Philco 49-906 Code 121 FM Oscillator Trimming Capacitor - Printable Version

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Philco 49-906 Code 121 FM Oscillator Trimming Capacitor - jrblasde - 07-04-2024

Good afternoon, folks! Has anyone had experience replacing the FM oscillator trimming capacitor (C400C, part of the C400 assembly; Philco part number 31-2724-2) on the 49-906 model radio? I've got a set which I have otherwise fully restored, but I am struggling to find any documentation regarding the capacitance of this trimmer. This is a hollow barrel of metal with a screw that can be driven further into or out of the barrel. Normally the barrel attaches to the body of the air core tuning capacitor (C400), but part of it has broken away. Without this outer barrel being fixed in place, I am noticing that the tuning drifts over time, likely due to thermal expansion of the remaining metal as the radio warms up. For reference, refer to the attached FM alignment chart from the 1948-1949 service manual.

Any suggestions on sizing of a replacement trimmer?

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RE: Philco 49-906 Code 121 FM Oscillator Trimming Capacitor - morzh - 07-04-2024

I do not know exact value, but I think 0-50pf range is good.

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RE: Philco 49-906 Code 121 FM Oscillator Trimming Capacitor - jrblasde - 07-04-2024

Thank you, that sounds pretty typical of FM receivers of the era. I was reading yesterday about a similar replacement on a different radio (I think someone was restoring an RCA), and the general consensus was in that same range. I'll give it a try, and adjust based on the results!

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RE: Philco 49-906 Code 121 FM Oscillator Trimming Capacitor - Arran - 07-16-2024

Given that it is high frequency the value of the trimmer cap will likely be quite small.
Regards
Arran

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RE: Philco 49-906 Code 121 FM Oscillator Trimming Capacitor - jrblasde - 08-07-2024

Just to update everyone, now that my other issue with the IF transformer has been resolved, I was able to test the 50 pF trimmer capacitor out for this FM oscillator circuit. Adjustment is a bit touchy, but it does work fine. 

Joseph

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