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-   -   Low hum on a Bush VHF64 (https://www.vintage-radio.net/forum/showthread.php?t=7861)

Jim - G4MEZ 7th Mar 2006 6:25 pm

Low hum on a Bush VHF64
 
This is nearly a success story but not quite..:wall:

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 8-o 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. :shrug:

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

Jim

yestertech 8th Mar 2006 12:48 am

Re: Low hum on a Bush VHF64
 
Hi Jim,
I noticed this problem on certain VHF64/94 sets - I believe it's a symptom of high audio gain being used to compensate for lack of signal coming through the HF end - Assume you have the volume turned up quite a way on VHF when this occurs ?
If so, then replce the cheapy lump of mains cable to the bass pot with proper screened cable. This should cure ....
ALso, I believe there was a post a while back about certain of the aluminium chassis sections not being bolted together tightly. Try retightening all the screws which hold the panels together.
Also worth checking the main electrolytic earthing as I seem to recall there is no earth lead and it relies on tightness of the clamp ! A.

PS the '94 when in good condition looks stunning !

Stewart 8th Mar 2006 6:25 am

Re: Low hum on a Bush VHF64
 
I had this exact same problem on one of my VHF64s - and as Setappeal says it was due to chassis members not making good contact with each other. I found this out by connecting a probe to the negative of the main electrolytic and moving around the various points that should be "grounded". An resistance check failed to reveal the problem - but this method did! If I remember correctly it was one of the side chassis panels that was not making good contact - even though it looked clean and secure.

Once attended to the set runs alomost silently! Good luck!

Stewart

Jim - G4MEZ 8th Mar 2006 8:52 am

Re: Low hum on a Bush VHF64
 
Thanks both,

Excellent stuff! ;D

I never even thought about the chassis despite spending hours checking the same thing on a Sound City PA head a few weeks ago. I will go do and report back...

Jim

Jim - G4MEZ 11th Mar 2006 11:50 pm

Re: Low hum on a Bush VHF64
 
Quote:

Originally Posted by Stewart
I had this exact same problem on one of my VHF64s - and as Setappeal says it was due to chassis members not making good contact with each other. I found this out by connecting a probe to the negative of the main electrolytic and moving around the various points that should be "grounded". An resistance check failed to reveal the problem - but this method did! If I remember correctly it was one of the side chassis panels that was not making good contact - even though it looked clean and secure.

Once attended to the set runs alomost silently! Good luck!

Stewart

Bingo!!! 'Twas exactly what it was. I have only so far tightened those I could easily get to by removing the back and the difference is quite marked. I'm sure whan I have the chassis out and do it properly it will then be 100% good. Some of the screws were so loose to be no more than hand tight. Thanks Stewart and setsappeal :clap:

Jim


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