philco: Transitone; 48-200 tunes only one station

My 48-200 has had all caps and resistors replaced, all tubes swapped out, and a thorough cleaning. It will only tune one station in the 640kc area. Tuning elsewhere on the dial decreases volume, but the single station is still there. Any ideas? Thanks from a newbie!

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Hi Steve,

When a radio will tune only one station, a possible cause is a nonfunctioning oscillator.  The mixer (or "converter") stage of the radio produces the fixed IF frequency (455 kHz) by mixing the station and the variable oscillator frequencies.  "Mixing" means that the mixer generates the difference freqency.  If the oscillator is not functioning, then the mixer can't do anything except pass through the original RF frequency.  Snice the IF is tuned to 455 kHz, only signals near the bottom of the dial have any chance to get through, and usually this is only possible if the IF alignment is not proper.

An easy way to tell whether the oscillator is working is to measure the oscillator grid voltage.  In this radio, that is pin 4 (marked with a big "D" on the schematic) of the 7A8 converter tube.  This point should be at least a few volts negative compared to the cathode (pin 7).  You must use a high impedance voltmeter when making this measurement (any modern digital multimeter will work, or an older VTVM or FETVM, but not a cheap analog meter).  If you have an oscilloscope, you can also use it to check the signal on pin 4.  It should show the oscillator frequency, which is the dial frequency + 455 kHz (e.g., 1045 kHz if the dial is tuned to 600 kHz).

If you confirm that the oscillator is not working, check the "oscillator plate" (pin 3) voltage (which should be around 90 V) and the screen grid (pin 5) voltage (which should be around 45 volts).  Check R400.  Make sure the tuning capacitor plates in the oscillator section are not touching.  Check for continuity on both windings of the oscillator coil (proper resistances are shown on the schematic).

Best regards,

Tom

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Thanks, Tom!

It took me awhile, but after fiddling with the oscillator section other stations finally spoke. Several voltages were low. Your assistance is very much appreciated!

Steve

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