General Radio 1214-A Unit Oscillator

I'm working on a General Radio 1214-A Unit Oscillator. 

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The oscillator is fixed tuned but can be switched between 400 Hz and 1000 Hz.

schematic

It's a cute Hartley LC audio oscillator using a single 117N7GT as the rectifier for the transformer-less power supply and as the pentode for the oscillator circuit. The output is taken from a secondary winding on the oscillator coil, so it's isolated from the AC power line. 

A "Thyrite" varistor limits the amplitude of the oscillation. The oscillator is only spec'd at 3% distortion, so the varistor clipping is acceptable.

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The output level is controlled by one of General Radio's high quality wire-wound potentiometers. It's a beauty. 

The original 117N7GT had a burnt out filament, so I procured a new old stock replacement tube from a vendor at the MIT Flea.

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I ran into an odd problem while testing the 1214-A. As I advance the potentiometer, the output level increases smoothly up to about 20 volts, and then jumps about a factor of 3 up to about 60 volts. I get similar behavior when decreasing the signal level. I managed to catch the transition in the scope photo above. At maximum output, I get almost 100 volts, while GR says it should only be about 60 volts.

The load was just the 10x scope probe, the 1 MΩ input impedance of my HP 3468A DMM, and the 10 MΩ input impedance of my newly-restored HP 400C AC VTVM, so it was only a bit under 850 KΩ. I can't see that such a high output load would change impedance of the output level potentiometer enough to affect the behavior of the oscillator.

I cleaned the potentiometer with some DeoxIT D100 applied directly to the point of contact between the wiper and the windings. (It just occurred to me that I should also try cleaning the rotating contact to the wiper.) I exercised the control a good bit to try to clean the contact surfaces, but it didn't eliminate the high-amplitude oscillation mode.

I'm stumped as to the cause and therefore the cure of this problem. Anyone have any ideas?

Update:

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I opened it up and disconnected one end of the potentiometer. Fortunately the connection was a solder lug on a nut and bolt terminal. 

The potentiometer measured the correct resistance, so the winding was intact. I hooked up my Simpson 260 to watch the resistance as I varied the potentiometer. This is one of those situations where an analog meter beats a digital one handily. The resistance changed erratically as I rotated the pot. I cleaned the wiper and the winding again and added a little more DeoxIT D100, but the resistance change was still erratic. 

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I finally remembered to add some D100 to the rotating contact on the wiper shaft. That fixed everything right up. It turns out the maximum open circuit output of my GR 1214-A is about 99.5 volts, much higher than the guaranteed 60 volts. The odd coincidence was that the dirty potentiometer contact would gave a maximum output of about 60 volts when the wiper was on the maximum resistance stop, but would usually give a higher output when turned lower and the wiper was on the windings. So I was fooled into thinking that the output was the spec'd 60 volts when the pot was out of circuit, and was somehow tripping into a higher output mode when the potentiometer was in-circuit.

The higher output voltage is handy, because I need a 100 volt AC source to calibrate the HP 400C that I'm restoring.

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Here's a top-view of the circuitry. The two Good All film capacitors form C1 which is part of the L-C tuned circuit.

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Here's the bottom view. The disk just above the Aerovox mica capacitor is the Thyrite varistor. Both sides of the incoming AC line are switched and fused. That's thoughtful since the GR 1214-A uses a transformerless power supply.

© Steve Byan 2011-2016