Amp "hums", disappears when touching case

[Mike W.]

Dear Community,

after successfully restoring a SX450 now i have a SA 610 amp on my desk.

It makes a "hum" sound (volume dependent) on both channels, but when touching the case or some of the pots/switches the hum became almost unaudible.

I checked the connection cable between main PCB and case, but all OK. The capacitors look OK (no deformation or cracks). The DC offset is -25 mV on both channels, maybe a bit too high...? Bias on test points ist 20 mV, that is OK according to the service manual. Pots and switches are all fine (no crackling noise).

So I hope someone could give me a hint where to start...

Thank you in advance
Mike.

 

[Watthour]

It may be worthwhile to check all chassis ground connections and C62.

 

[elvi]

Can you test with all other wall powered accessories disconnected?

Headphones, no speakers?

( chasing external ground loops, vs internal loops, per Watthour's nails on suggestion )

 

[larryderouin]

POWER OFF, CORD UNPLUGGED. Ohm out between board ground connection on board and the chassis, next to that ground point. Anything more than 0.5 to 2 ohms can cause a ground loop. If so, and it's riveted to the chassis, drill out the chassis, wirebrush a 1" area around the hole on the chassis AND the lead connector. add a little DeOxit SHIELD to the lead connector, and bolt and nut it with an internal lock washer or with blind rivets (if you should be so lucky). Check Ohms again! Continue on until you get them all. If the connection is good, you can (with a big soldering IRON) add solder to the mechanical joint.

 

[Mike W.]

Hi All,

thank you for your help so far. I substituted the ground cable (steps as described by larryderouin), resistance from chassis to PCB is <0.5 Ohms.

Regarding the C62 capacitor: This one is missing (factory side). Should I solder one in?

No other devices connected. The hum is also audible via headphone. And it's still there...

Greetings
Mike

 

[markthefixer]

Turn the plug around in the wall socket. Check hum level. The plugs are MADE to be able to do that.

 

[Mike W.]

OK, the hum is gone now! The cause? I just reassembled the bottom steel plate. Hum disappeared!
The amp is working well now, I like the fluoro VU meter

Grettings
Mike