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 Post subject: GE P780B Background Audio Hiss
PostPosted: Dec Mon 20, 2021 12:00 am 
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Greetings.

I have a GE P780B with the audio hiss issue heard with the volume all the way down. I have read posts talking about final transistors getting replaced, but I wasn't sure what was used, and any bias changes made to work with newer silicon transistors. I'd like to get my radio fixed, as I like how it receives and sounds. Please point me in the direction so that I can repair this set.

This radio has been re-capped and works fine otherwise. I do not have a schematic that I can share with you, otherwise I would.

Thank you for your time!

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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Dec Mon 20, 2021 3:04 am 
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Early 780B with oval Sylvania NPN outputs are commonly out of spec causing distortion. I've seen more noise issue from the driver.

Can't say as I've had a issue with late B that use GE PNP outputs. You need to verify which you have.

Scroll to 780B transistor wiring.
viewtopic.php?f=4&t=405145

Audio schematic for early B...


Attachments:
p780xistortype.JPG
p780xistortype.JPG [ 295.49 KiB | Viewed 462 times ]

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Last edited by 35Z5 on Dec Mon 20, 2021 3:25 am, edited 2 times in total.
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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Dec Mon 20, 2021 3:11 am 
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Try putting a a .1 uf cap across the 100 ohm thermistor ( R28) to see if that helps thermistors have been known to be noisy .

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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Dec Mon 20, 2021 4:06 pm 
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I had to replace X5 audio driver for the same reason


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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Dec Mon 20, 2021 4:43 pm 
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JoelHelms wrote:
I had to replace X5 audio driver for the same reason

Many of the early ger xixtors are now failing, Norm Leal & I discussed this on a couple occasions. With his knowledge of ger properties & design, said "they'll all eventually fail".

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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Dec Mon 27, 2021 8:22 pm 
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I finally had a chance to work on this radio. I let it warm up so the hiss was there. I sprayed the audio transistors one at a time, an X6 is the issue. Noise goes away and then comes back as it warms again. There was no change when the other transistors were sprayed.

The NTE substitution guide says a NTE 103 is the recommended replacement. Does this sound right to you guy’s? Wouldn’t there be a bias change required?

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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Dec Mon 27, 2021 8:30 pm 
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https://www.alliedelec.com/product/nte- ... lsrc=3p.ds

Its Germanium like the original .

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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Dec Mon 27, 2021 9:35 pm 
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Thanks for pointing out the obvious to me. Sometimes I wonder where my heads at. I’ll pick one up and give it a try.

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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Jan Fri 14, 2022 6:05 pm 
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Just a quick update. I replaced X5 and X 6 with equivalent NTE germanium transistors, and the background noise is much improved. Now it doesn't get louder as the set plays for a while. When troubleshooting, I isolated the audio path between X6 and the output transistors and the background audio was quiet, indicating the output stage was not making any appreciable noise. I was happy about that.

Now there is a slight hiss with the volume all the way down, which I find acceptable. Not having much experience with these sets, I am assuming that a little noise is normal.

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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Jan Fri 14, 2022 10:19 pm 
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That is good new. I am in the process of replacing the same two transistors for noise as well.


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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Jan Sat 15, 2022 1:00 am 
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Tbone wrote:
Now there is a slight hiss with the volume all the way down, which I find acceptable. Not having much experience with these sets, I am assuming that a little noise is normal.

I'm going to say not normal, but seems to happen fairly regurarly. One of these days I'll chase the noise in my P-783A again.

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 Post subject: Re: GE P780B Background Audio Hiss
PostPosted: Jan Sat 15, 2022 12:20 pm 
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The noise I hear is definitely from the first two audio stages. I suppose it could be noisy carbon resistors or possibly the non-electrolytic cap.

The good thing is that you have to be close to the radio to hear the hiss, and it no longer overrides the stations at low volume. Now to decide if it’s worth digging in further.

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