#1
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Philco 38-12 No IF.
Ive been working on a philco 38-12. When I first got it I knew right away this was going to be more than just a recapping job. There is audio output as in the output and amplifier stage are working. But the IF wont oscillate. I checked the oscillator coil and one side read open. And I ended up replacing it with a P-70-OSC which is a miller coil. I am having trouble wiring it up. There is somewhat of an improvement now. There is a buzz like the buzz you hear when you are tuning between stations, it gets louder on the upper end of the band. I went and did some voltage checks, all checked out fine except the screen grid on the 78 IF tube and the screen on the 6A7 oscillator/Det. Voltage checks 64 when it should be 90 on both grids which further supports my theory that the oscillator is not running. Any ideas? Here is the schematic go to pg 3 http://www.nostalgiaair.org/pagesbym...6/M0013236.pdf
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Admiral C322C2 Regent (Restoring) RCA CTC-7 Pensbury (Restored) RCA CTC-5 Westcott (Restored) CRA CTC--4 Director 21 (Restoring) |
#2
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If the LO (local osc.) is not running, try reversing the connections of either the primary or secondary (but not both).
FWIW, it's not "the IF" which needs to oscillate, but the LO. Think of the first two grids in the tube as a triode, with the second grid acting as 'plate'. The LO tracks 470 khz higher than the received signal. The two signals mix to generate the 470 khz IF signal. Say your station is at 600 khz; the LO will be at 1070 khz, and so on up the dial. In fact you can use a second radio to hear the LO of the problem radio, to tell if it's tracking correctly. Did you check that 40K resistor carrying B+ to those screen grids? Last edited by old_coot88; 10-11-2016 at 09:06 PM. |
#3
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Forgot to mention that, yes I did it measured 100k! I replaced it and went from 31v to my current 64v.
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Admiral C322C2 Regent (Restoring) RCA CTC-7 Pensbury (Restored) RCA CTC-5 Westcott (Restored) CRA CTC--4 Director 21 (Restoring) |
#4
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Chances are when you get the osc running, that screen grid voltage may come up somewhat (since the osc grid will be developing a few volts negative).
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#5
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This thing is driving me nuts. I've tried every possible wiring combination on this damn coil. Some bonehead was in the chassis in the past and screwed up the wiring on the filter caps and I can't seem to trace where they go on the chassis I have them in now but I don't think they are in the right place since I have to jump the B+ to another post to get any voltage at all on the grids of the 6a7 and 78. This thing is driving me nuts!
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Admiral C322C2 Regent (Restoring) RCA CTC-7 Pensbury (Restored) RCA CTC-5 Westcott (Restored) CRA CTC--4 Director 21 (Restoring) |
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