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Dynaco FM3 hum

68.108.173.65
Posted on November 26, 2023 at 12:23:19
ltman
 So it turns out after about 15-20 minutes a minor hum develops in my FM3. The big 40/40/20/20 can does get pretty warm, almost hot. I am thinking this is likely a contributor if not 100% cause of the hum.

Prior to the 15/20 minutes use, the FM3 is dead quiet, no hum or hiss.

I have a replacement on the way from Antique Audio. Due to aging and heat exposure I presume all the boards, traces, and solder joints are very fragile.

Two questions:

1: Any tricks to minimize causing new problems in the boards while R&Ring components?

2: Since I had the tuner powered up for a couple hours recently, I need to be concerned about the caps holding a charge after power was disconnected. Could I be sure they are discharged by switching the unit on with a load without being powered up? I do have a "snuffer stick" but I worry I could miss a cap and I am not very experienced reading schematics...


Any other tips to R&Ring components for this unit?

 

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RE: Dynaco FM3 hum, posted on November 26, 2023 at 12:37:59
fredtr
Audiophile

Posts: 2117
Location: Phoenix
Joined: January 4, 2005
Contributor
  Since: August 16, 2024
 Cap discharge, unplug it the night before you work on it.

 

RE: Dynaco FM3 hum, posted on November 26, 2023 at 15:05:02
ltman
 10-4, thanks!

 

RE: Dynaco FM3 hum, posted on November 26, 2023 at 15:29:54
fredtr
Audiophile

Posts: 2117
Location: Phoenix
Joined: January 4, 2005
Contributor
  Since: August 16, 2024
 You're welcome. Tube equipment is very forgiving. I'm guessing you don't have a variac. As an alternative, when you bring it back up, add a light socket in series with one of the ac lines going in. With a 100 watt bulb in series, the maximum the tuner will draw is less than an amp, low enough that you can observe, measure voltages if you have volt meter, before running it at full voltage. How intense the bulb glows will also tell you if you have a problem. If it looks like 100 watts, you have a short. It should be about 1/3 as bright as normal.

 

RE: Dynaco FM3 hum, posted on November 27, 2023 at 10:48:21
6bq5
Audiophile

Posts: 4592
Location: SF Bay
Joined: August 16, 2001
 The old electrolytic PS caps are notorious for going south!
if you want to keep the appearance, disconnect the can and place the individual caps under the chassis-
I have even seen detailed instructions for inserting them into the can so it looks 'original' -
Not worth it in my opinion, but...
Have fun!
Happy Listening

 

RE: Dynaco FM3 hum, posted on November 27, 2023 at 22:53:22
ltman
 that's helpful info, thanks.

I do have a variac and used it to slowly bring up the voltage for the first couple hours.

And a "load lamp" which I will use as testing proceeds. Aside from the big quad cap, I have a "restore" kit from the bay with a number of small caps and components.

I will not do the voltage tests described in the tuner manual though. Not going to mess with 400+ volts. I have to put a kid through college the next few years so I can't take the risk of doing something stupid yet...

 

: Dynaco FM3 hum - much reduced!, posted on November 28, 2023 at 17:22:46
ltman



for now anyway ;)

I had a replacement EZ80 (rectifier?), did some reading on the dynaco forum here, and learned that if that tube is going bad, it could cause hum.

Replaced it with a used telefunken from the bay, and much better now. Still a bit of hum but you have to really listen for it.

Next I will try resoldering existing ground connections and adding a few new
ground connections to all the boards.

Forum recommends and that will be much easier than replacing that quad cap. From what I read, the board tracings are extremely fragile. I could easily do more harm than good if I am not careful, and also lucky...

 

ESL has a power supply PCB, posted on November 28, 2023 at 19:00:23
Cougar
Audiophile

Posts: 4642
Location: SoCal
Joined: June 25, 2001
 for the capacitor upgrade/mod for the FM-3. I did buy and build in but never installed it due to the power supply section in my FM-3 was working fine. I did but some Film Caps from Erse before they stop production so I have all the value that are in the can but in film caps.

 

RE: ESL has a power supply PCB, posted on November 29, 2023 at 06:59:46
ltman
 ESL?

R&Ring a complete PCB seems like it would be a lot easier and likely to succeed than R&Ring individual components on those old boards.

 

Yes, the 6V4/EZ80 is the rectifier tube, posted on November 30, 2023 at 12:47:49
1973shovel
Audiophile

Posts: 10200
Location: Greenville SC
Joined: February 25, 2007



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."



 

RE: Yes, the 6V4/EZ80 is the rectifier tube, posted on December 2, 2023 at 12:07:51
ltman
 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.

 

Oh, sorry. You said Dyna-co, posted on December 13, 2023 at 09:49:04
M3 lover
Audiophile

Posts: 6687
Location: SW Mich
Joined: May 29, 2005
Contributor
  Since: July 4, 2007
 OK, silly response, but your Subject reminded me of this old Zappa number.

BTW, my first stereo included a FM-3, also Dyna Kit ST-70 and PAS-3. Along with AR speakers and Dual turntable I still have great memories of enjoyment with that system

"The secret of life is honesty and fair dealing, if you can fake that you've got it made." Groucho

 

RE: Yes, the 6V4/EZ80 is the rectifier tube, posted on December 29, 2023 at 20:28:36
ltman




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 :)











 

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