I have been stumped by this one for a while now. On an old (circa 1948) GE 356 AM/FM radio I'm restoring, AM plays well, FM was quiet until alignment. Now FM plays 88 to 94 MHz (weaker sound as freq increases). When warming up, I can receive stations up to about 98MHz, then they fade out (with some distortion).

Tube voltages measure good except some grids at -0.15 vs. -0.5 volt. Tried substituting 6AG7 and 6BE6 front end and osc/conv, no change. Raised AC voltage to 135v through Variac and a few more stations up to about 96MHz are tunable (and audio is higher), but the plate voltages are over the Riders spec'ed values when I do this.

Looking for additional troubleshooting ideas. Thanks,

Craig S

If you get more stations by raising the line voltage, have you checked for a weak tube, drifted resistor, or leaky condenser? Disconnect resistors and condensers before testing them. Condensers should have no leakage, not even in the millions. These sound like the most likely, because they are what would cause the oscillator to malfunction.

T.

Thanks, Thomas - I did check the RF amp and Osc/Conv tubes, and replaced a paper bypass cap on the RF amp, but will check the other R's and C's and see if there's a leaking cap or bad resistor in there.
I also should have noted that the plate supply voltage decreases slightly as I tune from low end to high end of FM band (92 volts to about 88 volts).

-Craig S
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:If you get more stations by raising the line voltage, have you checked for a weak tube, drifted resistor, or leaky condenser? Disconnect resistors and condensers before testing them. Condensers should have no leakage, not even in the millions. These sound like the most likely, because they are what would cause the oscillator to malfunction.
:
:T.

What is the voltage supposed to be? Is this an AC/DC set or an AC set? If it is an AC set, 92 volts sounds low to begin with. If this is the case, you probably have a resistor in this circuit (B+) that has drifted high in value.

T.D.

:Thanks, Thomas - I did check the RF amp and Osc/Conv tubes, and replaced a paper bypass cap on the RF amp, but will check the other R's and C's and see if there's a leaking cap or bad resistor in there.
:I also should have noted that the plate supply voltage decreases slightly as I tune from low end to high end of FM band (92 volts to about 88 volts).
:
:-Craig S
:___________________________________________________
:
::If you get more stations by raising the line voltage, have you checked for a weak tube, drifted resistor, or leaky condenser? Disconnect resistors and condensers before testing them. Condensers should have no leakage, not even in the millions. These sound like the most likely, because they are what would cause the oscillator to malfunction.
::
::T.

I guess I can just answer my own questions by looking at the schematic. It's AC/DC, and 90 volts is the approximate voltage to be found at the RF plates, so 92 to 88 volts is alright. Still check things to be sure. If resistors are not more than 15% off, don't worry about them.

T.D.

I replaced four more paper caps last night - also a 470 ohm resistor that measured 880 ohms(!), but still have the problem - I'm getting into the tight quarters around the RF tube sockets (and time) so will plink away at it slowly.

:I guess I can just answer my own questions by looking at the schematic. It's AC/DC, and 90 volts is the approximate voltage to be found at the RF plates, so 92 to 88 volts is alright. Still check things to be sure. If resistors are not more than 15% off, don't worry about them.
:
:T.D.