10-19-2022, 09:15 PM
Hi Boys and Gals:
Restoring a Philco 482 and discovered the 4th IF transformer resonating on 340 kHz, rather than 455 kHz. The mica trimmer has no effect on the frequency. I am sweeping it with a HP 3325B triggering a Tektronix 465M scope--e.g. Poor Man's Spectrum Analyzer. The curve is beautiful. But it won't move when I adjust the 455 kHz trimmer.
So I pulled the transformer from the chassis, opened it up and checked the mica cap inside (270 pf) which was reading 318 pf and leaking 200 uA with 600 VDC across. So I replaced it with modern silver mica (263 pfd). Swept it out of circuit and it is still resonating low--now about 315 kHz when stuffed into its can. The mica trimmer till has no effect.
Did the resistance checks on the coils with Heathkit analog IT17 and all resistances spot on. Took apart the mica trimmer partway, and pulled two of the mica insulating plates. Both have "tabs" on one end that are broken off. When I push down the plates after reinserting them, the mica trimmer now shorts. This is after I lifted one end off the coil to isolate it from the coil when making resistance measurements. So now I am trying to figure out how to "repair" the missing tabs on the mica insulators.
Just for fun, I swept the transformer with the 455 kHz trimmer disconnected from the coil, and it shows the same resonance--about 315 kHz--as before. And then also for fun I dropped a 100 pf across the 270 pf mica cap inside the transformer, and the resonance dropped in frequency slightly.
I predict after squaring away the mica insulation problem and putting the can back in the radio's circuit, the proper 270 pf mica cap installed will make the can resonte on 455 kHz and tune as per factory. Fingers crossed.
If I'm missing something, please let me know 8D
Restoring a Philco 482 and discovered the 4th IF transformer resonating on 340 kHz, rather than 455 kHz. The mica trimmer has no effect on the frequency. I am sweeping it with a HP 3325B triggering a Tektronix 465M scope--e.g. Poor Man's Spectrum Analyzer. The curve is beautiful. But it won't move when I adjust the 455 kHz trimmer.
So I pulled the transformer from the chassis, opened it up and checked the mica cap inside (270 pf) which was reading 318 pf and leaking 200 uA with 600 VDC across. So I replaced it with modern silver mica (263 pfd). Swept it out of circuit and it is still resonating low--now about 315 kHz when stuffed into its can. The mica trimmer till has no effect.
Did the resistance checks on the coils with Heathkit analog IT17 and all resistances spot on. Took apart the mica trimmer partway, and pulled two of the mica insulating plates. Both have "tabs" on one end that are broken off. When I push down the plates after reinserting them, the mica trimmer now shorts. This is after I lifted one end off the coil to isolate it from the coil when making resistance measurements. So now I am trying to figure out how to "repair" the missing tabs on the mica insulators.
Just for fun, I swept the transformer with the 455 kHz trimmer disconnected from the coil, and it shows the same resonance--about 315 kHz--as before. And then also for fun I dropped a 100 pf across the 270 pf mica cap inside the transformer, and the resonance dropped in frequency slightly.
I predict after squaring away the mica insulation problem and putting the can back in the radio's circuit, the proper 270 pf mica cap installed will make the can resonte on 455 kHz and tune as per factory. Fingers crossed.
If I'm missing something, please let me know 8D
