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Old 1st November 2008, 12:42 PM   #1
reiver is offline reiver  United States
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Default How do you mount TO3 transistors?

I'm building an amp from scratch using Exicon TO3 transistors for the outputs, all mounted to a thick heatsink. I have the install kits for each one with socket and thin plastic gasket. My questions:

- Should I drill one large hole for both the TO3 leads to pass through the heatsink, or should I drill two smaller holes?

- I'd like to use heat sink compound, what's the proper order of all the the mounting kits parts (T03, spacer, compound, heatsink, etc) when installed?
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Old 1st November 2008, 01:13 PM   #2
AndrewT is offline AndrewT  Scotland
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drill 4 holes for the To3.
Two for the mounting bolts and insulating top hats. Two smaller holes for the base and emitter leads.
If you can, make a drilling jig for the To3 layout. Preferably in steel to give sufficient life. Clamp it into place and drill your holes. They must be accurately located to ensure the semi casing bolts up correctly and that the leads do not short if bent slightly out of alignment.
The edges of the holes must be deburred to ensure the raised surface does not puncture the mica insulation.

The mica washer must be coated on both sides with thermally conductive paste, not silver loaded paste which is electrically conductive and can short the collector to the heatsink.
The assembly order is heatsink - paste - mica - paste - To3 - tophats - bolts - nuts.
One or both of the nuts should have a copper or tinned copper tag to connect the collector to the rest of the circuit.
regards Andrew T.
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Old 1st November 2008, 04:20 PM   #3
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Two follow up questions.

1. What is a good brand (or composition) of paste?

2. Without referring to a torques wrench settings, about "how much" force should be used to tighten the fasteners? IOW, not so much that it would deform the case, yet enough to ensure proper thermal conductivity. I am not sure how one communicates that in words.

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Old 1st November 2008, 04:47 PM   #4
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MG chemicals is ok paste. Mines a few yews old and is getting like cottage cheese now.

Gripping the screwdriver with your fingers (index, middle and thumb), not your hand and tightening till you can't turn it anymore works well.
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Old 2nd November 2008, 01:05 AM   #5
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Hi reiver

How are your outputs arranged? Are they along one edge of
the board or are they along opposite edge of the board? Or are you 'flying' leads from the board to them?

Four holes will be easier for you to do because if you are making 1 big one for the B,E, that would be a huge (presumably you can make only circular holes with your drill) one Expect to make a mess with the compound, its quite difficult to clean up after you mounted them. Have fun with the after-play.

If you try to 'google' your title, there should be some nice drawings showing you exactly how to do it.
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Old 2nd November 2008, 01:34 AM   #6
reiver is offline reiver  United States
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Default re:

I'm creating a Hafler xl280 copy, with 3 N Exicons on one heatsink and and 3 P Exicons on the other. I'll be daisychaining the wiring to mimic the wiring on the original Halfer so it should be fairly easy.

I have a use generic amp arriving in a couple of days, so I'm not sure 'exactly' what the heat sink will look like. I's going to be a very fun weekend though....
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Old 2nd November 2008, 04:17 AM   #7
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I am curious to see how it looks like too. Mind to post some photos when it arrived? I recall that I had, somewhere in time,
try to make pcb for a 200 but never had it done because of the
expensive outputs.
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Old 2nd November 2008, 06:27 AM   #8
djk is offline djk
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A healthy male can exert 60 foot-pounds of torque with a screwdriver, the proper torque for a TO-3 case is 6~8 inch-pounds (snug, but not tight).

You can tape an insulator onto the heatsink and use a center punch for the hole layout pattern.
Candidates for the Darwin Award should not read this author.
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Old 2nd November 2008, 07:08 PM   #9
eyoung is offline eyoung  United States
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Hello again,

My question is the heatsink flat ,or does it have the 2 ribs to mount the transistors to like a Hafler?? This could uncomplicate the suggestions alittle.

Regards, Elwood
"Lead me not into temptation...I can find it myself."
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Old 2nd November 2008, 10:12 PM   #10
reiver is offline reiver  United States
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The heatsink I'm using is flat on both sides where the mosfets sit, with ribs father down the sink. So I should be able to mount them solidly, I think I'll just use a template and scribe the holes before drilling.

If I drill carefully and make the B/E lead holes slightly oversize I think it will work out ok.

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