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eXStatA Build Thread II - Page 2

post #16 of 566

Re: the thermasil pads - yes you should use them even though the transistors have a plastic body. They help transfer the heat from the device to the heatsink.


Use a flat washer between the tran and the lock washer & nut.


Starting towards the side next to the big WIMA caps, it should be a sandwich as follows:












NOTE: Don't mount C3 & C4 until after you get the trans and heatsinks in place, or it may be difficult to keep the screwhead from spinning when tightening the sandwich of parts.


As far as orientation of the trans goes, you need to pay attention that the dot on the transistor is on the same side as the little dot on the silkscreen.  


Originally Posted by rbarth View Post


Thank for your response, Al.  I should have waited with my post, however, as I now have another question.  It's about the Q5, Q6 and Q7,Q8 transistors.  All four of them have some sort of ID printed on one side.  When I mount them to the heatsinks with this ID facing out (i.e., away from the surface of the heatsinks), their pins are too far apart to fit into the PS board.  If I reverse them and so that the printed ID sides are both facing towards the headsinks, their pins are too narrow to fit into the PS board.  (I checked this twice and believe I am right about this.)  So, how do these transistors get mounted? 


EDIT:  Also about transistors.  The mounting surfaces of some are made entirely of plastic and the special washer that comes with the Aavid mounting kit does not fit into their mounting holes.  Should I place a regular flat washer between the screw and outside surface of these transistors?  Also, I believe I heard somewhere that you don't need to use the Thermasil pads between the insid" surface of these plastic transistors and the heat sink.  Is that true or should I use the pads no matter what?

Edited by TimJo - 7/17/10 at 12:19pm
post #17 of 566

Looks like we posted at the same time. 


Okay, things look good. Bending pins is fine.


What's important is the orientation of the part is correct with regards to the dots on the part and the dots on the silkscreen.

post #18 of 566

Very, very helpful..  Many thanks.  (C3 and C4 are already mounted but I should be OK with a pair of needle nose pliers holding the screw while I tighten the nut.)  So, I can finish the PS.  On to the amp!

post #19 of 566
Thread Starter 

Yes, lettering to the outside and away from the heatsinks. You don't need the pads for plastic devices but if you don't use it then you need the thermal grease. Don't use the shoulder washer on the all plastic parts.


Sorry about the R10/R11. These indeed went away and are still in the BoM.


The two sets of pads for C1/C2 are to give you some choice for the lead spacing and size of these caps. Just use the pad spacing that fits with the caps you have.


Looks like you are just about finished. When you mount the mosfets pay attention to the leads of R1/R3/R5 and make sure that they are not contacting the heatsink. Push them away a little if you need to.


Edit: I see that my response is a little bit late.

Edited by runeight - 7/17/10 at 12:50pm
post #20 of 566

If I align the dots on the transistors so that they are on the same side as the little dot on the silkscreen, the printing on Q5, Q6, Q7 and Q8 is facing the heatsink, not away from it.  I can only see one dot on each transistor, which is on the side opposite the printing on the upper left hand corner.  So, I guess I'm still confused.  Right now, the transistors are mounted with the printing facing the heatsinks.

post #21 of 566
Thread Starter 

Yes, this is correct. Printing away from the heatsinks. Smooth side to the heatsink.


Check the MJE data sheets if you're not sure.

post #22 of 566
Thread Starter 

Maybe this will also help.


If you look at the devices from the bottom you'll see that the pins are closer to one side than the other. At least the should be.


The side that is farther away from the pins should be the blank side of the plastic and this goes against the heatsinks.

post #23 of 566

Thanks Alex.  That makes it crystal clear.

post #24 of 566

Sorry if I added to the confusion.


I'm back at my computer now and after googling the datasheet, I see the emitter doesn't have a dot. That's what i get for going on memory. 


Glad to see you have it sorted out. Just wait 'till it starts making music! 

post #25 of 566

Not at all a problem. All is well now.  On to the amp!

post #26 of 566
Thread Starter 

Does this mean that your PS is up and working? Or just assembled?

post #27 of 566


It's just assembled. My thought was to do all of the construction first and then return to the PS. Would you prefer that I test it first?

post #28 of 566
Thread Starter 

No. I just like hearing about when things work. Take your time.

post #29 of 566


post #30 of 566


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