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 Post subject: Magnavox AM-81 help and/or schematic needed
PostPosted: Feb Wed 24, 2010 7:18 pm 
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Joined: Feb Wed 24, 2010 7:09 pm
Posts: 12
Location: Corinth, MS
I'm trying to repair my wife's childhood radio. It will work with the volume pot about halfway. Anything above or below halfway and the audio get very distorted and disappears. The diode detector output has a DC bias that varies with the position of the volume pot. That doesn't seem right. I have replaced the electrolytic capacitors with no change. Any ideas or a schematic scanned from Sams #TSM-58 would be appreciated.


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 Post subject:
PostPosted: Feb Thu 25, 2010 6:24 am 
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Joined: Oct Mon 26, 2009 10:02 pm
Posts: 3018
Location: Indianapolis, IN
Hello,

Had nearly identical problem with a 1959 Delco "Auto-Portable"
transistor radio. Turned out to be a leaky audio coupling cap between wiper of volume control to input of 1st audio amp...
Symptoms were as you described...

Are you sure you got all the electrolytics ???

Welcome & good luck---let us know what you find.

John


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 Post subject:
PostPosted: Feb Thu 25, 2010 8:09 pm 
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Joined: Feb Wed 24, 2010 7:09 pm
Posts: 12
Location: Corinth, MS
Hi John,

Thanks for the reply. I can see where a leaky cap there would let the dc bias of the audio driver show up on the volume pot. Good idea!

I went back and checked the electrolytics again. I have replaced 3, the coupling cap you mentioned, the emitter bypass of the driver, and another one around the i.f. area. There is one more large one in the push-pull audio output that I have not changed. The driver is transformer coupled to the push-pull output, so I doubt it is the problem.

I lifted the coupling cap between the pot wiper and the driver stage. The dc bias was still on the detector diode and varied with the volume pot. The dc voltage peaks about mid range on the volume pot, still. Looking at the audio with a scope, at the detector stage, it still only works at mid-range on the volume pot.

I'm wondering if the little radio has agc, and perhaps there is a problem there? I'm kinda lost without a schematic at this point.

Thanks again...I'll post any updates if I get it working.

Fred - WB5CON


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 Post subject:
PostPosted: Feb Thu 25, 2010 11:16 pm 
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Joined: May Fri 19, 2006 8:24 pm
Posts: 426
Location: Keizer, Or
Fred, check your email. A PDF scan of the service info in TSM58 for your radio has been sent.


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 Post subject:
PostPosted: Feb Fri 26, 2010 2:11 am 
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Joined: Feb Wed 24, 2010 7:09 pm
Posts: 12
Location: Corinth, MS
Dave, thank you, thank you, thank you! It is printing out as I type. A beautiful, clear scan!

I'll put it to good use in the workshop tomorrow and post the results.

Fred - WB5CON


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 Post subject:
PostPosted: Feb Fri 26, 2010 8:37 pm 
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Joined: Feb Wed 24, 2010 7:09 pm
Posts: 12
Location: Corinth, MS
The little radio is now working! However, this was a weird one. The DC bias I mentioned in the first post is actually a floating ground. It should be 5.8 volts, but it varied between 0 and 4 volts, depending on the position of the volume pot. 4 volts if the pot is in the center and 0 volts if the pot is to either extreme. The wiper of the volume pot is connected to the mounting shaft. The pot shaft is insulated from the grounded mounting bracket with a shoulder washer. In this radio, the shaft was shorted to the mounting bracket because the shoulder washer was not seated properly and off center! The pot was dragging the floating ground down to 0 volts except when the pot was centered.

I don't believe this radio ever worked properly from the factory. There is no evidence of the radio ever being apart. The radio is like new. The leather case is like new. The earphone has never been used.

I think my wife got this as a present, found it to work, but not well, and put it away. At mid pot setting, it would pick up a local station with low volume. Now it has plenty of clear audio and good sensitivity.

I will touch up the alignment and put it back in its case.

Thanks for the help guys! I'm sure glad I found this forum. Maybe I can return the favor.

Fred - WB5CON


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 Post subject:
PostPosted: Feb Fri 26, 2010 9:29 pm 
Member

Joined: Oct Mon 26, 2009 10:02 pm
Posts: 3018
Location: Indianapolis, IN
Hello Fred,

That really is a bizarre fault !!! And, something to keep in mind...

Glad you got it going !

John


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