01-07-2022, 10:44 AM
It is very unusual that you have 10V of IF frequency signal on the plate pins of the 6H6, yet no DC rectified voltage at the cathodes. It is also unusual that the 85B secondary trimmer makes absolutely no difference in the DC output.
The only things I can think of is that either the 6H6 is bad or there is a open section of secondary winding in the discriminator transformer. First, as you mentioned you can try wiring up your spare tube base with a couple of silicon schottky or high speed switching diodes (1N4148, etc) and see if you get any output.
The other possibility is an open section of the secondary winding. If you look carefully at the secondary schematic, you can see that the resonating trimmer cap 85B is connected to the ends of the winding, while the 6H6 plate connections are tapped down toward the center. So you could still have signal on the plates, yet if one of the winding ends was open 85B would be disconnected and have no effect on tuning for resonance. Unfortunately, the only way to check would be to remove the transformer and check the full winding for continuity.
As far as the lack of AVC voltage, the voltages on the 6J5 AVC rectifier seem about right, and consistent with the ones of the similar circuit of the 37-690. Because the cathode is biased +11V and anode (grid) at -2.8V, it will take a IF peak voltage of 14V or more to generate any negative AVC voltage over the standing -2.8V bias. The 14V of IF signal seems like a lot, but with two IF amp stages, plus the gain of the RF stage and mixer, it should easily be attainable with a strong signal. If you can't generate that 14V of IF, it could be due to a weak tube in any of the previous stages, or other problem that reduces the overall gain of the receiver. Keep in mind that this radio has a lot of reserve gain when operating properly, so with reduced gain the radio will still receive fine, yet fail to generate AVC voltage. You can try exchanging one of the other 6J5 tubes in the radio with the AVC rectifier 6J5 and see if it makes any difference. There is also a separate independent AVC amp and rectifier circuit for the RF stage, so it would be interesting to see if that circuit is generating any AVC voltage.
The only things I can think of is that either the 6H6 is bad or there is a open section of secondary winding in the discriminator transformer. First, as you mentioned you can try wiring up your spare tube base with a couple of silicon schottky or high speed switching diodes (1N4148, etc) and see if you get any output.
The other possibility is an open section of the secondary winding. If you look carefully at the secondary schematic, you can see that the resonating trimmer cap 85B is connected to the ends of the winding, while the 6H6 plate connections are tapped down toward the center. So you could still have signal on the plates, yet if one of the winding ends was open 85B would be disconnected and have no effect on tuning for resonance. Unfortunately, the only way to check would be to remove the transformer and check the full winding for continuity.
As far as the lack of AVC voltage, the voltages on the 6J5 AVC rectifier seem about right, and consistent with the ones of the similar circuit of the 37-690. Because the cathode is biased +11V and anode (grid) at -2.8V, it will take a IF peak voltage of 14V or more to generate any negative AVC voltage over the standing -2.8V bias. The 14V of IF signal seems like a lot, but with two IF amp stages, plus the gain of the RF stage and mixer, it should easily be attainable with a strong signal. If you can't generate that 14V of IF, it could be due to a weak tube in any of the previous stages, or other problem that reduces the overall gain of the receiver. Keep in mind that this radio has a lot of reserve gain when operating properly, so with reduced gain the radio will still receive fine, yet fail to generate AVC voltage. You can try exchanging one of the other 6J5 tubes in the radio with the AVC rectifier 6J5 and see if it makes any difference. There is also a separate independent AVC amp and rectifier circuit for the RF stage, so it would be interesting to see if that circuit is generating any AVC voltage.
