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I finally have this amp pulled apart, and have removed all of the original BUZ901P N-Ch MosFET's and BUZ906P P-Ch MosFET's from the interesting dual Aluminum Extrusion, and with assistance from Jon Snell in finding suitable replacements for those obsolete parts, I'm now in the messy part of the operation. Putting the new MosFET's (Exicon ECX10N20 N-Ch & ECX10P20 P-Ch MosFETs) into place, and about to attempt getting discrete greased mica insulators between the TO-247 Parts and the heat sink.
What I found when I pulled this messy thing apart was a single long Hi Temp Mylar insulator strip with grease on the device side, and NO Thermal Compound on the heat sink side. I'm now guessing why they didn't bother greasing that side up, nor went to use discrete insulators, greased or not. The single PCB for the output stage runs down this long narrow extrusion, with the devices soldered in at the base of the PCB where the device terminals change from solder terminal width to the base width that go into the plastic body. Getting an insulator between the pwr devices and heat sink.....GOOD LUCK!! Great mechanical design. YOU GUYS ARE ALL FIRED!!
There was a punch-thru burn mark at the one end of the lower insulator, which was probably first, and then it took out four of those parts on that side and two of the parts on the opposite side.
In the mechanics of this assembly, if you solder the parts straight into the PCB, where they're seated all the way to where the device terminals taper down to the nominal terminal width from the base of each lead, there's no flexibility of the leads between the body of the parts with respect to the middle of the heat sink extrusion. Not sure why they didn't use something like Burgquist K6 or K10 greaseless insulator strips there. But there was no grease on the back side of these mylar strips. And with the parts soldered in straight, it forces the bodies of the MosFET's to stretch away from the leads going into the parts and then get torqued down with the screw-head and keps nuts on the other side of the extrusion. Not a lot of forced offset by the mounting screws/nuts, but more than I'd care to stress the power device package with under long term heating.
I pre-bent the new leads using a special Erem lead bending tool, which has 1" wide blades that close flat, so you can grip all three leads of the TO-247 part about a tenth of an inch below the body, bend it slightly forward, then at the transition to the narrow terminals, bend them back so to give a slight offset of the body so they mate up with the sides of the extrusion and NOT force the terminals to bend by the holding force of the mounting screws.
But still, I have to flex them back towards the middle to slip in the greased mica washers, then back upright again, and poke my guide tool thru the extrusion and center the washers to be aligned with the body of the devices. A real PITA to have to deal with, but hopefully completing the thermal path between the power devices and heat sink thru the greased insulator, it will be better than what I found.
Meanwhile sufficient cursing while being assaulted by the white greased extrusion during all of the procedure is mandatory to get it done! There's a top extrusion that screws down onto the two side extrusions, which is what all that grease is for. It's really there to get all over your hands and arms during this procedure!
More images to follow when I'm thru. And hopefully all will work afterwards.
What I found when I pulled this messy thing apart was a single long Hi Temp Mylar insulator strip with grease on the device side, and NO Thermal Compound on the heat sink side. I'm now guessing why they didn't bother greasing that side up, nor went to use discrete insulators, greased or not. The single PCB for the output stage runs down this long narrow extrusion, with the devices soldered in at the base of the PCB where the device terminals change from solder terminal width to the base width that go into the plastic body. Getting an insulator between the pwr devices and heat sink.....GOOD LUCK!! Great mechanical design. YOU GUYS ARE ALL FIRED!!
There was a punch-thru burn mark at the one end of the lower insulator, which was probably first, and then it took out four of those parts on that side and two of the parts on the opposite side.
In the mechanics of this assembly, if you solder the parts straight into the PCB, where they're seated all the way to where the device terminals taper down to the nominal terminal width from the base of each lead, there's no flexibility of the leads between the body of the parts with respect to the middle of the heat sink extrusion. Not sure why they didn't use something like Burgquist K6 or K10 greaseless insulator strips there. But there was no grease on the back side of these mylar strips. And with the parts soldered in straight, it forces the bodies of the MosFET's to stretch away from the leads going into the parts and then get torqued down with the screw-head and keps nuts on the other side of the extrusion. Not a lot of forced offset by the mounting screws/nuts, but more than I'd care to stress the power device package with under long term heating.
I pre-bent the new leads using a special Erem lead bending tool, which has 1" wide blades that close flat, so you can grip all three leads of the TO-247 part about a tenth of an inch below the body, bend it slightly forward, then at the transition to the narrow terminals, bend them back so to give a slight offset of the body so they mate up with the sides of the extrusion and NOT force the terminals to bend by the holding force of the mounting screws.
But still, I have to flex them back towards the middle to slip in the greased mica washers, then back upright again, and poke my guide tool thru the extrusion and center the washers to be aligned with the body of the devices. A real PITA to have to deal with, but hopefully completing the thermal path between the power devices and heat sink thru the greased insulator, it will be better than what I found.
Meanwhile sufficient cursing while being assaulted by the white greased extrusion during all of the procedure is mandatory to get it done! There's a top extrusion that screws down onto the two side extrusions, which is what all that grease is for. It's really there to get all over your hands and arms during this procedure!
More images to follow when I'm thru. And hopefully all will work afterwards.
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