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 Post subject: GE 860F Mixer Output
PostPosted: Jan Sat 07, 2017 11:49 pm 
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Joined: Nov Mon 01, 2010 1:14 am
Posts: 143
Location: Las Vegas Nevada 89131
Need some help. Losing my hair (and sleep) on this one. The collector of the mixer has a 6v peak to peak signal that tracks the oscillator frequency. I should see a much lower signal. Thinking the IF might be oscillating, I jumper-ed all the rf bypass capacitor with no improvement. No reason to suspect the mixer transistor, but I pulled and tested it anyway. Bias reading good on all transistor. If I tune to the high end of the band the signal goes away and I can pick up a radio station or two . Thanks for looking


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860_cut.jpg
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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Sun 08, 2017 4:44 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4382
Location: Indianapolis, IN
Hey Joel,

Did your IF's align well with 455 kHz ?

I worked on one of these a couple years ago: viewtopic.php?f=4&t=217887

Seem to remember that the oscillator output was fairly high amplitude-wise...

Hope that thread helps !

They are not fun to work with...

John


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Sun 08, 2017 6:01 am 
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Joined: Nov Mon 01, 2010 1:14 am
Posts: 143
Location: Las Vegas Nevada 89131
xrhonda91......I think I did see your post. I have been digging through the schematics of my radios to find one similar so I can do a comparison. The 455KHZ IF did peak out but the oscillator signal is so high at the first IF that it is making it way to the detector and creating havoc. The mixer feeds a IF transformer setup that different than what I am use to seeing. Maybe it will be higher.....

Thanks for looking.


P.S. Did you like the sound of your GE. Nice size speaker. I was hoping for a nice sound .......


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Sun 08, 2017 5:30 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4382
Location: Indianapolis, IN
Hello Joel,

The audio is pretty flat, but clear with plenty of usable volume when working correctly.

When my IF's were bad and I could only receive the stronger locals with very "broad" tuning, the sound quality was full & excellent (probably due to the wider bandwidth caused by the IF faults ?)...

How are the osc & mixer transistor voltages ?

Would it help if I pulled the cover on mine and scoped the oscillator output ?

John


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Sun 08, 2017 5:39 pm 
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Joined: Nov Mon 01, 2010 1:14 am
Posts: 143
Location: Las Vegas Nevada 89131
If you would put a scope on the collector of the mixer you would forever be my hero..... 1 million thanks


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Sun 08, 2017 5:50 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4382
Location: Indianapolis, IN
Getting ready to play in shop-room now---may be this evening before I get back on here.

John


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Sun 08, 2017 7:10 pm 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4382
Location: Indianapolis, IN
Hey Joel,

Wanted to do this first, so below are scope pics...

.1 V/DIVISION, 5 uS/DIVISION

Both the collector and emitter of mixer transistor readings are taken at both the bottom (550 kHz) and top (1600 kHz) of band with no signal present...

NOTE: I discovered that the Mixer E & C designations are REVERSED on the SAMS PCB trace side picture during troubleshooting back then and just noticed that I'd made correction on my printout.

Hope this helps !

John


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BOTTOM BAND MIX---EMITTER.jpg
BOTTOM BAND MIX---EMITTER.jpg [ 203.39 KiB | Viewed 397 times ]
TOP BAND MIX---COLLECTOR.jpg
TOP BAND MIX---COLLECTOR.jpg [ 226.32 KiB | Viewed 397 times ]
TOP BAND MIX---EMITTER.jpg
TOP BAND MIX---EMITTER.jpg [ 199.45 KiB | Viewed 397 times ]
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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Sun 08, 2017 11:01 pm 
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Joined: Nov Mon 01, 2010 1:14 am
Posts: 143
Location: Las Vegas Nevada 89131
most excellent. Just the info I needed. I want be able to take a look until tomorrow. I also noticed the error on the SAMs Photofact. Thanks for taking the time for the measurements and pics


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Mon 09, 2017 12:02 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4382
Location: Indianapolis, IN
You're welcome !

Good luck and please let us know what you find...

John


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Mon 09, 2017 8:20 am 
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Joined: Nov Mon 01, 2010 1:14 am
Posts: 143
Location: Las Vegas Nevada 89131
John your info was very helpful. As you can see my results are way off. As you can see I have a 6volt signal at the collector and ringing at the emitter Looks to be oscillating for some reason. maybe I will get some to time tomorrow. Thanks for the help


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scope_emitter.jpg
scope_emitter.jpg [ 123.89 KiB | Viewed 367 times ]
scope_collector.jpg
scope_collector.jpg [ 131.18 KiB | Viewed 367 times ]
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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Mon 09, 2017 9:03 pm 
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Joined: Mar Tue 03, 2009 11:12 pm
Posts: 1177
Location: Hutchinson KS
I'm sure you have checked this, but scope the electrolytics, it sure seems like one of them might be open in the power supply or/and agc line.


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Mon 09, 2017 10:56 pm 
Member

Joined: Nov Mon 01, 2010 1:14 am
Posts: 143
Location: Las Vegas Nevada 89131
Steve I replaced the capacitor on the agc line (first thing I did) and jumper-ed the filter caps as I don't have the correct replacement with no improvement. Clearly its oscillating
where it should not. I think you logic is correct. Maybe I missed one. Time to start all over again and check each one ..... thanks


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Tue 10, 2017 3:12 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4382
Location: Indianapolis, IN
Joel---I'm still wondering about the internal cap in 1st IFT. Just for curiousity, try jumpering the pins to which it is connected with a 100 pF or "tack" a 365 pF variable across it to see what effect it has when adjusted.

John


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Tue 10, 2017 7:29 am 
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Joined: Nov Mon 01, 2010 1:14 am
Posts: 143
Location: Las Vegas Nevada 89131
John , you and Steve seem to be one step ahead of me. I disconnected the resistor feeding the base of the mixer to see if the mixer was the culprit that was oscillating. Once the bias was removed I checked the collector again and the signal was still present. That makes be believe that it is the IF section that is oscillating. No time today, will try later in the week.

Thanks for the help


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 Post subject: Re: GE 860F Mixer Output
PostPosted: Jan Wed 11, 2017 2:35 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 4382
Location: Indianapolis, IN
Please remember to let us know what you find---am sure interested to know !

John


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