02-08-2014, 09:37 AM
One more thought. I'm assuming that you rewired and tested the IF transformers. IIRC, there are a number of rubber coated wires in those transformer cans that must be replaced or sleeved. There are also some internal mica caps and a resistor (37B, 37C, and 37D) in the 2nd IF can that should be tested and replaced if necessary.
There is a mish-mash of frequencies in the converter tube, 7J7. All of them should be filtered out except one, 455 KHz. If your IF transformers have opens or shorts, or if the internal caps and resistors are off their specified value, you won't get the filtering you need. Spurious frequencies might show up in the wrong places.
Mica capacitors don't usually go bad, but they do often enough that they are worth testing, especially if you have a problem. The trouble is that they are hard to test in the circuit because their capacitance is usually so small that the capacitance of your test leads overwhelms the measurement, assuming you have a meter that measures capacitance. You can subtract the capacitance of your leads from the measurement of the mica cap, but that isn't as accurate as one might hope.
I have a meter that I can plug capacitors into, eliminating the effect of test lead capacitance. The trick is, you have to remove the cap from the circuit to test it. Once I remove a cap to test, it is no more trouble to replace it with a modern silver-mica cap of known value. The upshot is that I replace a fair percentage of the old domino-style micas, some of which are OK, but enough of them are marginal or bad to make testing them worthwhile.
There is a mish-mash of frequencies in the converter tube, 7J7. All of them should be filtered out except one, 455 KHz. If your IF transformers have opens or shorts, or if the internal caps and resistors are off their specified value, you won't get the filtering you need. Spurious frequencies might show up in the wrong places.
Mica capacitors don't usually go bad, but they do often enough that they are worth testing, especially if you have a problem. The trouble is that they are hard to test in the circuit because their capacitance is usually so small that the capacitance of your test leads overwhelms the measurement, assuming you have a meter that measures capacitance. You can subtract the capacitance of your leads from the measurement of the mica cap, but that isn't as accurate as one might hope.
I have a meter that I can plug capacitors into, eliminating the effect of test lead capacitance. The trick is, you have to remove the cap from the circuit to test it. Once I remove a cap to test, it is no more trouble to replace it with a modern silver-mica cap of known value. The upshot is that I replace a fair percentage of the old domino-style micas, some of which are OK, but enough of them are marginal or bad to make testing them worthwhile.
John Honeycutt
