Motorola 77FM - Printable Version

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Motorola 77FM - vecher - 12-16-2020

All, I have a bad Silicon rectifier.. will replace with a 1N5408.. Question is do I need any additional resistance after the diode?

Here is schematic

[Image: https://live.staticflickr.com/65535/50726990263_9130e25bf7_c.jpg]


Thanks

Skip

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RE: Motorola 77FM - Phlogiston - 12-16-2020

I take it that you were replacing a selenium rectifier? ( I see that you stated silicon, which is what your replacement will be?). I would see what the voltage is after the last filter, then add resistance if necessary.

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RE: Motorola 77FM - Chas - 12-16-2020

There is 27 ohms in place now to limit the charging rate of the first filter cap.

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RE: Motorola 77FM - RodB - 12-16-2020

The original rectifier is a selenium. If you replace it with a silicon rectifier you should increase R33 by about 100 ohms or add a resistor. You will want to check the voltage chart for the 12AT7 plate voltage and adjust the resistor value accordingly. If someone already replaced the rectifier with a silicon then put your new one in and check if a resistor was added. FM front end amps are usually sensitive to variations in B+.

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