KiT 6550 Hum

[Mark]

Tim,

I had a similar problem with my KEL84 when I first built it, but not as bad as you appear to have it. IIRC I found that experimenting with the transformer mounting bolt tightness helped. Too tight or too loose caused a problem. Is the transformer tight or loose to the touch ? With the power lead removed (in the unlikely event that something shorts) can you move it ? It may be too tight or too loose. You might find that moving the transformer only a very little significantly reduces the vibration. It might have shifted slightly in transit. Try loosening the nuts slightly (with the mains lead out) and moving the transformer with the power on to see if there is a no, or reduced vibration sweet spot.

Also as Bob has already suggested you might want to experiment with the mountings. Again IIRC, my KEL84 was supplied with rubber grommets and white plastic sleeves that fitted in the grommets. I will check this evening. I think I found it was better with just the rubber grommets.

Finally I would like to underline what Bob has already said : there are high DC voltages inside which are lethal. Be careful !

Mark

[Richard]

Hi Tim,

It could also be the choke which has been a fairly common culprit over the years. Listen and check its mountings in a similar way to the mains TX.

Thin pieces of rubber strip (old bicycle inner tube would do) can be used to stop metal to metal contact.

If the main smoothing caps C1,2,3,4 look old/dusty I'd change them as a matter of course as they could be 20 years old and giving the choke a hard time with ripple.

[Mark]

Tim,

The gap between the Tx and the top plate of my KEL84 was too small to see what the mounting was so I took the bottom off. As I suspected it looks like a rubber grommet only mounting - see photo. Also note the nut is not central on the grommet indicating some room to wiggle the tx into a sweet spot.

It also occurred to me that the situation may be analogous to our office water cooler (!). When it stops dispensing water the solenoid valve slams shut and starts the thin metal case vibrating. This can be stopped with a gentle rap on the side with one knuckle. FWIW you might therefore try knocking your Tx on the front, back or sides gently with a wooden/rubber mallet before taking things to pieces.

If it's the choke as Richard suggests it could be, it might respond to the same treatment, but my KEL84 choke is rigidly bolted to the top plate, so yours may be too.

Mark

[TimN]

Hi Richard and Mark,

Thanks for your reply.

It does seem to be coming from the choke. If I put my fingers on the surface above it, I can feel a vibration. Also, if I press lightly above it, I can change the tone of the hum/buzz.

Looking at the Tx and the choke they both currently seem to be just bolted directly to the top cover without even any steel washers.

I think I'll try some rubber washers and/or gasket as damping/suspension. As you say, I may as well change C1,2,3 & 4 while I am there. Can you recommend a brand/type? Are Mundorf any good?

Thanks

Tim

[TimN]

Originally Posted by Mark

Tim,

The gap between the Tx and the top plate of my KEL84 was too small to see what the mounting was so I took the bottom off. As I suspected it looks like a rubber grommet only mounting - see photo. Also note the nut is not central on the grommet indicating some room to wiggle the tx into a sweet spot.

It also occurred to me that the situation may be analogous to our office water cooler (!). When it stops dispensing water the solenoid valve slams shut and starts the thin metal case vibrating. This can be stopped with a gentle rap on the side with one knuckle. FWIW you might therefore try knocking your Tx on the front, back or sides gently with a wooden/rubber mallet before taking things to pieces.

If it's the choke as Richard suggests it could be, it might respond to the same treatment, but my KEL84 choke is rigidly bolted to the top plate, so yours may be too.

Mark

Seems Mark was on the money straight away. Now identified as the transformer mounting. Cork gasket material and M5 rubber washers now ordered.

Thanks for your help, John.

What rating should the 47R resistors in the balancing circuit be, Mark?