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Philco 41-295 Output Tranformer - Printable Version

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Philco 41-295 Output Tranformer - TFB - 04-28-2010

I am working on a 41-295 that has a bad output/audio transformer. One leg of primary coil is open. It appears this unit has a tap on the secondary (not a CT) for an inverse feedback circuit. Must I find an exact replacment or is there a method to utilize a "standard/universal" replacement transformer and still achieve inverse feedback?

Part number on the schematic that I have is 32-7981, actual part number on the unit appears to be 32-9508.


Thanks,
TFB

Re: Philco 41-295 Output Tranformer - dberman51 - 04-28-2010

Usually negative feedback is taken from the top of the output transformer secondary. I don't see why you couldn't do that here, and use a resistive voltage divider to get the feedback level right.

-David

Re: Philco 41-295 Output Tranformer - TFB - 04-29-2010

David,

Thanks for the suggestion. Makes good sense to me.

TFB

Re: Philco 41-295 Output Tranformer - dberman51 - 04-29-2010

You'll find that the secondary will be sensitive to polarity, and unfortunately the markings on the replacement transformer won't help you determine which terminal to use for the ground side and which for the high side. One way will give you negative feedback and work, the other will give you positive feedback and bad oscillation. I would first set up the voltage divider for a tiny amount of feedback (maybe 5%) and then test to see which connection reduces the volume of the amplifier.

-David

Re: Philco 41-295 Output Tranformer - Ron Ramirez - 04-29-2010

A friend of mine has reported success by using a 100:1 resistor divider for feedback.

He connected one end of a 100 ohm resistor to the high side of the audio output transformer secondary. The other end was connected to the low side of the volume control. A 1 ohm resistor is added between the low side of the volume control and ground.

I have not tried this yet, but he reports excellent results.

Re: Philco 41-295 Output Tranformer - TFB - 05-04-2010

Thanks for the input. I believe that both of you gents are providing the same advice. For this unit, the tap on the secondary of the output transformer goes the low end of the volume control pot and also the wiper on the tone pot. The bottom of the output secondary is ground.

Have good day.