Dynaco FM-3

Click to enlarge

Upgraded and aligned by MPB 3-2007


This tuner was purchased used on ebay and modified and aligned in March 2007.  The basis for the modifications was primarily the James Lin article published in Glass Audio in 1991.  His recommendations include power supply changes, a number of capacitor and resistor substitutions, and correcting the equalization via the PEC modules.  Thanks to Charlie King for advice and Steve Marsh for tubes.

  Detailed references are shown at the bottom of this page.

Click photo to enlarge

Rear view without tubes, showing the mods, including: New PEC modules (3), new power supply PCB replacing original can capacitor, replacement of all audio coupling caps per the Lin article using polystyrene and polypropylene caps (C24-27, 29-31, and C71-83, note that the output couplers C82 and 83 were increased in value to 1mfd) .  Resistor replacement (again per Lin article) included  R2, 3, 6-8, 11, 13, 22, 25, 26, 74 and 82-84, all with tighter tolerances, and several with higher wattage ratings.  In addition, the captive power cord was replaced with a removable one, an F-connector was added for 75 ohm antenna input, and the RCAs were replaced with better quality ones.  All tubes were tested and 3 marginal or bad tubes were replaced.  Lastly, the unit was completely aligned.


Click photo to enlarge

Detail showing the new PEC modules for de-emphasis.  These were originally R-C networks in a single package, but had two problems: first, they were ceramic caps, and sounded bad, and second, the EQ networks were incorrect.  This was identified first by David Berning and published in  letter in Audio Amateur in 1988.  Several modifiers have noted dramatic improvement with this change alone.  These small PCBs and a third not shown here on the front circuit board were purchased from Chauncey at the FM3 Zone (see link below).  He sells many parts for the FM-3, including fully populated PEC modules, and power supply boards to replace the original can capacitor.  I purchased blank PCBs, as I wanted to use different parts than normally supplied - polystyrene caps (selected to 1%) on the PECs and larger value caps on the power supply filter board.  I left the capacitance of the first section at the stock value due to the tube rectifer, but doubled or tripled each of the following sections.  I also used 105degree caps due to the fact that the FM-3 runs very warm, hoping they'll be more reliable over the long term.


Stock unit before modifications, click photo to enlarge.


Top view of the completed unit, showing changes, click photo to enlarge.


References, for more information:

FM-3 Pictorial Diagram, with bottom wiring diagram and PCB layouts thanks to David Lee for this scan

Pictorial Diagram - part 2 - top wiring diagram, again thanks to David Lee

Full FM3 manual, including schematic and alignment instructions, courtesy of Joe Curcio.

Van Alstine Audio's article on Keeping the FM3 working

Charlie King's paean to the FM3

SDS Audio Labs, Sheldon Stokes site is another source of PEC and power supply PCBs.

Greg Dunn's FM3 infomation page

Yahoo FMTuners Group links for the FM-3 (you may need to join the group to view these)


The following articles are available as back issues from AudioXpress:


Improving Dyna's FM-3, article by James Lin, Glass Audio 1/91, pages 18-24.  This is the most comprehensive and best article on modifying the FM-3, and it includes most other suggested modifications discussed elsewhere.

Modifying the Dynaco FM-3, article by Benjamin L. Poehland, The Audio Amateur 4/86, pages 26-28.

More on FM-3, letter by David Berning, The Audio Amateur 2/88, page 52.  This letter first identified the problems with the PEC modules and includes a new PEC design that has been widely used to correct the PEC problems.


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