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Old 7th Mar 2006, 6:25 pm   #1
Jim - G4MEZ
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Join Date: Feb 2006
Location: Newport Pagnell, Buckinghamshire, UK.
Posts: 85
Default Low hum on a Bush VHF64

This is nearly a success story but not quite..

I acquired a VHF64 as I really love them and mine takes pride of place in the lounge. It joins my fleet of two VHF81s (kitchen and bedroom) and an Ultra U930 Mistral in my workshop at work.

I have done a lot to the '64. All new decouplers in the IF stages, replaced THAT capacitor in the EL84 grid as it was leakier than a colander and a new smoothing block as the old one was bulging and someone had bodged some axial electrolytics across it. A new EM81 and a good clean and polish completes the basic job on it.

It now goes a treat except that there is a low hum present on the audio. It's not really bad but just enough to notice on speech and just enough to irritate me. It's not the HT as shorting the EL84 grid to earth makes it fully silent.

I have noticed that the hum level goes up an down with the tone controls. If one backs off both the Bass and Treble controls the hum practically disappears. Turn them both full-up and it is worse. I have also noticed that moving the bit of mains lead around that connects the Bass pot to the grid circuit of the EL84 makes the hum level change a bit, suggesting it's pick-up.

I've tried earthing the chassis to mains earth, changed the EABC80 and EL84 for known good ones and no difference.

It's irritating as my VHF81s are both as quiet as mice despite half-wave rectification and being simpler sets.

Is this a VHF64 trait or have I missed something? All thoughts much appreciated...

Jim
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