High Plate Voltage on IF tube 39-116 - Printable Version

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+--- Thread: High Plate Voltage on IF tube 39-116 (/showthread.php?tid=10954)

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High Plate Voltage on IF tube 39-116 - jeffgniffke - 11-14-2014

In restoring a 39-116 with considerable rewiring. Injected AF a 2nd Detector/1st Audio produces a tone. From there back nothing...checking voltages I find the plate voltage of the IF tube at 260v (In looking at documentation for the 76 tube near its limit it looks like) with a schematic indicating 160v. I did rewire both the IF cans due to bad rubber wire....replaced the resistors in the 2nd IF can.
I'm still learning on this so any help is appreciated here.

Thanks.

Jeff

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RE: High Plate Voltage on IF tube 39-116 - Ron Ramirez - 11-14-2014

Hi Jeff

A look at the schematic indicates 160 volts is incorrect - it should be 260 volts. Notice on the schematic, the plate of the IF tube connects to the screen grids of the 42 output tubes through the primary of the 2nd IF transformer.

Try injecting a 455 kc signal through the control grid of the IF tube. If you get nothing, then use a 0.1 uF cap in series with your signal generator to block the DC from going into your generator, and inject 455 kc into the plate terminal of the IF tube. If you still have nothing, then there must be a problem with your 2nd IF transformer.

Don't forget to check continuity of the 2nd IF secondary (between the 51K resistor and the green wire).

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RE: High Plate Voltage on IF tube 39-116 - jeffgniffke - 11-14-2014

(11-14-2014, 08:49 PM)Ron Ramirez Wrote:  Hi Jeff

A look at the schematic indicates 160 volts is incorrect - it should be 260 volts. Notice on the schematic, the plate of the IF tube connects to the screen grids of the 42 output tubes through the primary of the 2nd IF transformer.

Try injecting a 455 kc signal through the control grid of the IF tube. If you get nothing, then use a 0.1 uF cap in series with your signal generator to block the DC from going into your generator, and inject 455 kc into the plate terminal of the IF tube. If you still have nothing, then there must be a problem with your 2nd IF transformer.

Don't forget to check continuity of the 2nd IF secondary (between the 51K resistor and the green wire).

Yes see it now Ron, the red wire out of the 2nd IF to the 42 tubes. Injected the 455 kc signal in in both methods as you had indicated....very distant faint crackling. I will pull the 2nd IF transformer and see what I have. I did get readings in tolerance (primary and secondary resistance) but something may have shifted in reattaching it.

Thank you Ron.


Jeff

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RE: High Plate Voltage on IF tube 39-116 - jeffgniffke - 11-16-2014

I pulled the 2nd IF transformer and in examining it it appears one of the solder joints of the new wiring was grounding to the exterior shield of the transformer. Readings are all good and I attached it with patch cords and now there is considerably more static etc.
What has anyone used to firm up the tiny coil wires? I have one that the wax has come off and it is separated into multiple tiny strands. I was thinking maybe nail polish?

Thanks for your assistance.

Jeff

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