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 Post subject: GE P865B AM/FM common faults.
PostPosted: Jan Tue 08, 2019 2:32 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 96
Location: Marquette, Michigan, USA
Hey guys, I have an old GE P865B set that only plays (a little ) on AM. Looking for tips on common faults and how to get it working better. I've cleaned the contacts on the band switch, and fixed a cold solder joint on the tuning capacitor, also changed a bad coupling capacitor in the IF chain, and thought it was solved. Any help is much appreciated.


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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Tue 08, 2019 5:25 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
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Location: Annapolis, MD
can you post a link to the schematic?

A brief standard checklist:
--check all coils and transformers for continuity**.
--replace electrolytic capacitors--especially power supply filters
--clean and check(measure the DC resistance) all switches
--check voltages

EDIT: Is this it?
https://www.radiomuseum.org/r/general_el_p_865b.html

they have the schematic for the A version, but not the B

**
an admittedly ambiguous term---to me, it means "verify a rational DC resistance" If the schematic does not tell you what to expect, then look at similar parts or ask someone.

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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Tue 08, 2019 8:20 pm 
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If it's playing weak/poorly on both AM & FM, likely it has bad coupling caps in audio section. Usually value(s) around 6 to 8 mfd.

AFAIK 865A & B are basically same radio.

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Tom


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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Wed 09, 2019 12:12 am 
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Location: Mountains of Mourne. Ireland.
Bob, shoot me your e-mail address via PM (private message) and I will send you the service manual.

Greg.


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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Wed 09, 2019 9:08 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 96
Location: Marquette, Michigan, USA
Greg, I have a Sams Photofact for the P865A model, so don’t know if I need another, can’t believe there were that many changes to the B model.
I have changed the coupling caps, and all but the big electrolytic, it is better on AM, still no FM, but the problem may be the big cap. Putting 9 volts in and only getting 5.3 volts down stream of the 150 ohm dropping resistor, so the big filter is likely bad and pulling the voltage down. I’m building up a replacement for it using two smaller caps in parallel, so we’ll see how that goes. For only having a tad over 5 volts on the audio section, this thing sounds remarkably good and gets LOUD.

Thanks everyone, I’ll keep ya posted.

Bob


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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Thu 10, 2019 2:48 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 5738
Location: Indianapolis, IN
Hope the cap replacement fixes it, Bob !

Watching for your report...

John


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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Thu 10, 2019 9:42 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Marquette, Michigan, USA
John, and all, still tinkering with the GE, I was going to pull the big filter cap and replace it , disconnected the positive lead, and checked it with my ESR tester, it measured good. I jumpered in my replacement and it made no difference, so I just reconnected the original one. I finally did a current draw test, the specs say 24 ma at idle, it’s pulling 84 ma !
The voltage at the output stage is nearly the same as supply volts, so I think the output stage is fine. Where it’s loosing it’s steam is in the preamp and driver stages, that take their juice post filter and dropping resistor, the first 150 ohm one. Whatever is going on is draining the electrons headed elsewhere. I might have narrowed it to the preamp stage, the voltages on X8 are messed up. Collector and Base read nearly the same. There is a 100k resistor between them and ohms checking it measures 35k one direction. Reverse the meter leads and it measures 63k. ?????
I suppose the collector base junction is altering that reading, but haven’t pulled the transistor to verify, as I really don’t want to pull the chassis. All the work I have done is top of the board work, so it is harder to troubleshoot that way.
I just cleaned up,the case and fixed the broken handle, it’s staring at me on the shelf, not sure where I go from here with it.

Bob.


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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Thu 10, 2019 9:56 pm 
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Finding point of excessive current draw is a good start, in fact was major part of early transistor troubleshooting. Almost always indicates a leaky/shorted transistor.

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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Thu 10, 2019 11:12 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
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Location: Indianapolis, IN
Bob,

If the audio is of good quality & loud and the AM works somewhat normally, it's possibly a transistor or other component in FM section is causing fault since it's dead. Have you felt each transistor to see if any are warm to touch ? At current you mentioned, that would be possible. Does the current draw change at all when switching bands with volume at minimum ? Have you checked any of the transistor voltages in the FM circuits ? Still a lot you can do from the topside...

Don't give up too easily as that is a great radio and well worth the effort for repair !

John


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 Post subject: Re: GE P865B AM/FM common faults.
PostPosted: Jan Thu 10, 2019 11:54 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 96
Location: Marquette, Michigan, USA
Guys,

Thanks for the hints and encouragement. I haven’t really sorted out any of the FM section, thinking it was the audio chain and it’s high current parts that were the culprit. Someone had been inside it before I got it, one of the transistors had been changed, X1,the am/fm RF amp. It had been done from the top also. I didn’t consider that as a possible culprit ( duh ! ! )
Can’t figure the voltage issue with the af amp ( X8 ) emitter voltage is supposed to be 7 volts, it it’s only getting about 5.7 it has 5.5 on the base, 3.7 on the collector. It’s like the transistor is acting like a resistor. I wouldn’t think the audio chain would work if X8 was dead, or that leaky ? Maybe.... We’ll sometimes I wish I had a dual channel O-scope so I could see what is happening in a circuit. I had a friend who was a slick tech, only used a scope, he could find a fault so easily it made your head spin. Just freakin BRILLIANT ! Anyway, it’s a hobby, so mucking about is part of the fun. Thanks again guys. I’ll give her another look tomorrow.

Bob.


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