On Thursday, February 6, 2014 2:30:05 PM UTC-5, Ralph Mowery wrote:
wrote in message

...

The original fsu board was tied down only on the edges by that black

plastic which was clipped by some small fins of the aluminium radiator .It

did have a thermal paste .i also used a paste .

The +12 i took from a main +12 and used a 2.5 amps fuse .

The transistor i think disipates about 50w to 80w of heat at low speeds

that makes the heatsink to small to dissipate that heat and the airflow of

the blower at that speed is too little.Its true the transistor is made for

250 degrees but lets be true they dont make like they used.



The heatsink of the unit needs to be where air can circulate past it. The

fan is a good idea. I am not sure how the transistor is fastened to the

heat sink. It needs to make good mechanical contact with it.


The power transistor is not fastend to the heat sink at all.
If you look at the pics, it's on the top side of the PC board.
The bottom of the PC board has copper over all of it. Then the
bottom of the PC board contacts the heatsink. So, you have the
PC board material in between, which is not a good heat conductor.
And the transistor is closer to one side of the heat sink and no
bolt through the usual bolt location provided.

And to top it off, these things have been failing for 10+ years
and not one of the many suppliers has seen fit to make some of
the simple changes that would fix it.