I can turn on this set and listen to it for between 2 and 3 minutes before with a "pop" the audio gets very weak and there is a steady crackle of static under the low audio.
Since I believe this to be a bias problem, or something closely related, I took voltage measurements before the problem starts at all candohm points and got the following numbers:
257, 81, 1.2, 0, -19.4
Then after the "pop" I get these numbers:
345, 109, .04,0, -10.2
Also, the voltage differential between the screen and plate on the 42 output tube disappears. With the radio working I get these numbers:
42 Plate 257 VDC
42 Screen 239 VDC
With the radio not working I get these numbers:
42 Plate 345 VDC
42 Screen 345 VDC
I tried swapping out the speaker and audio transformer, 75 tube and 42 tube but no luck.
Can anyone point me in the right direction?
David
Since voltage goes up when the radio gets weak something opens. I would expect the wire wound resistor. May be section between 109 and .04 volt reading?
Notice your B+ numbers increase, 345 and 109 volts but the next resistor voltage goes down in value, .04 volts. This could indicate the 5700 ohm section opens?
Norm
I have a Zenith 805 that has been restored with all new caps. A few resistors have been replaced as well for being out of spec.
:
:I can turn on this set and listen to it for between 2 and 3 minutes before with a "pop" the audio gets very weak and there is a steady crackle of static under the low audio.
:
:
:Since I believe this to be a bias problem, or something closely related, I took voltage measurements before the problem starts at all candohm points and got the following numbers:
:
:257, 81, 1.2, 0, -19.4
:
:Then after the "pop" I get these numbers:
:
:345, 109, .04,0, -10.2
:
:Also, the voltage differential between the screen and plate on the 42 output tube disappears. With the radio working I get these numbers:
:
:42 Plate 257 VDC
:42 Screen 239 VDC
:
:With the radio not working I get these numbers:
:
:42 Plate 345 VDC
:42 Screen 345 VDC
:
:I tried swapping out the speaker and audio transformer, 75 tube and 42 tube but no luck.
:
:Can anyone point me in the right direction?
:
:David
:
:
:
:
:I can turn on this set and listen to it for between 2 and 3 minutes before with a "pop" the audio gets very weak and there is a steady crackle of static under the low audio.
:
:
:Since I believe this to be a bias problem, or something closely related, I took voltage measurements before the problem starts at all candohm points and got the following numbers:
:
:257, 81, 1.2, 0, -19.4
:
:Then after the "pop" I get these numbers:
:
:345, 109, .04,0, -10.2
:
:Also, the voltage differential between the screen and plate on the 42 output tube disappears. With the radio working I get these numbers:
:
:42 Plate 257 VDC
:42 Screen 239 VDC
:
:With the radio not working I get these numbers:
:
:42 Plate 345 VDC
:42 Screen 345 VDC
:
:I tried swapping out the speaker and audio transformer, 75 tube and 42 tube but no luck.
:
:Can anyone point me in the right direction?
:
:David
It would appear that maybe the output tube stops drawing current after the "pop". Even tho the cathode is grounded directly to chassis, look at it and the grid. If the grid opens between it and the candohm resistor, the tube will eventually cut off from the electrons buntching up on it and having no way to get to ground (chassis). Keeping an eye on the grid Voltage (with a hi-Z meter) might show some change that will lead you in the right direction. Resolder all the tube socket connections, and the candohm joints. Let us know, we are here to help.
Lewis
P. S. I've learned how to put play-prettys in my posts like Edd and Doug do, so here,
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I'm driving myself batty looking for this problem. I'm almost thinking it might be a wire somewhere with a break inside the insulation. Zenith used that solid core wire and after being twisted, bent, pulled-on, stripped and resoldered, there could be a break I don't see.
David
What happens to grid #1 on the 42 tube when plate voltage goes up? Seems like a lot of plate voltage. Unless grid #1 goes very negative would expect the 42 tube to conduct and bring this voltage down.
Still strange that voltage goes up on one end of the 5700 ohm resistor and down on the other. Did you disconnect the original candohm?
Norm
:I have replaced all sections of the candohm with 10 Watt wire-wound, sand-block resistors. I have resoldered these connections several times, as well as the connections on the 42 socket.
:
:I'm driving myself batty looking for this problem. I'm almost thinking it might be a wire somewhere with a break inside the insulation. Zenith used that solid core wire and after being twisted, bent, pulled-on, stripped and resoldered, there could be a break I don't see.
:
:David
This problem was indeed a bad 42 tube. I located another and it fixed the problem entirely. Thanks again for everyone's help!
David
One can make an adapter using the 6-pin base from a defective tube and an octal 8-pin socket minus its mounting hardware and use a 6F6G/GT. Can do the same for the 41 tube using a 6K6G/GT
Radiodoc
**************
:I always hate it when someone abandons a thread when they find a solution. Anyway, thanks to all who sent me suggestions.
:
:This problem was indeed a bad 42 tube. I located another and it fixed the problem entirely. Thanks again for everyone's help!
:
:David