zenith: Issue with Special Temperature Compensating Resistor

I’m restoring a Zenith (ModelH72422) AM/FM radio and have run into a problem with a special temperature compensating resistor (R34) [supposed to measure 100 Ω hot and 750 Ω cold].  R34 is connected to pin 4 of the 6BJ6 tube and pin 3 of the 12BA6 tube and I’m getting readings of around 1.7 mΩ hot and around 167 Ω cold. The radio is receiving stations but at a very low volume.  I have no idea how to upgrade this resistor.  Can anyone help?

Thanks!

• Zenith Schematic (1792 KB)
• Zenith H723z2 Schematic (1792 KB)

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This resistor is just a dropping resistor for the heater chain. The reason for the original designer to choose a NTC resistor was probably to limit the inrush current when the set is switched on with cold heaters. I would suggest that you measure the current in the heater chain, if it is near 150 mA when the set is hot, you dont have to bother about that resistor.

The cause of low volume at both AM and FM would probably be in the AF amplifier, unless the B+ voltage for the whole set is grossly too low. Check that there is no positive voltage at the control grid of the output tube 35C5, if so, C29 has probably leakage.

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Also check all screen grid decoupling capacitors. Leakage reduces gain. RF, IF and AF.
All Anode to following grid capacitors. Leakage makes grids too positive.

They will test OK on a low voltage tester, but not at 100V, if paper dielectric.

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I think at least C6, C7, C9, C18, C20, C25, C26, C28 and C29.
1 mmF = 1 picoFarad

so
1000 mmF = 1nF
10,000 mmF = 10nF = 0.01 μF 

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Thank you!  I will incorporate your suggestions and let you know what happens!

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Thanks for your input!  I will check out your suggestions!

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