How to repair the poor stereo performance for Dynaco FM3 - diyAudio
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Old 1st March 2010, 12:05 AM   #1
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Default How to repair the poor stereo performance for Dynaco FM3

Dear sir
My FM3 just working in the mono mode
I got the schematic of FM3 in the web,
but how to repair it ????
Thanks!!!
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Old 8th March 2010, 01:30 PM   #2
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Default FM3 obsolete

Not what you want to hear, but I gave up on my FM3 when I found a $10 pocket radio that worked and sounded better. The FM3 was okay for 1968, but that was a long time ago. Depower the tubes and transformers in the FM3, and use the beautiful case to install a carefully selected pocket radio with remote tuner pot for the varactor and line battery eliminator. The FM3 stereo section was always a black box. I haven't seen a labeled schematic on the internet.
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Old 8th March 2010, 01:52 PM   #3
Bill_P is offline Bill_P  United States
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My modified FM3 sounded wonderful - something must be wrong with yours if a $10 radio is better. The manual has a full schematic and alignment procedure so I don't understand the black box comment.

Olaychang, check all the tubes and replace those that are weak. Get a copy of the manual and do the alignment procedure. There were also many modifications in Audio Amateur and Glass Audio for the FM3. I recommend James Lin's article from Glass Audio 1/91.
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Old 8th March 2010, 02:45 PM   #4
Bill_P is offline Bill_P  United States
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Here are some links that may help with FM3 repair
AVA - Audio Basics - Keeping the FM3 Working
Tuner Information Center - Tube Tuners
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Old 8th March 2010, 10:18 PM   #5
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Hi Bill
Thanks for your infomation!
I have solved this problem, the R75 was open..but it looks well..
anyway, I'm very exciting now.
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Old 10th March 2010, 04:33 AM   #6
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Default radio repair unobtainium

Nothing like a good argument to bring out the help. Mr OlayChang went 10 days without help until I exhibited my ignorance. Thanks for listing the web resources available. Mr Van Alstein's page on radio repair is interesting, and resoldering joints and replacing bad caps is pretty basic, but the part about taking the 10.7 mhz coil to a radio shop for alignment with special tools is what I object to. There is no such thing in 2010, not here in the middle of the US. I could probably easier build a 10.7 mhz digital synthesizer out of Schottky TTL IC's and build a discrete squarewave-sinewave converter and signal multiplier, than find a set of FM alignment instruments. I built a 13 MHz synthesizer and sync detect circuit for a tester installed in the NASA JSC in 1981, all very simple except getting the test equipment to read the signal (burnt ground in the logic probe caused the clock frequency input of the analyzer to ring). My FM3 had poor sensitivity requiring a folded dipole antenna for local stations, and it would drift off as it heated up. I have a scope, but not being a "ham" I don't intend to buy FM alignment instruments.
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Last edited by indianajo; 10th March 2010 at 04:37 AM. Reason: get name right.
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