03-23-2025, 08:35 PM
Hi OnlyRick,
Yes, they are notorious for drifting. Several ways of dealing with them:
Resistors 1 -7, the 250 Ohm section of the volume control and the speaker field comprise the main "voltage divider" of the radio where all B voltages and bias for the output tubes are derived from. If any of these resistors open, use 5 Watt or better yet, 10 Watt resistors to repair the break in the divider.
If you have the newer paper spider type speaker, this is a nice sounding set. IMHO, the metal spider speaker is disappointing.
Re the 950 Ohm resistor, you can check by CAREFULLY measuring the various plate and screen voltages. If they are off, the problem is the resistor that all voltages "above" it are too high and all voltages below it are lower than they are supposed to be. I have measured the voltages on 3 Model 20s and on all 3, the detector plate is much higher than documented, bit all other voltages are close.
Although the Model 20 schematic is in our library, the Nostalgia Air schematic for the 20 has good pictorials and a list of voltages.
BTW, check out the schematic for the Model 3 car radioat the end of the NA document. Early car radios actually used "B" batteries like the first radios. Deluxe models had a motor-generator for HV. It was only around 1933 or so that the Mallory Vibrator type power supply was introduced, eliminating the B Battery. With 4 45V batteries, this set put out 0.7W. How does one hear this over the engine and exhaust noises of your Model A Ford Roadster? With the depression, this radio was more of a luxury than the car itself!
Keep us posted, and hope this helps. Good luck bringing this historically important radio back to health.
Yes, they are notorious for drifting. Several ways of dealing with them:
- If authenticity is not an issue, replace them with modern metal oxide resistors. Non "voltage divider" resistors can be 1/2 Watt. For an added safety margin, use 1 Watt resistors in cathode and plate circuits.
- If authenticity is an issue, search the Phorum for articles on reproducing dog bone resistors.
- You can use Ohm's law or simple math to determine the value of a resistor needed to bridge the resistor in question. This bridge resistor can be soldered to the underside of the dog bone and hidden. The issue here, is that the dog bone can continue to change value.
Resistors 1 -7, the 250 Ohm section of the volume control and the speaker field comprise the main "voltage divider" of the radio where all B voltages and bias for the output tubes are derived from. If any of these resistors open, use 5 Watt or better yet, 10 Watt resistors to repair the break in the divider.
If you have the newer paper spider type speaker, this is a nice sounding set. IMHO, the metal spider speaker is disappointing.
Re the 950 Ohm resistor, you can check by CAREFULLY measuring the various plate and screen voltages. If they are off, the problem is the resistor that all voltages "above" it are too high and all voltages below it are lower than they are supposed to be. I have measured the voltages on 3 Model 20s and on all 3, the detector plate is much higher than documented, bit all other voltages are close.
Although the Model 20 schematic is in our library, the Nostalgia Air schematic for the 20 has good pictorials and a list of voltages.
BTW, check out the schematic for the Model 3 car radioat the end of the NA document. Early car radios actually used "B" batteries like the first radios. Deluxe models had a motor-generator for HV. It was only around 1933 or so that the Mallory Vibrator type power supply was introduced, eliminating the B Battery. With 4 45V batteries, this set put out 0.7W. How does one hear this over the engine and exhaust noises of your Model A Ford Roadster? With the depression, this radio was more of a luxury than the car itself!
Keep us posted, and hope this helps. Good luck bringing this historically important radio back to health.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
"Let us begin to do good"- St. Francis
Best Regards,
MrFixr55
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