KiT 6550 Hum

[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?

[Richard]

Originally Posted by TimN

What rating should the 47R resistors in the balancing circuit be

3W wirewound would be fine

[Mark]

Tim,

I have a KEL84. The comments/suggestions that follow assume WD/WAD used the same TX mounting system as the KAT 6550, but I may be wrong.

If you have grommets you might like to try and get them to work properly. The advantage of the rubber grommets over washers is that there is always rubber between the bolt thread and the metal edge of the bolt hole so the metal thread will never contact the metal case. The other advantage is that they can be "tuned" by adjusting the tightness of the nuts. The nuts may simply have been over-tightened, which increases the likelihood of vibration transmission from TX to case. Or as John has suggested they may not be tight enough ! Start with them finger-tight or less and gradually tighten until the vibration starts, then back them off a little.

You should also try slowly/gently moving the transformer from side-to-side to make sure all the bolts are in the centres of the grommets and that one or more bolts haven't squashed the rubber hard up against the sharp edge of the bolt holes and increased the likelihood of vibration transmission.

IIRC my KEL84 kit came with some plastic shoulder washers which centralised the bolts in the grommet but I may have taken them out as they made the whole spring/damper TX mounting system too stiff. If you have those in your KAT 6550 try it without them.

Mark

[TimN]

Thanks Mark. I see your point regarding the grommets. I have some on order.

All that was there when I examined it were some hard plastic washers. They may have had shoulders at some points but they are now squashed out of recognisable shape.

I think there is enough length on the screws for plenty of isolation in the form of gaskets, grommets and washers.

Thanks again.

Tim

[Mark]

Ah, that explains everything. I should have asked what there was in place at the beginning instead of assuming it was the same as my KEL84. Lesson learned. Plastic shoulder washers on their own, with or without the shoulder removed, tightened so much they have been distorted beyond recognition would almost guarantee vibration transmission from the power TX to the case !

There may be some KAT 6550 owners on this forum who could tell you what works for them to save you reinventing the wheel. Someone may also have a copy of the original instructions in their archives which may tell you what the original factory power TX mount configuration was.

[bob orbell]

Be willing to help, the original build instructions show a side view of the mains transformer elevated from the chassis and the top cap elevated above the transformer, four plastic top hat washers fit into the top of the top cap, four more are fitted into the chassis from underneath, there is NO gasket, nor should there be one, the four long 5mm screws are fitted from the top, the assy. then lowered to its final position and four 5mm nuts fitted, this arrangement, although fiddley now isolates the transformer completely from the chassis, it does have its own earth connection. The output transformers receive the same attention, if memory is correct the KEL 84 is the same part and Matthew can supply the plastic washers, do not over tighten, just firm pressure from you driver. Bob

[Mark]

Thanks Bob. Top hat washer makes a better mental picture than shoulder washer !

Apologies - I said KAT 6550 in my post when I should have said Kit 6550. I don't know what the difference is.

[Greg.]

KIT= Intergrated Amplifier Kit (multiple switching input with volume control).
KAT= Power Amplifier Kit (single input, no volume control).

[Mark]

Thanks Greg - so the power TX mounting should be the same for the KIT 6550 and the KAT 6550.

FWIW I have looked at my KEL84 instructions (no date and no revision number, but I finished building it early 2007) and they say :

"Pass the mounting bolts through the transformer cover, transformer, and the grommets in the chassis. Secure using the flanged nuts. Add a solder tag to the bolt nearest the earth tag (of the transformer) and secure with a nut and a washer". This is different to the KIT 6550 instructions Bob has quoted. Who knows why WD changed them.

There is no mention in the KEL84 instructions about what to do with the top hat/shoulder washers which were supplied with the kit. 4 x "M5 x 5mm nylon screw insulators" are in the parts list in the manual, and next to them I wrote in pencil "What are these for ?" :-)

[Richard]

Tim, there are a couple of other things to check which may influence the noise,

What is the number of your mains transformer and how high is the stack of laminations (not inc cap)?

What value 6550 cathode resistors are fitted (if original you may be looking for 4 white ceramic box wirewounds)?