10-28-2023, 08:40 AM
I wanted to update this thread with what I've found trying to solve this problem in case it's helpful to others. First of all, it turns out that I caused most of the problem myself. I experienced this problem with everything assembled in the cabinet as well as on the bench, so I dismissed the idea of the built in loop being involved. It turns out that when the loop was directly attached with its short leads, I still experienced the problem because I also had my external antenna adapter connected. And if I had the adapter disconnected, I had the loop extended on 12" clip leads. That was critical. And not so obvious, because it did not impact the BC and lower SW band in a readily observable way.
So it turns out that the BC and 2.3 MHz bands do have the LO on the high side, and the 9.1 MHz band uses LO on the low side. The LO/dial tracking was not very good on the 9.1 MHz band (only), and that was ultimately resolved by moving C19, 84pF, up and back ~1/2" from its original spot near the group of wires leaving the band switch. The LO/dial tracking improved significantly and LO was always 420-450 KHz below dial frequency across the dial. C19 is only in the oscillator circuit in the upper SW position.
In my original work on the radio, I'd soldered a few of the critical RF riveted chassis connections, like the oscillator compensator assemblies and a couple terminal strip commons, but this time I soldered all of them, and there are a couple I hadn't even considered before, like the one near the antenna coil.
The reality check on this was receiving known off air broadcasts at 9.3, and with the loop extended on 12" clip leads, or with the loop directly attached but with the adapter also attached, 9.3 on the dial had the correct LO frequency but it was barely possible to pick the broadcast out of the noise at 9.3 on the dial, while it could easily be heard at ~10.2. With the loop (only) directly connected, performance was as expected with strong reception at 9.3 and a weak image at ~10.2. And, the adapter was problematic regardless of whether it was set for "local" with no external antenna, or set for "distant" with a 10' wire. Now, the adapter is one that I made from the diagram for the Philco 76-1134, so it's completely possible that it's not built correctly or that it's not correct for the 41-255. My final item to resolve is trying to make sure that the loop antenna resonance is tracking the dial, which I've been able to see with my scope, but I'm certain my scope is affecting resonance so I can't really confirm how well that's tracking. C9, 98pF, is measuring 108pF, so I may replace that.
So it turns out that the BC and 2.3 MHz bands do have the LO on the high side, and the 9.1 MHz band uses LO on the low side. The LO/dial tracking was not very good on the 9.1 MHz band (only), and that was ultimately resolved by moving C19, 84pF, up and back ~1/2" from its original spot near the group of wires leaving the band switch. The LO/dial tracking improved significantly and LO was always 420-450 KHz below dial frequency across the dial. C19 is only in the oscillator circuit in the upper SW position.
In my original work on the radio, I'd soldered a few of the critical RF riveted chassis connections, like the oscillator compensator assemblies and a couple terminal strip commons, but this time I soldered all of them, and there are a couple I hadn't even considered before, like the one near the antenna coil.
The reality check on this was receiving known off air broadcasts at 9.3, and with the loop extended on 12" clip leads, or with the loop directly attached but with the adapter also attached, 9.3 on the dial had the correct LO frequency but it was barely possible to pick the broadcast out of the noise at 9.3 on the dial, while it could easily be heard at ~10.2. With the loop (only) directly connected, performance was as expected with strong reception at 9.3 and a weak image at ~10.2. And, the adapter was problematic regardless of whether it was set for "local" with no external antenna, or set for "distant" with a 10' wire. Now, the adapter is one that I made from the diagram for the Philco 76-1134, so it's completely possible that it's not built correctly or that it's not correct for the 41-255. My final item to resolve is trying to make sure that the loop antenna resonance is tracking the dial, which I've been able to see with my scope, but I'm certain my scope is affecting resonance so I can't really confirm how well that's tracking. C9, 98pF, is measuring 108pF, so I may replace that.
