zenith: Low Voltage at Tube

I'm working on a restoration of a Zenith tabletop radio/turntable that I bought at the closing of an electronics store.  After doing a recap of the most obvious capacitors, I tested the voltages of the tubes according to the scematics and all of the voltages came in almost right on with two exceptions:

1. 6AQ6 - Pin 7 should be 48V and it came in at 30V.  I bought a new tube and the voltage measured 37V, an improvement, but clearly something still wrong.

2. I was not able to get any reading out of the Phono Pre-Amp (12J5) even after buying a replacement bulb.  Of course, I have the radio part of the device out of the shell and the turntable is not connected so that may have something to do with this.  Right or wrong?

3. I am unable to get a signal on the radio, just some static and a few spots where it perks up, but no meaningful sound.  I don't think that there's an antenna connected so would that be the reason for this or is there a problem elsewhere?

I'm looking for any direction to help with these items.

thanks you in advance!

craig.

 

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Hi Craig.

Chances are that if your voltages are incorrect, it is the result of a component whose value has changed.  Since you replaced all the capacitors, the low voltage on the plate of the 6AQ6 would probably be the result of resistor R16 or network U1 (your set has one or the other).  I wouldn't bother changing anything though, because if you have audio output, then the 6AQ6 is working fine.  Looking at the diagram for your set, I see that the voltages on the 12J5 would be dependent on whether switch S1 is in the Radio or Phono position.

Voltages can typically vary by 10% of what the voltage chart reads, so don't spend money on a new tube just because the voltage is not what the diagram states.

The fact that you have sound, even if it is just static, means you have made great progress!  You mention that there is no antenna connected, but the diagram shows a loop antenna.  The loop antenna needs to be connected to receive not because it picks up the signal, but because it is part of the tuned circuit.

Let me know how things work out.

-Howard

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Thanks for the feedback, Howard.  I didn't see the antenna until I looked in the cabinet and there it was.  So I put the chassis into the cabinet, plugged in the radio, and here come the radio stations!  It works, which is a big step, now for a couple of minor questions:

1. I only tune about 4 channels and, living in Chicago, there are probably 20 on the AM dial. I am doing all this work in my basement so perhaps that's the issue, but would like your opinion.

2. The volume knob needs to be turned all the way up to hear the sound well, but I wonder if there's something slightly off.  Could this be a resistor like you indicated in the previous message?

Overall, I'm pretty excited that it's starting to come together.  thanks for the tips so far!

craig.

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