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N_Cook

On 08/07/2016 20:10, wrote:

Other than replace the 2 core mains cable with 3 core ,any ideas ?

add a suitable safety rated Y cap.

Google Y cap.

M

It makes you wonder how they can use guitar amps in the USA, with just 2
pin plugs and that cap to chassis to offset ac leakage.
This is a Roland Cube 15X , so not worth any more ****ing about, other
than this exploration , before replacing the factory fitted 2 core
cable, with 3 core.
Touching chassis with 18 Meg and will reduce the hum, touching with 3
Gigohms and no noticeable drop in hum.
Ordinary DVM-V(ac) between good ground and 12-0-12 secondaries , measure
attenuated 13,17,21V of the floating 50Hz leakage.
Replacing the Tx bolts with nylon and retaining the original gaffer tape
insulation (not that heatsink cloth, the black stuff used for holding
back small wiring ) and touching the Tx ,then drops the hum to nearly
touching chassis level.
Placing 1/4 nylon spacers under as well as gaffer then drops to 18M
touch level to chassis.
Someone has obviously graunched up the 2 Tx bolts, so perhaps that was
precisely set torque plus threadlock for minimum "hum", or being PAT
tested, or old age has disturbed whatever factory fudge they found for
offetting the leakage for a particular batch of mains Txs.