Motorola 77FM.. - Printable Version

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+--- Thread: Motorola 77FM.. (/showthread.php?tid=21993)

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Motorola 77FM.. - vecher - 01-02-2021

Started to work on this.. All recapped . Getting lots of stations on AM.. But when switch to FM all I get is a Hum. Hum gets loader with volume..

Where do I start? Have not worked on FM circuits in a long time.. Even if I inject a FM signal I still get nothing..

Skip

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RE: Motorola 77FM.. - RodB - 01-03-2021

You can start with the 2 tubes that are used in handling FM, 12AT7 FM converter and 19T8 Detector. If they are OK then S2 the AM/FM switch switches B+ to the 12AT7 converter. You should see plate voltages on that tube when in FM. Measure the voltages and compare to voltage chart. You should see a negative voltage at the FM discriminator circuit when feeding a 4.3 MHz carrier in the FM section. The voltage should vary with signal strength. That should keep you busy for a short while. GL.

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RE: Motorola 77FM.. - vecher - 01-03-2021

Ok Replaced the 12AT7 and the 12BE6.. FM started to work many stations..

So not sure which one fixed it.. and really do not care.. But both tubes were tested in tube tester and showed good..

Just goes to show.. the real test in in circuit..

Skip

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RE: Motorola 77FM.. - RodB - 01-03-2021

The 12BE6 is used as the mixer for AM so it's probably the 12AT7. Yes, at that frequency a tube tester is a marginal instrument.

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RE: Motorola 77FM.. - David - 01-03-2021

These  tubes hate me 12AT7, 12DT8, 19T8, 19V8,  always good to have a few extra! David

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