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FM-3 hum is back and question about impedances.

68.104.11.135
Posted on March 9, 2024 at 08:20:43
ltman
 60hz hum is back. Installing the "RCA ground upgrade" eliminated audible hum along with replacing the rectifier tube. But only for a while; the hum has returned. I do play the tuner 5-10 hours per day...

The hum does not change amplitude when volume pot is rotated. And no change regardless of signal level input.

However, it only becomes loud enough to be audible about 20 minutes after power up. I am guessing it's the power supply quad cap.

I have a brand new replacement 40/40/20/20 . That will be next to-do I guess. Although I expect the dern boards are very fragile and getting that sucker out without damage to the board will be a challenge.

Has to be expected to replace that can after 50+ years, and the tuner does sound wonderful otherwise...

QUESTION: I now have the FOSI V3 amp in the system. It has a "pass-through" line out which can be used while the amp is playing normally. I have connected this pass-through to another Class D amp in another room.

Would this arrangement put any electrical strain on the output of the FM-3? Maybe lowering impedance the tuner sees?

 

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I have the FM3...., posted on April 24, 2024 at 07:45:41
Cougar
Audiophile

Posts: 4634
Location: SoCal
Joined: June 25, 2001
 I did the PEC upgrade and coupling caps with a few resistors that were in the signal path and out of tolerance. I built the newer power supply cap board for the replacement of the quad cap but haven't done it yet as the the voltages are still in tolerance and no humming so far.

Have you cleaned or replaced the turn on switch? Sometime when a switch has carbon build up it can cause humming in the signal path like you describe.

 

RE: FM-3 hum is back and question about impedances., posted on April 24, 2024 at 08:13:42
ltman
 Switch, yeah that's an interesting idea. Hadn't thought of that. I did give it a deoxit bath when I first received the tuner. That's SOP for me for all vintage equipment. And as is well known with the FM3, corroded and/or poor ground connections with any component or connection in the FM3 can induce hum.

I ended up working on the tuning ganged capacitor due to obvious corrosion and unpleasant noises when tuning- removing, cleaning out corrosion thoroghly with "no residue" cleaner, relubing the bearings with that dern expensive silver-bearing conductive grease, removing corrosion from the mounts to the chassis, and resoldering the tuning cap's grounding tabs.

That fixed the hum.

I bought components and perf board to build the pec mods but have been occupied recently with restoring some reel-to-reel decks...

 

RE: FM-3 hum is back and question about impedances., posted on May 18, 2024 at 17:51:56
tomservo
Manufacturer

Posts: 8615
Joined: July 4, 2002
 jumping in mid thread the 40/40/20/20 sounds like a tube equipment?.
If you want a quick fix, get two modern 40 or 50mf electrolytic caps with the same Voltage rating and solder these right onto the lugs of the existing cap and ground (watch polarity!). Use some heat shrink if needed but "back then" neat wiring was normally "enough insulation".

Oh the issue is the old electrolytic caps tended to dry out and the MFD goes down and the die-electric liquid goes away.

 

RE: FM-3 hum is back and question about impedances., posted on July 11, 2024 at 21:16:44
ltman
 I did order a replacement quad cap and will replace it in the fall. too hot here now in Vegas to use any tube equipment.

I did the "rca jack" grounding upgrade and that helped to reduce the hum.

Also I resoldered the ground shield that covers the variable tuning capacitor and regreased the bearings with that dern expensive silver bearing conductive grease.

Oh and I replace the somewhat corroded front panel screws and that must have helped the grounding also because tuning is much less noisy.

The FM-3 sounds better than my modded Sonys that's for sure although the sonys sound very good. The FM-3 is just in another league...

The hum is gone now even though I have not yet replaced the quad cap, but I will do so eventually...

 

RE: I have the FM3...., posted on July 12, 2024 at 10:21:14
ltman
 Cleaned? that's my specialty. I am a certified DeOxit techncian :)
And I do have a NOS replacement but probably will keep on the shelf for now.

I have been experimenting with the FM-3 alignment on a different FM-3 I have that sounds somewhat distorted.

Got 1/2 way through the alignment steps in the manual but the instructions for what to look for in some steps don't match what I was able to see. Will probably start over and try again...

 

RE: FM-3 hum is back and question about impedances., posted on July 16, 2024 at 16:19:34
6bq5
Audiophile

Posts: 4559
Location: SF Bay
Joined: August 16, 2001
 When you did the PS upgrade did you add a 3 pronged mains plug?
Happy Listening

 

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