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 Post subject: Tough dog GE E-86
PostPosted: Feb Sun 27, 2011 10:46 pm 
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Here's the schematic.

I would say it's just a dog, but it actually worked quite well in a brief power up to confirm it was worth doing. There were only a few obvious repairs in the past, but they were about one rung above hacks.

I rigged up a substitution for the remainder of the candohm which had been only partially bypassed and replaced a lot of petrified rubber wire in addition to the paper and lytic caps and a number of resistors.

When it was next powered up, the audio was noticeably distorted. After just a moment of poking around, with a sharp shriek the audio disappeared entirely. The problem seemed to be in the first audio stage so, it wasn't long before I discovered that removing the cap connection from the 6F5 would partially restore the sound. The closer the connector was to the grid cap, the louder it was, so I put a capacitor between them and voila, audio was restored. But that's no fix.

When the grid cap is connected to the 6F5 it's almost as if that shorts it to ground, as it will not hum when touched. I just don't know where to start looking because the symptom is so perplexing.
Ideas?

:?:


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 Post subject:
PostPosted: Feb Mon 28, 2011 12:18 am 
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Hi David

Is cap C34, .02 mf, shorted? Is the grid connected through a shielded cable?

If C34 is shorted negative AVC voltage may cut off the 6F5 tube. If you have a shorted shielded cable won't have a signal. Believe C34 is shorted as the signal is there but wire may be DC negative?

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 Post subject:
PostPosted: Feb Mon 28, 2011 1:41 am 
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Hi Norm,

the wire to the grid cap is only about 2 1/2 inches long, the volume control is right behind the tube. It is not shielded and there's nothing for it to touch.

C34 was not shorted, but I found out that with C34's connection cut, the 6F5 still acts like it is shorted to ground with the grid cap connection on. Remove the connector and there's a loud hum when the top cap is touched, connect it and there's almost total silence when touched. Other than letting the radio signal through, there is no difference whether C34 is in or out of the circuit.


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 Post subject:
PostPosted: Feb Mon 28, 2011 1:47 am 
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Hi, David

Maybe there is a short in the volume control. Check the resistance from the grid to ground.

Here's a link to the schematic: http://www.nostalgiaair.org/PagesByMode ... 007786.pdf

I rebuilt one of those radios for a customer last year; it was a real performer when recapped and realigned.

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 Post subject:
PostPosted: Feb Mon 28, 2011 2:16 am 
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Did you get the radio playing pretty good before you replaced all those parts and did you check by plugging it in throughout your work steps ?


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 Post subject:
PostPosted: Feb Mon 28, 2011 2:32 am 
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Hi Tim,

No short in the volume control. In fact, it works just fine as does the tone control just as long as I put a capacitor between the connector and the grid cap of the 6F5. But that's a Band-aid.

I'm listening to it right now and it sounds great, but I can't call it done the way it is.


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 Post subject:
PostPosted: Feb Mon 28, 2011 1:07 pm 
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The bias for the 6F5 is developed across the 210 ohm resistor, which was part of the Candohm. I would check all of the wiring associated with your replacement of it.

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 Post subject:
PostPosted: Feb Mon 28, 2011 2:50 pm 
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Tim, you win the award! I immediately noticed two boneheaded math errors, corrected them and all is well. Thank you sir.


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