Hum problem....

[Chris]

Am not sure how it's happened..but I have managed to get a very loud hum, I guess it's some kind of earthing problem..
Tried to isolate the problem, removed the pre amp and fed the Wad 6550 directly from CD player as it has digital pre amp. Completely silent while the cd is playing no hum, soon as the Cd player is stopped the hum returns very loud..
Any ideas much appreciated..

Chris

[Richard]

Does it hum with no source or pre-amp connected at all?
(Preferably try this with shorting plugs fitted in the relevant RCA input sockets.)

[Chris]

Thanks Richard, will try with just the amp and speakers connected tomorrow.

Chris

[Chris]

Hi Richard

Fitted shorting plugs to both rca sockets, no noise or hum, each channel loud hum once shorting plugs are removed..

Chris

[John Caswell]

Hi Chris,
As the amp hums with the shorting phonos plugs removed from the inputs I would expect this some degree.
However if you have a DMM, or multimeter with the amp off check for continuity/resistance between the phono socket outer and chassis, it should read 0 ohms, or if you have an "earth lift" resistor, the value of that resistor.

John

[Richard]

Yes as John says, some amps will hum with open inputs anyway if they pick up RF noise. Shorting the inputs suggests the hum is getting into the amp as RF or as a line signal.


Work from there; is the hum one or both channels, plug in the pre amp and try the same test with the shorting plugs in the pre's relevant input, does the hum change?

[Chris]

Hi John the phono earth to chassis resistance is 10.2 ohms on both channels, Richard using the rca shorting there is no hum, but both channels hum loudly once rca shorting is removed.

Chris..

[John Caswell]

Silly question, have you got the bottom cover on? and is it earthed via a screw and lock washer between bottom cover and chassis?

John

[Chris]

John that is a very good question, I've lost a couple of screws and one of the threads on the chassis has dropped out...I will order some new screws, are these 3mm pan heads? also I will put a lock washer and remove paint if needed to get continuity to the chassis from the bottom cover.

Chris

[Chris]

Hi John

Just checked and there is continuity from earth post to bottom cover screw and bottom cover vent.

Chris