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Mains hash problem

UK, 240V low power practise amp with 2 wire cable and no earth wire. Loud mains hash but finger touch the metal chassis and mains hash noise goes. To a lesser extent of noise reduction , touching guitar body( via guitar lead). 0V of 12-0-12 Tx secondary to chassis ground.

6 100nF, 25V disc ceramics between various points and chassis. Ground lines are all fine, input socket fine. Battery powered source , into aux input, is fine , no hash. Someone has been inside messing about, but nothing obviously missing. As found, the plastic barrel Jalco guitar input socket, does not have a local bond to ground. So i created one and cut the ground line from Prea to PS+PA , as that line is over 1 foot long, no change to mains hash. What is the function of heatsink insulator fibre reinforced matting between Tx and chassis under the 2 mounting flanges, held with 2 steel bolts and washers, damps acoustic resonanace/vibration? Other than replace the 2 core mains cable with 3 core ,any ideas ?
Reply to
N_Cook
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add a suitable safety rated Y cap.

Google Y cap.

M
Reply to
makolber

** Bad idea.

.... Phil

Reply to
Phil Allison

** The mains noise is being injected BECAUSE the chassis is not earthed.

Having no earth allows the chassis to float at about half the AC voltage - due to primary/secondary leakage capacitance in the AC tranny. Any guitar plugged into the amp will also be floating at the same voltage, about 120 VAC above ground.

Now, unless that guitar has *perfect* shielding on all its wiring and pickups ( they never do ) the high frequency part of the mains voltage wave will be injected direct into the wiring by any stray capacitance to ground - then amplified by 60 to 80dB.

Non earthed guitar amps are only noise free IF they are run from battery power.

.... Phil

Reply to
Phil Allison

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 pissing 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.

Reply to
N_Cook

** Not since the very early 70s have 2 pin *only* outlets plugs been common in the USA.

Prior to that, mains hum with guitar amp was considered *100%* NORMAL.

The use of polarity switches and DEATH caps was lethal and is now banned.

You stinking, lying retard.

** Really ?

Post a bloody schem.

.... Phil

Reply to
Phil Allison

One more observation. The mounting bracket under the Tx is made of mu-metal (place a small steel screw above and moving a magnet below has no effect). So that in conjunction with the gaffer tape must be something to do with minimising hum/hash, factory set on the torquing-up. So any "undesigned" stressing via messing with the mounting bolts or dropping the whole cab for instance, could destroy the mu-metal effect and lead to a noise increase I suppose.

Reply to
N_Cook

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