OK, Gotcha. Affirm. That's the story on some amps that run the feedback line out to the preamp section with balance controls on the user panel. You have to split the difference that a pot would provide at the center position. Different amps will have a different pot value. You get the idea.
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If you are building it to re-sell then just redo it according to the Magnavox schematic. If you want better performance and a really unique amp then build El-Cheapo.
Get the trafo rewound either back to stock specs for a stock amp or the way I recommended for El-Cheapo use.
Ok, I looked above where the amp sits in the original console....nothing dripped on the trafo from above - so I'm assuming it's oil from the trafo getting too hot. I am planning on rewinding for El Cheapo - but please be ready for ?'s. I "will" need help. Thanks in advance Eli, Jim, Dave & all others
@Jim, the link for Gary Brown is DOA....I'll search for him
@Jim, the link for Gary Brown is DOA....I'll search for him
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Since you are assuming the PT is toast, it gives you the 0 cost option of pulling the tubes and firing it up to do some A/C voltage checks at the rectifier socket and for heater voltage at one of the other tube sockets.
With the rectifier PULLED out of the amp, you do not need to have speakers hooked up as no DC voltage will be developed in the amp. Look at the partial schematic and place your VOM where the red lines are drawn, and you should see the ac voltages as indicated. Dont worry about the winding at the bottom of the transformer, it supplies the pre-amp when the turntable is being used.
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Look at the transformer diagram, you will see that there are two(2) 300 volt windings. actually its a 600 vac winding with a center tap. you should get 600 volts between the two red wires, and 300 volts between the red/yellow and the either red wires (check both). Remember with NO load the voltages will be a little high and thats OK.
Since line voltages are higher these days the B+ will be 10-15V higher than the diagram voltages. You expressed a concern that the 5U4 was bad, if you replace it you could or even should go with a 5R4GYB type which has a lower output than the 5U4, (higher voltage drop). It will help keep the B+ closer to the original values. The original 9302 B+ (325v) pushes the 6BQ5's into their high range from the start so it's a good idea to not let them get any higher if you can avoid it. Not a critical concern, just something to be aware of. If the 5U4 is still good, it's no biggy to keep using it to test and troubleshoot.
Bruce, that's very interesting. Do you happen to know the RMS current rating of that winding? If it's good for 20 mA. RMS or more, the winding could be used to energize the "El Cheapo" B- supply. Switching the B+ rectifier to a "potato masher" 5R4 would put the B+ rail voltage just about perfect for "El Cheapo". Stay safe and make the 1st filter cap. 15 μF.
The above remarks are made assuming the OEM power trafo is, after all. functional.
Bruce, thanks. For some, stupid, reason I thought that winding was "120" V. Still, it will be very useful for energizing the heater of a 7Y4 that will be part of the B- PSU.
Now I've got to find something of reasonable cost for the B- supply.
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