11-18-2025, 11:48 PM
Hi Dan,
Neat radio. Shame that only 1 of the 45s is a globe tube, the other is the "ST" style. This is a relatively high end TRF (Tuned Radio Frequency) set.
The volume control on this set is quite interesting. It is at the very beginning of the circuit and consists of 2 functions.
The first function is to "shunt" the antenna to ground at minimum setting. The resistance element is connected to the antenna and the wiper is connected to ground. This is a common method of regulating volume in sets before AVC (Automatic Volume Control). The second function is to bias the RF amps. The lower the volume, the more the bias. However, Zenith did not vary the cathode bias to regulate volume. It seems that they vary the resistance to one leg of a capacitor whose other leg connects to an RF coil in the plate circuit. I assume that the closer that one lead of the capacitor is to ground, the more filtering of RF from the plate circuit of the first RF (and conversely the lower the signal to the grid circuit of the succeeding RF amp and detector).
Servicing the set should be rather easy. Verify that the coils are not shorted or open. Verify both sections of the volume control. Replace the caps, and you should be good to go.
Because this is a Non-AVC set, there is some strangeness to volume. First off, if the volume control is set rather high for a distant station, and you tune to a strong local station, the RF amp may become saturated, and you may stop hearing stations. Just turn the volume to minimum for a few seconds and slowly raise volume. Some stations may be so strong that they will be picked up by the wiring between the tuning cap and the grids of the RF amp tubes. Make sure that all shields are in place.
Hiope this helps.
Neat radio. Shame that only 1 of the 45s is a globe tube, the other is the "ST" style. This is a relatively high end TRF (Tuned Radio Frequency) set.
The volume control on this set is quite interesting. It is at the very beginning of the circuit and consists of 2 functions.
The first function is to "shunt" the antenna to ground at minimum setting. The resistance element is connected to the antenna and the wiper is connected to ground. This is a common method of regulating volume in sets before AVC (Automatic Volume Control). The second function is to bias the RF amps. The lower the volume, the more the bias. However, Zenith did not vary the cathode bias to regulate volume. It seems that they vary the resistance to one leg of a capacitor whose other leg connects to an RF coil in the plate circuit. I assume that the closer that one lead of the capacitor is to ground, the more filtering of RF from the plate circuit of the first RF (and conversely the lower the signal to the grid circuit of the succeeding RF amp and detector).
Servicing the set should be rather easy. Verify that the coils are not shorted or open. Verify both sections of the volume control. Replace the caps, and you should be good to go.
Because this is a Non-AVC set, there is some strangeness to volume. First off, if the volume control is set rather high for a distant station, and you tune to a strong local station, the RF amp may become saturated, and you may stop hearing stations. Just turn the volume to minimum for a few seconds and slowly raise volume. Some stations may be so strong that they will be picked up by the wiring between the tuning cap and the grids of the RF amp tubes. Make sure that all shields are in place.
Hiope this helps.
"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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