05-13-2021, 04:50 PM
The dual section volume control on the model 20 and 70's work like this. The 5K section shorts the rf signal to ground coming down the antenna as the control is turned downed down. The 250 ohm section controls the gain of the rf amp stages by lowering the cathode resistance to ground. This increase the gain. The trick is that if you just use the 250 ohm pot to vary the gain you can't get it low enough so you can't hear any stations. That's why the 5k pot section is used as a backup to killoff the signal.
Folks that sell replacement volumes controls for these sets what they consist of is a a fixed resistor and a 250 ohm pot. The 5k section isn't particularly important in terms of the set working or not. If it was shorted at the low end that could be a problem.
I reread your question and I think the answer is that you can't measure the 5k pot in circuit as the ant coil is in parallel with it. It's a much lower resistance.
Folks that sell replacement volumes controls for these sets what they consist of is a a fixed resistor and a 250 ohm pot. The 5k section isn't particularly important in terms of the set working or not. If it was shorted at the low end that could be a problem.
I reread your question and I think the answer is that you can't measure the 5k pot in circuit as the ant coil is in parallel with it. It's a much lower resistance.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got it!
Terry
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