Ground hum, Yamaha C4

[avionic]

Just checked. Don’t think I tapped it sufficiently hard that it touched the heat sink, although it is a possibility. Certainly, though, it was the cause of shutting the unit down and blowing the fuse in the previous photo.

I clean the old glue off of those heatsinks and glue them down with Permatex.

 

[avionic]

Reflow solder, change fuse and power on?

 

[Hifibythekg]

Thanks. Yes, on that one I’d cleaned the old glue of, re pasted the heat sink interface and glued down with a glue gun. Presume permatex is better though.
If it does power up ok, given its hum symptoms should I just try for a total recap of the main boards? Does the headphone socket behaviour suggest anything?

 

[avionic]

Headphone socket..
Headphones get no signal unless volume for speakers is turned up a little, at this point the volume pot for the headphones works fine.

This is normal..

 

[Hifibythekg]

Ok, well that’s good news.
Really, the unit sounds and operates so well apart from that hum.

 

[avionic]

Check and reflow these eight solder connections.. 5 of the 8 completed a ground circuit.

 

[Electone]

Just out of curiosity, have you tried reversing the AC plug in the socket? I know my C4 is just ever-so-slightly hummy when the AC cord is inserted one way than the other.

 

[Hifibythekg]

Thanks.
Seasons greetings everyone.
Will let you know how it goes.

 

[Hifibythekg]

Have resoldered points the as shown in post 26. Unfortunately it didn't change anything.
But we have a New Clue!!
With INPUT selecting Phono the following occurs:
When PHONO is set to Phono-3 MC the hum is equal on both channels.
When Phono 1, or Phono 2 is selected the hum is much stronger through the left speaker.
Does this allow one to determine the area of the amp where the hum fault is located?

 

[avionic]

https://audiokarma.org/forums/index.php?threads/yet-another-yamaha-c4-hum-problem.982732/

 

[Hifibythekg]

A to B on DMM beeps.
C to D on DMM reads 1200 and climbs to 1450 ish. Volume pot at zero.
If volume pot turned up about two thirds of the dial the DMM beeps.
Is this all ok or is there a problem?
Hum still present after much further cleaning, checking etc etc..

 

[Hifibythekg]

Should have added that this is on the board to which the volume pot is attached.
NA 07151-1
LC68730-1

 

[Hifibythekg]

Hum persists, especially on Phono.

Turning ‘pF’ adjustment has no effect on sound at all.. not even static or clicks, is this correct?

(However, ‘Cartridge Load’ switch adjustment does affect the sound.)

 

[Yamaha5000]

Do you realize you have have two completely different devices here?

 

[Hifibythekg]

Thanks, yes I do realise.
Also, the problem is now resolved and the hum is banished.
It was caused by a faulty transistor ( TR626 or 628) and covered by another thread I started around the same time.

 

[Peter S]

Just out of curiosity, have you tried reversing the AC plug in the socket? I know my C4 is just ever-so-slightly hummy when the AC cord is inserted one way than the other.

Hi Folks, I have a C-4 on the bench now. It hums regardless of the volume control position. Reversing the plug reduces the hum level. Connecting a ground wire from the C-4 to the power amp seems to 'pretty much' solve the problem. I measured 72 VAC from the chassis to the building ground, this reduced to 62 VAC by just lightly touching the chassis with my hand. Reversing the line plug reduced the chassis voltage to about 6 VAC.. I am assuming the power transformer is leaky and conclude that the only resolution would be to orient the plug for least hum and then connect a ground wire....Any thoughts?

 

[avionic]

I am assuming the power transformer is leaky and conclude that the only resolution would be to orient the plug for least hum and then connect a ground wire....Any thoughts?

Bingo..