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Yes, the 6V4/EZ80 is the rectifier tube

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In Reply to: RE: : Dynaco FM3 hum - much reduced! posted by ltman on November 28, 2023 at 17:22:46

Glad that replacing the rectifier reduced your hum. I would also consider replacing the 'hot to the touch' quad (can) capacitor in the power supply, either with a direct replacement, or preferably I'd follow Cougar's advice and see if SDS Labs (link below) still has any of their FM-3 power supply boards for sale, although he no longer lists them.

There's also a site called Dynaco Designs offering the cap boards, here:

https://dynacodesigns.wordpress.com/fm-3-kits-and-parts/

Also, if you're soldering anyway, you might consider replacing R-11 and R-13 on PC-8 (both 10K @ 2 watt) with 5 watt equivalents. I believe it was the late Bill Thomas who said here on the Asylum:

"On the FM3, one resistor as-supplied is 2-watts, and runs at about 1.9999-> watts. Going to 5 watts is a very good thing."

SDS Labs (Open in New Window)

Follow Ups:

RE: Yes, the 6V4/EZ80 is the rectifier tube - ltman 20:28:36 12/29/23 (0)
just completed the RCA jack enhancement (new RCA jacks insulated from the chassis and grounded to the center ground connection of the 12AX7 tube socket) and that completely eliminated all remaining hum.

I did not really expect that fix to work, but it sure as heck did.

The tuner is dead quiet now, no hum at all even with my ear right up to the speakers. And no hiss either. I have to say it sounds fab. In stereo, even better than my Scott which is hissy in stereo, although great in mono...

So now I wonder if I should go through with replacing the 40/40/20/20 cap and the other caps provided in the restore kit I purchased on the bay.

I might do more harm than good continuing to screw around with the unit. All it takes is one ruined trace on these old boards and I will be out of business since I do not have the technical knowledge to troubleshoot safely and effectively. Especially not with 400+ DC volts!

Maybe I should pick up another FM3 and try the restore on a second unit.

But that would leave me with 7 tuners, which is hard to justify.

Maybe I can sell 6 of them to Rod M :)
RE: Yes, the 6V4/EZ80 is the rectifier tube - ltman 12:07:51 12/02/23 (0)
I have already on the way the quad cap, from Antique Electronics.
And I did buy an FM3 "rebuild kit" from a guy on the bay.

We'll see how it sounds after those replacement assuming I am successful with the component replacements.

I have read that the boards are super fragile at thei age now, and would not surprise me if develops into an issue for me.

After careful listening vs. my Sony ST-J75 the FM3 seems rolled off in the highs, which I have read is not unusual complaint for it, although the
FM3 bass was more "weighty" and the midrange more airy and realistic to my ears.

I did recently order a vacuum desolder gun, a small cheapo version, not the $350 HAKKO. So that will be an interesting test also.

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