10-20-2023, 05:32 PM
I restored a Philco 41-255 some time back and all appeared to work well, though I recently noticed the upper SW band (9-12 MHz) seemed to receive far fewer stations than my other radios, and apparently at the wrong place on the dial. Performance on the BC and 2-7 MHz band is as expected. I revisited my alignment, IF and oscillators, and all was as it should be. The alignment instructions for the 9-12 MHz band specify adjusting the oscillator compensator to the fundamental frequency, and that seems to work as described - starting from the closed position on the trimmer then backing out CCW for the first peak. When I check the alignment with my generator, dial tracking is very poor on the mid/low end of the dial, and received signals are weak, with a much stronger image received about 1 MHz higher on the dial. I'm assuming it's 910 KHz higher.
I used the spectrum analyzer loosely coupled near the chassis to watch the LO frequency vs dial frequency, and for the BC and 2-7 MHz bands, the LO is 455 above the tuned frequency and tracks well. But on the problematic 9-12 MHz band, the LO is 455 below the tuned frequency. So now, if a known strong station is at 9.3 MHz, I can barely know it's there at 9.3 on the dial, but receive it normally at 10.2 MHz.
I can't find any way to make the upper SW band LO run 455 above dial frequency, which is counter to alignment info but would match the other two bands. If I open the trimmer to get the LO at 455 KHz on the high side at 12 MHz, it tracks so incredibly poorly that by 11 MHz on the dial, the LO itself is at 11 MHz, then it's soon farther and farther below the tuned frequency. I'm attaching the Philco schematic, as well as the alignment instructions for the similar model 41-280, which is what Philco specifies as instructions for the 41-255.
I've triple checked my original work. That included replacing the oddly packaged dual capacitor C21/C21A with two discrete 360 pF silver mica capacitors in the osc./mixer circuit. The other discrete silver mica capacitors in that circuit appear to be at the correct value. I've also measured continuity of the various band switch connections related to the upper SW band, including coils, etc., and soldered every riveted chassis connection, including the chassis connections of the oscillator trimmers. And of course I've cleaned the oscillator tube/socket pins. The oscillator trimmer is one half of a dual trimmer, and it measures 0-90 pF, though no value is given in the parts list.
I don't have enough hair to pull out, but that's where I'm at. I'm hoping someone can point me in the right direction on this one.
EDIT: original 1st post text replaced with this more accurate description of the problem
I used the spectrum analyzer loosely coupled near the chassis to watch the LO frequency vs dial frequency, and for the BC and 2-7 MHz bands, the LO is 455 above the tuned frequency and tracks well. But on the problematic 9-12 MHz band, the LO is 455 below the tuned frequency. So now, if a known strong station is at 9.3 MHz, I can barely know it's there at 9.3 on the dial, but receive it normally at 10.2 MHz.
I can't find any way to make the upper SW band LO run 455 above dial frequency, which is counter to alignment info but would match the other two bands. If I open the trimmer to get the LO at 455 KHz on the high side at 12 MHz, it tracks so incredibly poorly that by 11 MHz on the dial, the LO itself is at 11 MHz, then it's soon farther and farther below the tuned frequency. I'm attaching the Philco schematic, as well as the alignment instructions for the similar model 41-280, which is what Philco specifies as instructions for the 41-255.
I've triple checked my original work. That included replacing the oddly packaged dual capacitor C21/C21A with two discrete 360 pF silver mica capacitors in the osc./mixer circuit. The other discrete silver mica capacitors in that circuit appear to be at the correct value. I've also measured continuity of the various band switch connections related to the upper SW band, including coils, etc., and soldered every riveted chassis connection, including the chassis connections of the oscillator trimmers. And of course I've cleaned the oscillator tube/socket pins. The oscillator trimmer is one half of a dual trimmer, and it measures 0-90 pF, though no value is given in the parts list.
I don't have enough hair to pull out, but that's where I'm at. I'm hoping someone can point me in the right direction on this one.
EDIT: original 1st post text replaced with this more accurate description of the problem
