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Old 1st March 2018, 04:32 PM
bikerhifinut bikerhifinut is offline
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Join Date: Jan 2006
Location: Penrith, Cumbria
Posts: 1,089
Lightbulb Re: Phono3S it's humming again.

Quote:
Originally Posted by John Caswell View Post
Andy, with all inputs removed does it still hum? If so try putting phono plugs with 100R soldered across them on input sockets to see if it still hums. If so, you need to look at the phono stage, if not it is hum pickup in the cartridge/leads/arm.

John
Problem Solved John, as you pointed out there was no appreciable hum with the inputs disconnected and on mute (inputs shorted at the relays?) it was silent apart from a bit of valve noise.
I had the blessed thing apart this afternoon and everything metered fine, I reckon it did no harm as it had a good clean out in the process.
So back in the system. On the solid state commercial pre the hum was much reduced, this is a very quiet bit of kit and any extraneous noise shows up clearly. I moved the Phono vertically up whilst connected and when level with the TT the hum reduced to a very low level but still faintly in the background.
So I thought it has to be something like an unsheilded mains transformer transmitting a hum field.
I reconnected the homebrew valve pre amp (Pre3 casework with an Aikido ACF cathode follower buffer stage and Hi fi collective remote volume controller) and the hum was loud again. Moving the phono stage had much less effect this time.
Whatever it was had to be pretty close to the phono stage to do this.

Then the lightbulb moment occured. Theres a tiny plastic case with the Michell Gyro DC PSU in it and because the umbilical wire to the motor is a bit on the short side it sits behind the FM tuner on the second shelf of the equipment stand. It had somehow been disturbed and moved sideways no more than about 3 inches but that was all it needed, the hum field from the tiny transformer in it was enough to send the phono 3 into orbit, I suspect from the hanging loop of arm cable that dangles from the TT shelf and back up to the phono stage.

I had not thought to check that as I haven't moved the TT PSU during any other system alterations but somehow its been disturbed and that very small sideways and forwards shift was enough to couple its hum field into the arm leads.

I now have a silent phono stage again, all I get is valve hiss now.
And a practical eddication in how EM fields can propagate.

I am considering the practicalities of moving the Michell PSU into a metal diecast box to screen the innards. And of course when the snow melts and I get off the drive again, maplins in carlisle will be gone and they had a good selection of diecast boxes in store.

Thanks for all the offers of assistance and suggestions gents, it was a big help to me as I didnt fancy surgery to the relay board, it looks a right pig to remove.
When I redo the transformer pre in the big case i think I'll omit the WD relay board and make my own relay board up with very short pigtail leads from the RCA sockets on the back panel.

Andy

Last edited by bikerhifinut; 1st March 2018 at 05:37 PM.
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