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Check and make sure you have plate voltage on the 6K7 magnetic tuning (AFC) amplifier tube. With no IF signal on its plate no AFC signal will be developed.
Check the voltage across each side of the AFC defeat switch to ground. As you tune through a station the DC voltage should swing from positive to negative or vice versa. If no voltage is seen there is a problem in the AFC amp or discriminator transformer.
Read the description of the Philco AFC circuit starting on page 93 of Rider's book. Although based on the 37-116 circuit it is practically identical to the 38-690. He goes into a lot of detail on how the AFC voltage is developed and applied to the 6N7G control tube to tune the oscillator frequency.
https://worldradiohistory.com/BOOKSHELF-...-Rider.pdf
Messages In This Thread
RE: 38-690 Code 125 - by Mondial - 01-02-2022, 07:59 PM
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