Those are plastic, or a very thin Mica plate, the stress marks indicate thin mica. and no, I would not advise the reuse of them. because those stress points can brake during snugging of the screw on assembly and allow potential a short even with non conductive grease, as the transistors heat up, that grease becomes malleable (soft like a fluid gel -pliable) and can ooze away allowing a metal to metal contact short. And the broken one, did it come off like that? or was it stressed then broke rather easily after removal?
As for the wire used, that will be ok, as it only enables the field produced by the board in operation to be absorbed so to speak to a close by return point. That large block of aluminum would act as an overgrown antenna, holding a low charge, especially from being any where near that speakers magnet, and the pcb being so close, this field would play havoc on near by components that were sensitive to it. Transistors primarily in general have an amazingly low static transfer rate, until energized or connected in a shielded manner. The manufacturers method is just to use a gnd plane for return. As long as the leads reach, and able to be installed, and are not too long, as this could bunch up between the board and sink, it will work fine. As for tinning the ring leads, would help to increase the transfer, tho not really needed. As for tinning to help then hold together better, that would help, however I have not seen if those leads (Oring) would be kept away from the other contacts on the power pcb when everything is assembled together.
If possible to get the numbers from these transistors between the pcb and part without removing them? or is that glue in the way that I see on them thus far. (the red dots)
Post 85, no need for that, getting the amp back together and it decides to work will be the direction that is the objective.
If the glue is too well applied to those transistors (it may hold them at the board), then a large size thermal plate can be considered, however, if they have all ready been ordered, those pads mentioned earlier can work, it will just be that the thermal pad potentially being smaller will still insulate the transistor from the heat sink, but have the edges exposed, non conductive thermal grease can be used to fill in as with the plates as is, resulting in the plate suspending the transistor from the heat sink, and the grease being able to stay at the edges via the thermal pad acting as a spacer for the center upper portion of the transistors exposed plate on the back of it.
That's just glue// My mistake, It's not just glue..
I inspected these big flat transistors, and can't find any numbers on them. But I did get a measurement with my little metal ruler. I'll upload the pics
View attachment 102647
View attachment 102648
Which is roughly:
Transistor Width 20/32 - 15.875 mm
Transistor Height 25/32 - 19.84375 mm
Then the T Pads:
View attachment 102654
View attachment 102655
Which is roughly:
TPad Width 25/32 - 19.84mm
TPad Height 1" - 25.5 mm
At least now I can find the correct sizes
The width of the thermal plate will be the ruling factor on the replacement plates. the length is the latter only as the transistor metal exposed tab ends near the base, so for the case of best heat transfer as the thermal pad is a part between the transistor and the housing would need to cover as much as possible so that the heat can transfer evenly across the transistor to heat sink, just without any sparks from electrical contact. The Mouser Aavid Thermalloy has better in thickness being less than the Digi, Keystone pad. overall the Aavid is a better of the two listed thus far, not only is it thinner, the material shows better strength in operation points. However it is an Elastomer based pad. that Elastomer material can be used, it just has a bit of a sponge like quality to it when stretched, tho not going to be a problem when installed on transistor itself with grease, then applied to the heat sink, (think of, dropping an engine into a car when it needs to be applied straight into the engine compartment) So I doubt that the pads would have to deal with any slide stress that would cause this when heated under operation.
As for the width, those mica that were removed are quite a bit wider than the transistor, and not all the width is necessary. Main area to observe for a thermal pad would be the top part of the Device that has a T shape to the heat transfer plate.
the Mousers Thermalloy type shows 2.925mm over the 15.875mm width at transistor. total 18.8mm pad width.
The Thermalloy Height, 3.394mm over the 19.846mm height at transistor. total 23.24mm pad height.
I went over by .001 on the transistors height.
even with material differences pad per pad, I doubt that would vary much. the thinner the thermal plate the better, however the grease tends to go in the other direction on heat transfer by small amounts. Just used for thermal contact adhesive, and for filling in the gaps between parts. It was used prior on that amp, so the mica being thinner (the worn out removed plates) of the lot looked at thus far. the temperature difference based on how the amp is driven (how loud) would be much thermal difference. so thus far the Mouser thermalloy looks the part that fits.
If my measurements are off, I'm not at quality with math very well, however, nice 32nd/64ths caliper ruler.
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