09-02-2013, 12:44 PM
Actually I was quite surprised at the performance of the 48-482 on FM. It is much better than any other 40's FM receiver I have tested. It does have 3 IF stages, an RF stage, a triode mixer, and the quadrature detector so maybe it should not be so surprising.
If you have low sensitivity, check the 33K plate resistor of the 7W7 RF stage. It was open on mine. Philco underrated this resistor at 1/2 watt as it dissipates significantly more than that and therefore fails regularly. I replaced it with a 2W metal oxide resistor. The 100K screen resistor is also marginal and usually increases in value.
The first thing I would try is to sweep the Fm1000 detector. Connect the generator set to 9.1 MHz, 1MHz sym sweep to the grid of the 7H7 3rd IF, marked point "A" on the schematic. Use the fastest sweep rate range of the generator. Connect the AC coupled scope probe to the junction of C304 and C305 in the FM detector section. Point is marked "B" on the schematic. As you increase the signal level you should notice the detector lock in and see a sloped waveform with noise at each end. You can decrease the sweep width with the sweep vernier to get a better expanded view. Then adjust the detector slug on top for the most symmetrical waveform with the straightest , most linear slope on the center portion. This will be the optimal adjustment for the detector.
If you have low sensitivity, check the 33K plate resistor of the 7W7 RF stage. It was open on mine. Philco underrated this resistor at 1/2 watt as it dissipates significantly more than that and therefore fails regularly. I replaced it with a 2W metal oxide resistor. The 100K screen resistor is also marginal and usually increases in value.
The first thing I would try is to sweep the Fm1000 detector. Connect the generator set to 9.1 MHz, 1MHz sym sweep to the grid of the 7H7 3rd IF, marked point "A" on the schematic. Use the fastest sweep rate range of the generator. Connect the AC coupled scope probe to the junction of C304 and C305 in the FM detector section. Point is marked "B" on the schematic. As you increase the signal level you should notice the detector lock in and see a sloped waveform with noise at each end. You can decrease the sweep width with the sweep vernier to get a better expanded view. Then adjust the detector slug on top for the most symmetrical waveform with the straightest , most linear slope on the center portion. This will be the optimal adjustment for the detector.
