Well, tell that to Adcom, Carver, etc because they were all the original factory pads...
Thermal paste for output transistors?
- Thread starter Oleg LED
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Blue Shadow
Waiting for Vintage Gear from this century
Pads are fine if you don't need the best heat transfer. If the pads are squished thin they were not used properly which includes a torque spec for installing the transistor to the heat sink.
That torque spec is VERY LOW, 4-7 Inch-pounds and this shouldn't squish a thermal pad.
Learn or refresh if you want, just one manufacturer's comments on mounting TO-3s
http://www.ti.com/lit/an/sboa020/sboa020.pdf
That torque spec is VERY LOW, 4-7 Inch-pounds and this shouldn't squish a thermal pad.
Learn or refresh if you want, just one manufacturer's comments on mounting TO-3s
http://www.ti.com/lit/an/sboa020/sboa020.pdf
RICKCARY
Active Member
I have a question. I pulled my heatsink to gain access to my caps for a swap out. The transistors on the heatsink dont appear to have white compound on them but there is also a clear thin square plastic piece thats stuck on there too. When i put it back together do i put the thermal paste on the transistor face and also under the plastic square piece? Or just under the clear square plastic? cheers.
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Blue Shadow
Waiting for Vintage Gear from this century
clean all surfaces. Transistor, mica top and bottom and the heatsink.
use as little as possible to get as thin as possible layer of non-conducting thermal transfer compound on both sides of the mica putting it on whatever you want of the 4 surfaces.
The compound is only there to fill in the non-smooth parts of the metal and mica as the metal-mica interface is better at heat transfer than the metal-compound-mica interface.
use as little as possible to get as thin as possible layer of non-conducting thermal transfer compound on both sides of the mica putting it on whatever you want of the 4 surfaces.
The compound is only there to fill in the non-smooth parts of the metal and mica as the metal-mica interface is better at heat transfer than the metal-compound-mica interface.
RICKCARY
Active Member
Oh, ok. So a thin layer sandwiched on trans/mica/heatsink and screwed on? I have seen YT vids where there was no mica and its just trans/heatsink. Thanks for the tip.
Blue Shadow
Waiting for Vintage Gear from this century
Very LOW torque. The big all metal TO-3 transistors use less than 10 INCH-pounds torque.
You can look up the torque in spec sheets or just don't use a lot. Don't want to deform the transistor which would decrease the surface to surface contact area.
RICKCARY
Active Member
OK. Yes i remember when i took them off they were finger tight and unscrewed quite easily. Good tip again, because i usually put stuff on snug, so i better re check my tascam 122 mkii which i did a while back. It has similar transistors and a big fin of a heatsink. I was going to go back to it anyways to add some paste because i didnt have any at that time. Cheers. 
Good tip again, because i usually put stuff on snug, so i better re check my tascam 122 mkii which i did a while back. It has similar transistors and a big fin of a heatsink. I was going to go back to it anyways to add some paste because i didnt have any at that time. Cheers. 
Oh, ok. So a thin layer sandwiched on trans/mica/heatsink and screwed on? I have seen YT vids where there was no mica and its just trans/heatsink. Thanks for the tip.
The mica is there when insulation between the transistor and heatsink is required. The heatsink is sometimes grounded and the transistor's metal is directly connected to the collector which means it has a voltage. If an insulator such as a mica is needed and it's omitted then the transistor would be shorted out.
Blue Shadow
Waiting for Vintage Gear from this century
Is very fragile hard mineral that can crack or break. It isn't plastic in most units.
Mica - Wikipedia
RICKCARY
Active Member
Good point. I wondered about that. I have seen some vids where the transistor is just screwed right to the heatsink, with no mica between. I'm fairly sure the the HS is grounded on my decks. I didn't realise they are fragile, so i should be careful when i clean them off before reapplying them. ISO should be OK to use on them i presume?
RICKCARY
Active Member
I'll keep that in mind as i spread it on the transistor.
Blue Shadow
Waiting for Vintage Gear from this century
and replace any broken ones with new ones from Mouser/Digikey with same (usually thicker) or the newer plastic sheets that some like since it does not need donkey cum.
