Refresh

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

Hallicrafters S-120, Refreshed
Hallicrafters S-120, Refreshed

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.


4 thoughts on “Hallicrafters S-120 Gets a Refresh”

  1. My WR 600 has the incoming AC isolated from the chassis. The negative side of the multi-unit electrolytic capacitor is tied to one side of the AC. The S-120 schematic indicates multiple chassis grounds for various circuit connections.

    Should one side of the AC be grounded ?

    1. I don’t think so – if it’s isolated, keep ‘er that way.
      Is the WR600 same (mostly) circuit as the S-120?

      The S-120 *DOES* have one side to chassis. And then they (attempt) to isolate the chassis from
      the outer cabinet – with plastic and fiber washers & such.

      Basically – I don’t think the power plugs on these are polarized. So, you might not be able
      to control WHICH side gets HOT and which side gets Neutral.
      OR – plugs can be wired wrong.

      I do ‘polarize’ the plug, by snipping the side that goes to chassis in the middle. This makes THAT side of
      the plug go to Neutral.
      By ‘snipping’ – I don’t mean ‘cut it off’ – I make a cut (with some wirecutters) in the middle of whichever side
      is connected to chassis. Then I spread it a bit.
      OR – you can do the same by using a modern plug – brown extension cords are great – cut off the ‘extension sockets’,
      and use the polarized cord. WIDE side to the chassis.

      STILL – make sure all the little plastic and / or fiber washers are good and in place, and any plastic feet, since you *still*
      can’t account for mis-wired wall sockets.

  2. Mike,

    Fascinating write up – thanks for the insight.

    I snagged a used 120 at a thrift store for $8. It does power up, but has a nasty, loud AC hum. Any ideas as to which capacitor to replace?

    Again, thanks for your work on these radio’s.


    1. The large, can-type, capacitor is the culprit. There’s plenty of room under the chassis to put ‘modern’ replacement capacitors.
      Alternatively, Hayseed Hamfest is a website that supplies brand-new replacements for these 3-in-one or 4-in-one capacitors. Specify which model you have (Hallicrafters S-120) and he should have something equivalent.

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