Our "Second Dose" of "Fine Tuning" - A TOTAL Dynaco FM-3 reconstruction!

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Bill Thomas

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     I think you're onto something, but first...  "It's Philosophy Time!"

     When attempting to evaluate a potential "improvement" in a unit's performance, you need to run a "baseline test" on the unit, but you NEED to make sure the unit is operating as well as it can be.  (Of course, one could argue that a *true* "baseline test" would include running a test prior to alignment, but we have some "prior knowledge" about the FM-3's alignment.)

     Throughout the entire Monaural section of the tuner, adjustments are fairly broad.  (It is important to be as accurate as possible with the Discriminator alignment, but otherwise, things are pretty simple.)

     When you get to the multiplex board, the adjustment to T-71 and T-72 is broad and well-pronounced.  T-73 is a bit of a bear.  EXTREMELY small movement of the slugs can make a BIG difference in the Stereo Separation and noise rejection.

     So, before you go any further, I would STRONGLY recommend doing an alignment of the Multiplex board.  I think you will find the separation will improve - even with the strip antenna.

     As for your research, I think you are DEFINITELY on the right track.  The earlier FM-3 that I modified for 50 microsecond response didn't seem to suffer from the same issues.  Sure, it was "bright" when listening to US stations' 75 microsecond pre-emphasis, but it was no more pronounced than listening to a dolby B encoded tape with the dolby decoder off.  (That's probably the closest I can "describe" a sound quality in print.)

     By the way, I wouldn't bother aligning the rest of the tuner at this point, but if your Multiplex section is WAAYYY off, I'd consider doing at least a Discriminator alignment before aligning the Multiplex board.

     Either way, it *does* appear that the 100 pF capacitor should *probably* be reduced to 68 pF for proper performance.  Next time I do an FM-3, I'll try to do a comparison, but in theory, it looks like you're right on this.

     I LOVE to learn!

Sincerely,

Bill Thomas

handsome

thnaks for the reply bill.

I did try to realign the mulltiplex board but using a multimeter as described in the application note on replacing the eye tube with an EM84. There the instruction ask one to adjust T72 & T71 until one gets a as high a voltage on TP75 as possible. I started at 14V and ended off with 30V....but it didn't make a huge difference. Some fiddling with T73 made a very small difference. I will return to that. I will also try replacing the 6BL8. By the way my stereo indicator closes fully.

I am beginning to suspect the pull out switch as well. Looking at the schematic surely if one engages the mono switch the PECs are completely bypassed? Then shouldnt their contribution (brightness) be minimised? I briefly measured the resistance of the switch (in circuit) and it appeared a bit high perhaps this is the issue?

I cant say that there was a huge change from the 100pF to the 68pF but i feel more comfortable with it. Still it would be great if someone could work out a PEC layout that matched the 50uS time constant AND suppressed 38kHz.

I have a problem aligning the discriminator; where i live there are no mono stations! Any suggestions as to how i can proceed?

Thanks for the great assistance!
regards
johnny

Jampot

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Quote
The earlier FM-3 that I modified for 50 microsecond response didn't seem to suffer from the same issues.  Sure, it was "bright" when listening to US stations' 75 microsecond pre-emphasis,

No sign of 'brightness' here in UK Bill, sound's great :thumb:

Jim

guzeev

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Hi, I'm looking for files of Dinaco FM-3 circuit boards(Gerber files and drill file) for my project of restoring this receiver. Who can share the link?. Or buy ready-made :D
Thank you.

dB Cooper

The FM3 dates to decades before there was anything like Gerber files, so these probably don't exist.

If you are trying to maintain and/or restore an FM3, I strongly recommend checking out the following resources:

"Keeping the Dyna FM3 Working" By Frank van Alstine (he has a forum here). A MUST-read for anyone trying to, well, keep the Dyna FM-3 working. Article starts at the bottom of Page 4. There is another version with suggestions from a reader for additional "fixes". Not in the linked file or on the website as far as I can tell but I can email you a copy if you PM me.

Link for upgrade parts (EQ modules, power supply etc)

Another great source for parts ranging from substitute PEC de-emphasis modules and an LED pilot light that reduces the load on the power supply all the way up to a full CAD-designed replacement motherboard that replaces almost all point-to-point wiring.

I also have a copy of the Dynaco service bulletin detailing how to substitute the EM84 tuning-eye tube if anyone needs it.

If you really want everything you can get from the stock circuit, you might want to consider pulling the boards and replacing the ancient carbon-comp resistors with modern film types. They have probably all "cooked" out oof spec and there are several places where Dyna originally used tighter-tolerance parts (some in pairs) for performance reasons. If you don't mind spending a little time with a multimeter, you can match these parts much tighter than Dyna was willing to pay for. (Due to the cramped layout, many of these parts are hard to get to with the boards installed.)

guzeev

  • Jr. Member
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Thank you, this is very useful information!

dB Cooper

One other thing, when I had mine, a little research turned up 'mil-spec' versions of many of the tubes, mainly in the RF and IF sections and the MPX switching tube. I lacked test equipment to validate possible performance improvements, but they worked well and didn't cost much more than the 'street' tubes you find.