Fairbanks Morse motorboating - Printable Version

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Fairbanks Morse motorboating - 37silverstreak - 01-13-2013

I've been working on a 1936 Fairbanks Morse 72C2. I've done a complete recap as well as replacing all of the resisitors and some bad wires. I've also aligned the set and overall it's playing very well. the exception is when I reach the 800 to 900 Khz area. in this area the radio starts to motorboat loudly and the tuning eye pulses in brightness. Once I pass either side of this band the radio works normally again. It only does this on the broadcast band. Police and shortwave are unaffected. If I place my finger on the grid cap of the RF tube (6K7) the motorboating goes away and I can pull in a station in that area. Once I take my finger away the problem returns. I've tried switching tubes with no luck, anybody have any ideas? Thanks!
Kevin

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RE: Fairbanks Morse motorboating - Radioroslyn - 01-13-2013

Sounds like you need better bypassing of the RF stage. Double check the screen and plate bypass caps (maybe bad new ones) Also the lead dressing. Lead placement may be critical on this set. You can up the value of the bypass cap too to see if that helps. If your set uses metal tubes make sure that pin 1 is grounded at the socket.
GL
Terry

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