Hello. I have a Model 60 run 7 that has been re-capped, re-resistored, and ant coil rewound. It has low sensitivity and is noisy below 650 KHz. Looking at the 6A7 oscillator waveform using a non-contact probe on an oscilloscope shows considerable distortion and and about a 5:1 attenuation of the signal as the radio is tuned towards the higher frequencies. A check with a spectrum analyzer shows multiple harmonics of the oscillator fundamental present. Is this distortion and attenuation normal? What might be the cause? It does not seem to affect the quality of the reception at the higher frequencies. I have tried two different "good" 6A7's with the same result. The radio works fine at frequencies above about 650 KHz. Thanks.
-Bill Kushner
Arl.Hts IL
Welcome to the Phorum!
Hi Bill and welcome!
I probably would not be too concerned abt the distortion but would but looking at the amplitude of the oscillator output. Does it drop off significantly from one end of the band to the other?? If so I have a look at the osc coil and rewind the feedback winding. I've had some that weren't open but were flaky.
Back in the days when is old guy was built an o scope was bleeding edge technology much less than one that have good response up to a mc. Point is there is no data on the distortion and such via Philco. As for the harmonics some of these sets use the second harmonic to give SW coverage w/o tapping the coil (don't necessarily mean the 60 but the model 81 comes to mind).
GL
Hi Terry,
The oscillator coil checked out ok. However, I found out that moving and separating the wires connecting the 6A7 to the oscillator coil, antenna coil, and band switch from each other as much as possible, I was able to eliminate almost all of the distortion and reduce the across-band attenuation considerably. Now the oscillator waveform is pretty clean and the amplitude only drops about 3 dB from 550 KHz to 1600 KHz. I guess there was some stray cross-coupling that was causing the distortion. The factory wiring layout of the stock Model 60 leaves something to be desired. The radio works great now. Thanks.
The distortion is important. The 6A7 is not moding properly and at the point where the distortion occurs, the oscillator frequency will go out of track all the way down the tuning dial. I've seen it several times and it came down to wrong voltages on the tube. For a quick solution, tack a resistor across each resistor in the 6A7 circuitry. At some point the addition of the resistor will make the problem worse or less. That's going to lead you to adjusting a resistor value up or down. Don't be afraid to make some changes in resistance values. Not all tube perform perfectly in any given application. Tweaking doesn't hurt.