My SB-104A has been on my workbench for the last week as I work on the talkback and the 60Hz line noise appearing on the transmit audio. I received a report that my transmit audio was almost unreadable due to the noise. I first thought it was bad filter capacitors on my power supply since I could see the 60Hz noise on the 13.8V power rail during transmit. It turns out I can see it there for two reasons:
- My shack has bad 60Hz EMI and poor radio shielding was letting that appear on my input audio.
- My output RF was appearing on my 13.8V rail because of the SB-104A’s well known talkback issue.
I decided to try my hand at both problems.
The source (and hopefully resolution) of the bad 60Hz EMI in my shack will be the subject of a later article.
The noise, it turns out, is mostly appearing from my microphone. The microphone cable inside the SB-104A which runs from the front jack is shielded. The shielding is supposed to be soldered to the ground of the nearby headphone jack while the center goes to the (originally a 2-pin but now a) four pin microphone jack. On my unit this was not done. It seems like whomever built it thought that grounding the shield on the other side of the cable was good enough. And when the radio was new, it might have been. But time and oxidation have left some of the physical fame connections better than others. Connecting up the microphone shielding as it’s supposed to be has helped the 60Hz noise a lot.
Another thing which has improved matters is loosening and retightening as many of the frame connection screws as I can, and running some more contact cleaner on the board ground sliders has also helped. I have reports of excellent audio now.
The appearance of RF on the 13.8V rail, isn’t causing transmit audio problems, but it is a little disconcerting. It was the heavy 60Hz hum modulated on that RF and seen on the DC power rail that made me originally think I had a bad power supply. It is also what causes the talkback issue in the radio, where you hear your transmitted audio over the speaker.
Heathkit has a “fix” for talkback that you can see in their service bulletin. I’ve copied that bulletin below. It seems to address the issue on two fonts. It tries to reduce the amount of RF appearing on the 13.8V rail to begin with, for the most part by adjustments to the grounding (the chassis wiring section below). The other thing it does is make the received audio pre-amp deaf (F board changes).
I don’t actually care about the “talkback” symptom. Hearing a tiny bit of my transmitted audio isn’t a problem. In fact, when I first operated this radio I thought that was by design. So I’m not about to deafen the receive audio on my F board just to make my speaker absolutely quiet on transmit. The only talkback issue I want to address is the RF getting on my 13.8V rail to begin with. The chassis wiring changes do seem to help, but not entirely.
I haven’t yet attempted (or even analyzed) the changes in the bottom “only do this if you have an angry customer” section.
May 24, 1978 SB/HW-104(A) Bulletin No: Solid State Transceiver SB-104-67 Reduction Of Talk Back The following steps should be taken in units received for servicing because of a "talk-back" problem: CHASSIS WIRING CHANGES - Move the two large orange stranded wires from J1 to F19. - Remove and discard the small orange wire between J1 and K3. - Locate the small orange wire between J1 and relay lug 11. Relocate the J1 end to K3. - Connect a new heavy stranded wire between J1 and H6. F BOARD CHANGES - Change R-581 to an 18M ohm resistor [PN 6-186]. - Change R-578 to an 1800 ohm resistor [PN 6-182]. The above procedure should remedy most "talk-back" problems. However, if a customer is not sufficiently placated; and only to avoid a refund; the following procedure may be performed: SB SERIES - Move the gray wire from the CW switch, lug 2 to Tune switch, lug 3. - Remove the jumper wire which is between Tune switch, lug 3 and CW switch, lug 2. - Relocate the white-violet wire from CW switch, lug 4 to HI switch, lug 6. - Remove the jumper wire from between CW switch, lug 6 and LSB switch, lug 6. - Relocate gray wire from CW switch, lug 6, to LSB switch, lug 6. - Install a 100 ohm, 1/2 watt resistor [PN 6-101] from HI switch lug 6 to CW switch, lug ? ((Fiche is unreadable, but could POSSIBILITY be lug 3)). - Locate the orange wire from F19 to relay lug 11. Relocate the wire end at relay lug 11 to relay lug 3. - Install an orange wire from F19 to CW switch, lug 4. - Install an orange wire from K3 to CW switch, lug 6. - Install a 560 ohm resistor [PN 6-561] between F19 and ground.