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Old 19th August 2014, 10:51 PM   #11
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I soldered MOSFETs to the boards, planning to suspend the boards by the MOSFETs horizontally. Though the heat sinks are drilled to mount the boards to the sinks using spacers, I prefered horizontal placement simply for the ease of adjusting bias with P1 and P2 with the top of the chassis off. Take a look at the 3rd and 4th photos, in which I had loosely mounted the MOSFETs to the sinks to see what it would look like. I am sure that most all of you immediately see the problem that was not obvious to me.

(Christian - great to hear from you. I have received great feedback on your boards, and as you can see, I am finally getting around to using them, myself.)
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Old 19th August 2014, 11:25 PM   #12
Zen Mod is offline Zen Mod  Serbia
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Quote:
Originally Posted by toxic ingestion View Post
I soldered MOSFETs to the boards, planning to suspend the boards by the MOSFETs horizontally.........
nono

you must suspend pcbs separately
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Old 20th August 2014, 01:00 AM   #13
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nono

you must suspend pcbs separately
And why is that, ZM?

(There is yet another problem in this horizontal suspension.)
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Old 20th August 2014, 02:59 AM   #14
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when you add wires , combined physical load and thermal cycling will cause boom

big badaboom

ZM , AKA LeelooDallasMultiPass

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Old 20th August 2014, 03:00 AM   #15
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be creative - support these pcbs
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Old 20th August 2014, 04:23 AM   #16
a.wayne is offline a.wayne  United States
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will cause boom

big badaboom

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Old 20th August 2014, 05:36 AM   #17
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Thanks for the insight regarding suspending boards that can get both heavy and hot. Having one of those droop down and shorting out on the sink or elsewhere would not be desirable. I have attached a photo of a previous build where I used horizontally mounted boards but suspended them from below, as they should be in that situation.

As I said at the beginning, I make new errors and learn new things with each build. In addition to ZMs important point about how wrong it is to suspend a board from MOSFETs by themselves, the other problem I had not anticipated was that when sitting horizontally, the boards hit the transformers and PS caps, preventing the chassis from fitting together (pictures below). This would not have been an issue with a single transformer and single PS board, but certainly was with dual mono construction.

Lesson learned. 6L6 prodded me to simply mount them as designed, which is what I should have done to begin with. MOSFET leads were clipped close to the boards. After using copper braid to clean up the boards (about 20 minutes), I could easily use these same MOSFETs with the boards mounted correctly on the heat sink with aluminum spacers. The last photo shows the board and MOSFETs loosely secured to the sink prior to soldering them into place.
Attached Images
File Type: jpg F5Previous build.jpg (48.1 KB, 141 views)
File Type: jpg F5#11.jpg (401.7 KB, 102 views)
File Type: jpg F5#12.jpg (520.4 KB, 114 views)
File Type: jpg F5#13.jpg (280.4 KB, 109 views)
File Type: jpg F5#14.jpg (418.8 KB, 100 views)
File Type: jpg F5#15.jpg (438.6 KB, 89 views)
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File Type: jpg F5#16a.jpg (211.5 KB, 105 views)
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Old 20th August 2014, 08:49 AM   #18
cviller is offline cviller  Denmark
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I actually find it easer to do the adjustments with the boards mounted to the sturdy sinks as you have now done.
To make it easier to reach the posts and optimise the mosfet placement on the sink, I normally mount them upside down (with mosfets in the bottom).
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Old 20th August 2014, 02:41 PM   #19
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To make it easier to reach the posts and optimise the mosfet placement on the sink, I normally mount them upside down (with mosfets in the bottom).
Hi. I don't understand exactly what you mean by your statement that you mount "them upside down (with mosfets in the bottom)." Might you explain a bit or provide a quick photo or sketch?

Thanks.
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Old 20th August 2014, 03:00 PM   #20
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mosfets at lower half of heatsink , pcb upwards

locate mosfets at approx. 1/3 of heatsink height
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