On May 27, 10:53 am, Palindrome wrote:
SparkyGuy wrote:
Fixed a 26" Gateway LCD TV/monitor (bad solder joint in the SMPS).

But exposing the component side of the power supply requires pulling off 2
heat sinks that are glued to the tops of the 2 transformers. Some of the glue
pulled off of the top of one transformer (about 3/16 inch, or 5mm thick) and
a copper strap that winds around the laminates came loose.

Is this really a heat conductor? Seems to be not possible, but then I'm no
glue chemist.

Should the missing glue be replaced? What should I use?

Is it important to relocate the end of the copper strap?

Regarding the other transformer (the glue is intact), should anything be
placed between the glue and the heat sink (ie, heat sink compound), or just
clamp down the heat sink?

The "glue" is white and seems to be the same stuff used to attach some of the
components to the PCB. It is a bit flexible but not very; it crumbles when
scraped off or when a sliver of it is bent. Not epoxy, no way.

Here's the top of the transformer:

http://www.uploadwiz.com/WIZ2755487483

Here's what's left of the material, stuck to the heat sink:

http://www.uploadwiz.com/WIZ3413654658

I'm not sure exactly what that band is for. It could be there to reduce
radiation of the transformer's eddy currents and thus satisfy emission
regulations. It could be there as part of the heat transfer mechanism.

However, they wouldn't have put it there without good reason and the
mechanical design of the transformer is key to its electronic
performance - so I would try to get it back in position and re-join it,
idealy across the complete width, to form a complete electrical loop -
if that was how it looked to be intended to be. It, or something else,
could massively over-heat if the transformer's characteristics are altered.

For "glue", I would use silicone adhesive/sealant eg maplin N71BZ. It
may not be what they used originally, but it should do the trick nicely.

Farnell stocks two thermally conductive adhesives. The Electroline
ETCR75S (order code 130-485) is probably just rubber loaded with
finely divided zinc oxide. I used something very similar twenty years
ago, and it worked well.

The Loctite 1705052 (order code 537-020) is a "self-shimming"
adhensive, where a methacrylate based adhesive has been loaded with
0.15mm diameter balls, and is intended for applications whee there is
a relatively high potential difference across the ahdhesive layer.

Both adhesives are quite expensive.

snip

--
Bill Sloman, Nijmegen