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Old 26th September 2006, 06:25 AM   #1
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Angry Kapton Tape problems

My pretty much abandoned RF 250.1 (it's the one without MEHSA) got a present in mail - Kapton Polymide tape to replace badly damaged heatsink insulators. Replace went great, tightened heatsink screws, fired up amp... Bam! Sparks from one heatsink, my power supply cut of indicating a short. Unscrewed heatsink, replaced tape. Amp is working. for a 10 minutes just fine. Switched it off-on - Bam! Sparks from under another heatsink, power supply cuts off
May be I need 2-3 layers of 1mil (B6-5) tape? Something else? Am I overtightening screws? Is there torque specs for them? Thanks
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Old 26th September 2006, 10:43 AM   #2
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Kapton is frighteningly easy to cut, and 1 mil is mighty thin. It's possible that extremely small burrs on the edge of the holes are cutting through it. Make sure everything is really, really smooth, and if you can get it, use some slightly thicker tape.
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Old 26th September 2006, 12:26 PM   #3
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The surface where the tape is applied should be relatively smooth. Any burrs should be knocked down with fine emory cloth. Sand just enough to level the surface, no more. Using a light oil will help keep the emory cloth clean and cutting longer. Remove all oils before assembly.

Sometimes the transistors are damaged and have a small burr on their edges. This can cut the tape.

I don't think that you can tighten the screws too much. I've used kapton for approximately 8 years and have never cut it from tightening the screws too much.

The surface must be absolutely free of metal shavings (sometimes created if you cut new threads in the screw holes). Before applying the Kapton, lay down some packing tape. When you remove the packing tape, it will remove all debris.

You shouldn't need thicker tape. I've always used 1 mil tape. Don't use multiple layers. It won't transfer heat as efficiently.

If you can't find the point where it's shorting, use a current limiter or small fuse and with the amp on, begin tightening the screws. When you get to the point that's shorting, the amp will begin to draw more current.
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Old 26th September 2006, 01:44 PM   #4
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Thanks!!! Mounting surface is not really smooth at all, and I will check all TO220's mounting surfaces tonight too. I blindly assumed that tape will survive that abuse. Never had experience with that material before. Live and learn
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Old 26th September 2006, 05:25 PM   #5
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RF uses a thick ~3 mil rubberized type insulator, due to the fact they saved money not machining the heatsink face area.
Kapton would not be the best choice in this app.
Sorry to hear you had troubles

Try finding some Silver nitide compound type insulator material. Its kind of thick and somewhat rubber like. The problem was the rough surface on the sink.
That surface and your torque combined to defeat you in your efforts.

Those RF amps must have a soft fairly thick rubbery like insulator.

If you still worried by what you find then attach the kapton glue side to the fets base as a extra insulator surface against what ever softer membrane material you can find.

This should work worse case.

hope this helps
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Old 26th September 2006, 07:13 PM   #6
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Thanks! Yes, that was soft, rubberized insulator. I'll try either polishing surface or "softening" insulator by any means. There is no chance for retirement for this "Diablo" amp
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Old 26th September 2006, 07:29 PM   #7
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Why not use the correct thermal pad material?
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Old 26th September 2006, 07:43 PM   #8
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Quote:
Originally posted by BlackCatSound
Why not use the correct thermal pad material?

Possibly very difficult to find as RF does little support of amps over 5 years old. (Just my experiance with them in the past.)

Plus I believe he would be better of with a silver nitride based rubberized material. I have a bunch but its too thick for this amp it came out a 42 inch flat screen that had a broken screen. Seems they use that stuff everywhere nowadays, even behind your flat plasma screens to sink heat away from the screen.

I am extra careful when working on that era of RF amp just because I don't have a source for that exact type and thickness of insulator material.

Hope this is helpful
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Old 26th September 2006, 08:09 PM   #9
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Ceramic loaded thermal pads are available from most good electronics suppliers in a range of thicknesses and K.

Something like a car amp that might be subject to vibration is not the place to use a very thin and flimsy insulator like kapton tape.
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Old 26th September 2006, 11:39 PM   #10
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Well Orion first used Kapton back in the early 90's so its nothing really new to the car amp scene. (1 mil thin can be thin, I use 2 mil myself)
I never saw any failures back then, or now. Hifonics and Soundstream used Kapton sheeting materials coated with silicone paste for years.
I found a few (1 or2) SS amps that had burn thru on there Kapton, but I also found metal debris under the Kapton indicating unclean assembly or reassembly from after market repair services from less qualified tech's. So I don't hold the Kapton at fault here because poor assembly is just that poor assembly.

The main issues with this situation is the un-finished surface that Rockford left inside the amp. Which is only found inside Rockford and Zr series kicker amps.
Everybody else at least tries to utilize some form a smooth surface for mating Semi's to.
Rockford went to MESHA style panels just after this model and has stayed with that production method since. But even now under the MESHA the surface is very poorly finished, they just use a thicker paste like heatsink transfer compound that is almost like thickened concrete.
It has a semi mud like consistency. It thins and spreads like normal when hit with regular silicone grease.

Kapton has served me well for many years now with zero failures on properly prep'ed surfaces. The Fosgate in this post is not a acceptable use of Kapton. And I would not have used it just because of the poor surface flatness issue.

The current market uses Sil-pads and silver nitride rubberized compounds, and others. That hot form once and can be a real source of failure if reused since they have formed to install once under compression, and most probably won't seat properly the second time around.
The current market is also built on speed of production so Sil-pads are considered to be the answer to rapid assembly needs.

I myself worry about properly seat semi's as I see when I pull the amps down where the sil-pads are compressed off center on one side and not properly seated from the less then diserable clamping methods used currently.
I see fets with less than 35% surface mating with sil-pads, so I know more than anything else proper attention must be paid to proper reassembly, again mostly due to warped clamps and single pressure point assembly methods.

I still keep 5 different sized rolls of Kapton on my bench. If it was good enough for NASA and the moon mission and all of our current and past satelites, Its good enough for me and my customers who enjoy a 1 year warranty on all my work performed.

Hope all this info has been of help to somebody out there, it pays my bills everyday
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