Phil,
Thanks so much for the detailed response. I took the xfomer out of the unit
and found the following.
-The transformer was repaired. The duck tape wraps around the coil
-The wedges are present.
-The laminations fall away too easily.
-Some of the windings are tied together with lacing tape and others are not.
-Some coil ends where splices are made to the external wires are laying on
other coil ends.
Here is what I will try if it makes sense and pending any other suggestions
ie potting etc:
1-Use paper masking tape to insulate the wire ends from each other.
2-Put the transformer back together without tightening up on the four bolts.
3-Drown the transformer windings with the oil based polyurethane varnish by
pouring the stuff though the shell opening made during the "labodomy". I'll
wait a few minutes and drain of the extra varnish.
4-Paint the outside of the transformer at the lamination ends using the varnish.
5-Tighten the screws.
What do you think?
Len
At 07:50 PM 2/18/99, Phil T. (VE3OZZ) wrote:
>Len, one cause of hummy power transformers is loose laminations and
>sometimes loose windings. Loose laminations can be cured by tightening
>down the screws or bolts that hold or clamp the lamination stack together.
>The fact that someone's been in there already suggests the possibility that
>they didn't tighten things up completely when they were done.
>
>Loose windings will also vibrate and hum. This usually isn't a problem
>because most windings are tight to begin with, and if the unit has been
>vacuum impregnated with varnish, the varnish 'glues' everything in place
>fairly well. If the previous owner disassembled and reassembled the entire
>transformer, he may have forgotten to re-install the wedges between the
>winding tube and the core. The wedges are pieces or strips of fiber board
>that are jammed in between the inside of the winding tube and the faces of
>the core leg. If you're able to wiggle the whole winding assembly while
>its in the core, its too loose; look for missing wedges. I'm not familiar
>with the SB-220 transformer but its possible that wedges were used.
>
>If some of the individual turns in the windings became loose and therefore
>noisy over time, the previous owner may have been trying to correct it with
>the duct tape (if that's what it is). It is normal to see the occasional
>1/2" or 3/4" wide piece of cloth tape in the windings. These are used to
>anchor the first and last one or two wraps on a winding. They may also
>cover a soldered-on lead comming out of the transformer.
>
>Shellac can be used but its normally not recommeded because of moisture
>content. Admittedly, commercial shellac (like Zinsser's Bullseye shellac)
>is made with very high quality denatured alcohol that has only minor
>mositure. Good quality "oil based" polyurethane varnish is a better choice
>but it can take several hours to dry. You can identify oil based varnishes
>(just in case you're not familiar) by reading the instructions on the can.
>If it isn't identified as oil based on the front label, you'll know its oil
>based if the clean-up instructions tell you to use "mineral spirits" (ie,
>Varsol, paint thinner, etc). The suggested drying times for oil based are
>usually 4 hours or more. Water based polyurethane varnishes - which you
>don't want - will either say 'water based' on the front label, or the
>clean-up instructions will tell you to use water. Water based products
>typically have drying times of only 1 or 2 hours (or less) - which is
>another clue that its water based.
>
>Rich may reply with advice about potting, which is something I haven't
>tried yet. But potting would certainly quiet loose winding-turns as long
>as the potting material can reach the noisy area.
>
>Phil T
>
>At 04:56 PM 2/18/99 -0500, you wrote:
>>
>>I have a very noisy SB220 power transformer that sounds like a bad
>>florescent ballast. Other than being noisy, the amp works fine. I've taken a
>>good look at the transfomer and noticed that the back surface of the
>>tranformer has been hacked off. It looks like someone was trying to make
>>more clearance between the transformer and the back plate of the amp but I
>>doubt that that is the reason for cutting the transformer. I removed the
>>back plate and pushed asside the paper insulator on the windings and noticed
>>that there is some grey tape ( looks suspicially like duck tape) wrapped
>>around the windings.
>>
>>I'm tempted to just loosen the screws, paint the laminations with some
>>shellac and see if I can get rid of the noise.
>>
>>Anybody ever hear of this kind of transformer surgery or why anybody would
>>do it?
>>
>>Thanks in advance.
>>
>>Len
>>
>>
>>
>>
>>--
>>FAQ on WWW: http://www.contesting.com/ampfaq.html
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>>
>>
>>
>
>
>
--
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