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Which tuner to get and getting the most from it. Thank God, for the radio!

RE: 10.7 mHz Discriminator Transformer Repaired!

As a final update on this issue, my unit although working, had weak audio and very poor swing of the center of channel tuning meter. RF and IF tubes had been replaced with new tubes and realignment done. I researched technical information on the Foster-Seeley Discriminator circuit and paid particular attention to any mention of the coupling capacitor between the plate of the limiter stage and the secondary of the discriminator transformer. Discussions refer to the capacitor as C8. Nowhere was a value for the capacitor stated. I had used a 33pFd, which was similar to the value used across the secondary of the transformer. The only thing mentioned about the capacitor was that its reactance was low at the operating frequency. That 33pFd value calculates to about 450 ohms at 10.7mHz. I had a 560pFd mica capacitor on hand and tacked it across the coupling capacitor. Immediately the audio strength improved and the tuning meter had much greater deflection when tuning across a station. A 560pFd has a reactance of about 26 ohms at 10.7mHz. I finally settled on using a 1000pFd capacitor which has a reactance of about 15 ohms at 10.7mHz. The value of the capacitor had no effect on the tuning of the primary or secondary of the transformer.


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  • RE: 10.7 mHz Discriminator Transformer Repaired! - strijw426 07:02:24 12/16/12 (0)

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