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Dynaco FM-5 modification

wlgrant6

wlgrant6

AK Subscriber
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Just picked up a "long unused" Dynaco FM-5 Tuner in nice condition. Along with it came the original assembly manual (says it was factory assembled) and also the paperwork where a VSM AUDIO MPX-2 kit was installed in it. Anyone have any idea what this is all about? I googled it, and didn't find anything on it.
 
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In the old days mono tuners were sold with a decoder output which could be plugged into a separate MPX decoder when the owner wanted to upgrade. Tube examples of these can bring hundreds of dollars on the used market. I remember hearing about the VSM decoder somewhere and recorded this information:

VSM Audio, PO box 114 Maspeth, NY 11378

That's all I know. It is possible they offered a new decoder as an upgrade to the Dynaco. IC MPX decoders got a lot better after the FM-5 was designed.

Good luck with your search and let us know what you find out!
 
KiM3Ce's recollection is right on target. The FM-5 used the MC1307P multiplex decoder chip to extract the left and right channels from the composite signal. The VSM kit replaced the 1307P with an MC1310 phase-locked-loop multiplex decoder, reducing distortion on stereo signals and providing improved separation.
 
Thanks KiM3Ce, and Dr. Tinear for the info. It sounds pretty good, but since I have never heard another FM-5 that was not modified, I can't attest to it being an improvement. The downside is if I have any problems, it is no longer wired like the schematic.

I wonder if this was a common mod. Anyone else out there have an FM-5 with this update?
 
I too remember the mod, I think it was even published somewhere (Popular Electronics, or more likely, Audio Amateur). I think I even have an uninstalled kit somewhere in the dark reaches of storage.
 
I believe the same kit worked for both tuners. I considered buying one for my AF-6 when I was in college, but decided not to because I was satisfied with the stock circuit's sound quality.
 
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