S-20R General Specifications:
Band 1: 550 - 1780 KHZ AM/CW IF = 455 KHZ
Band 2: 1.74 - 5.4 MHZ AM/CW IF = 455 KHZ
Band 3: 5.3 - 15.8 MHZ AM/CW IF = 455 KHZ
Band 4: 15.5 - 44 MHZ AM/CW IF = 455 KHZ
1 2 3
1: Hallicrafters S-20R receiver.
2: Replaced all paper caps with orange drops.
3: Fix for band 4 problem (see paragraph 6 below).
I purchased the S-20R at same Antique Radio Swapmeet in Kutztown, Pa where I picked up my Hallicrafters SX-62. Checking out the S-20R at home, a loud hum was heard regardless of where the audio volume control was set. No stations were received on any band. While turning the main tuning and bandspread tuning, both old dial cords broke. (I hate dial stringing !). My revitalization project involved getting the receiver to perform as it did when it was new, no effort was made to make it better cosmetically.
Steps taken in the revitalization:
1. ELECTROLYTIC CAP REPLACEMENT: The original power supply filter had dried out and opened up. I could have purchased a replacement from a parts store but I had a couple of new 100 MFD 450 volt electrolytic caps on hand so I jumpered them across the original dried out filter. Result: Hum gone completely but still no stations received. (I should have removed the old caps but this is not a serious receiver for my shack).
2. TUBES: Out of 9 tubes the following tubes were bad: 6K8, 6SQ7, 6H6. All replaced by New Old Stock (NOS) from ebay sellers.
3. PAPER CAP REPLACEMENT: All paper caps looked real bad so I replaced them all with orange drops. With new caps installed, stations were now being received.
4. IF AND RF ALIGNMENT: This made a remarkable improvement in the receiver performance. Interesting note: Because of no dial cords, I could easily move the tuning dial from one end of the band to the other while performing RF alignment. Secondly the IF was sitting on 453 instead of 455 KHZ. After alignment, stations are now received very well on bands 1-3 but still no band 4 output.
5. SPEAKER: Audio from the in-case speaker was distorted, similar to a torn speaker cone. I paralleled an external speaker and the sound was good. I unmounted the speaker from the front panel and the speaker didn't have any rips but the paper cone was very weak. I tried to stiffen the cone with several coats of KRYLON Crystal Clear Acrylic and while it did make a big improvement it was not enough of an improvement to warrant keeping it, so I replaced it with one I had inside a Radio Shack bookcase speaker.
6. BAND 4 FIX: Found no B+ on the RF amp tube. Found an open winding on the primary of the output RF tank. The hair thin wire could not be repaired so I wound my own coil with #21 magnet wire. Band 4 now works fine.
7. ANTENNA CONNECTION MODIFICATION: I drilled a new hole on the back of the chassis an installed an SO-239 coax connector. (Hey, we're not talking Collins here). After this mod I re-did the complete RF alignment to work with the new connector; however, not much change was noticed.
8. RESTRINGING DIALS. I had no dial map so I had to figure things out. The main tuning dial was not nearly as bad to re-string as the bandspread dial because of space limitations on the bandspread shaft area. I hate dial stringing and spent 4 hours on this step alone.
9. PHONE JACK MODIFICATION: I rewired the phone jack from a high impedance, taken off the grid of the audio power amp stage, to a low impedance which is taken off the audio output transformer when the speaker is switched out.
One evening I was listening to an amateur radio AM station in the neighborhood of 3885 kHz and suddenly another AM station came right in over the top crushing the first station like a wet blanket. The first station made no comment at all about interference. Puzzled, I tuned the stations in on my Drake R8B and discovered that the first station was on 3885 kHz and the second station was on 3875 kHz. I now know why the "famous" now silent key owner of an S-20R always complained about buckshot. The S-20R is as broad as a barn door and has no filters at all!!
Well, it was a fun project and since it is not a serious communications receiver for crowded bands, it is now in my backyard shed, so whenever I need tools or my bicycle, I stop briefly and tune around the bands for enjoyment.