Restoring a Heathkit SB-101

Restoring the SB-101 transceiver that I received together with the HP-23 power supply was more challenging than restoring the HW-32A, mostly because the 101 is mechanically much more complex. Like the restoration of the 32A, this restoration was not 100% successful, but the transceiver is working. It receives just fine. It also transmits, but there is still a remaining problem on the transmit side.
When I got it, it was really in a terrible shape. It was covered with a thick layer of dust. The dust covered not only the outside of the transceiver, but also everything on the top side of the circuit boards; dust entered the enclosure through the perforated cover. The bottom side of the boards was clean, thanks to gravity I guess.

 

 

Cleaning up the dust is easy (and satisfying; it fun to see the electronics come out of the dust). I tried to remove all the knobs so I could remove the front panel, but I could not release the set screw of one of the knobs. I did not want to ruin the knob, so I left it attached. I did not manage to remove the front panel, but it did not matter at the end.

With the unit clean, I replaced the electrolytic capacitors, as I did on the HW-32A. There aren’t many of them and most were easy to replace. One 10uF 10V cap was burried under a jungle of wires near the front panels. It was very hard to get so I initially left it in the circuit. It’s an audio-frequency cathode bypass in the microphone amplifier, and I thought that even if is shorted, it would not cause further damage. Eventually I replaced it too.

With the electrolytics replaced, I powered the transceiver. It received okay on most bands. The only band that was dead was the 29.5-30MHz. I think it’s heterodyne oscillator is not working; maybe the heterodyne crystal is bad. There’s only FM activity in this band and the transceiver does not support FM, so it did not seem worth it to try to fix this. Also, the VFO dial was 100kHz off, but I left this for later.

The next step was to fix the mechanical problems. There were several. The most difficult one to fix was a missing shaft coupler for the finals tuning capacitor. The 101 used an insulated coupler. In the unit I had, it was badly broken and parts of it were replaced by some solidified goo, perhaps old epoxy. The remains of the coupler were not working: the panel shaft rotated without turning the capacitor. I removed all the broken pieces and the goo and tried to find a replacement coupler. I did not find out. I did have a suitable aluminum coupler, but it had a large diameter so it would not go through the hole in the shield of the final amplifier. I tried to make a coupler myself from soft aluminum tubing, but it did not work (the set screws slipped). I tried to remove the cover but it’s held by hard-to-reach screws. Eventually, I removed the variable capacitor (which required disconnecting an inductor and a couple of capacitors that were soldered to it), attached my coupler, and mounted it back.

 

 

The next step was to replace the three o-rings that couple two front-panel knobs to three variable capacitors (final loading and the driver/preselector tuning). The o-rings that came with the transceiver have completely disintegrated. I tried to locate replacements (they are available on eBay), but decided eventually to use a nylon cord instead. Diagrams and pictures on the Heathkit Yahoo group helped a lot.

 

With o-rings replaced, I could try the transmitter (without the o-rings, there is no way to tune the driver and the loading capacitor of the final tank circuit). The transmitter seemed to work fine on 3.5, 7, and 21MHz, but experienced bad oscillations on 14MHz. On 28MHz, it didn’t oscillate but it only produced a few watts (as opposed to over 100 on 3.5, 7, and 21). You see that it oscillates because after you tune it for maximum output, it keeps producing output even if you turn the drive knob all the way down.

I described the situation on the Yahoo group and many folks responded with useful advice (many thanks to Bill Harris, Bob Burns, Mike Waldrop, Timothy Bolbach, and Kevin Schuchmann). They included adjusting the heterodyne coils, the driver/preselector coils, ensuring that the driver PC board is making good contacts with the chasis, and improving the grounding of the PCBs that carry the crystals and coils that the band switch switches. I adjusted the coils to the extent that was possible. This included doing the initial receive-mode adjustment of the heterodyne coils (at first I thought that this was it) and of the driver coils. This cured the problem on 14MHz but oscillations appeared on other bands. I tightened the screws that attach the PCBs to the chasis, but I could not check that there are spring washers between the PCB and the chasis, because most of the nuts on the screws that hold the driver boards are nearly impossible to reach. I did improve the grounding of the band-switch boards, but without soldering them to their brackets or replacing the brackets (this mod was recommended by Heathkit decades ago, but my brackets were both impossible to solder and very hard to remove). In the picture below you can see these boards. The exposed wire that connects them near the bottom of the chasis is from the original build. I added thicker multi-strand ground wire. At some point I tried to add a copper sheet on this side to ground the boards even better, but it didn’t seem to do any good.

I also discovered a couple of other problems associated with the driver. The wire that serves as a gimmick neutralizing capacitor was not placed in the hole it should be in. Also, the shield of the driver was missing. I corrected the positioning of the wire and moved a shield from another tube (V1) to the driver. I build a homemade shield for V1.

These improvements eliminated the oscillations, but the transmitter was still a bit unstable. The instability shows up as unstable grid current in tune mode (and CW). Plate current and output power are also somewhat unstable. This instability amplitude-modulates the CW or tune carriers and I can hear it in an AM receiver. It sounds like scratching or a bad contact in an audio system.

The unstable grid current does not allow me to do the fine tuning of the coils, which is supposed to be done by peaking the grid current. I also tried to diagnose and perhaps to fix this in a couple of other ways, such as improving some suspect solder joints and trying to adjust the final’s neutralizing capacitor (this seemed to do more harm than good), but nothing eliminated the problem. I also tried to replace the driver tube but this didn’t help either (it’s kind of a miracle that I had a spare, but I did) and to clean and re-solder the anode caps on the finals.

At this point I gave up trying to fix this. The textbook solution of trying to replace the brackets that hold the bands-witch boards, solder the boards to the replacement brackets, and checking the screws and nuts and washers that hold the driver board to the chasis required more disassembly than I was prepared to do. These boards are not designed to be removed, and some of the screws that I had to remove to disassemble this part of the transceiver were really jammed. I guess it is possible to fix this problem, but it’s not easy.

Also, from all the Heathkit service bulletins and all the experiences of the people on the Heathkit Yahoo group I realized that the SB-101 suffers from some serious design problems in the driver and band-switch. This kind of discouraged me.

I replaced two connectors as part of the restoration. I replaced the UHF antenna socket with a BNC. The old socket was not original (the original was an RCA) and was not well attached to the chasis. A chasis-mount BNC socket fit the hole, is well attached, and more convenient. I replaced the weird Heathkit microphone connector with a more conventional 4-pin socket for which I had two plugs available.

The end result is a transceiver that receives and transmits on all bands (apart from the 29.5MHz sub-band), but with amplitude variations on transmit. Transmit performance was sufficient to chat with a friend for a while, and this was good enough for me.