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output transistors - mica washers look cracked
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Old 17th June 2018, 10:44 PM   #1
MaccAu is offline MaccAu  Australia
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Default output transistors - mica washers look cracked

I have just about put my Technics Su-V9 back together, before soldering the heatsink back in i check continuty the Sankens and heatsink.

It was only while going over the photos to check on the PSU wiring, i noticed on the sanken the MT200 mica washers look a bit cracked?

Should it be okay, will I have to remove heatsink and apply new ones? A bit of work to removed them again but if i have to i shall

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Old 17th June 2018, 11:17 PM   #2
scottjoplin is offline scottjoplin  Wales
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If pieces broke away entirely it would reduce the contact area, otherwise I don't see that it should be a problem.
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Old 18th June 2018, 12:26 AM   #3
avtech23 is offline avtech23  Australia
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output transistors - mica washers look cracked
Might just be the thermal compound has dried and cracked. Had that on my old NAD rebuilds.
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Old 18th June 2018, 12:33 AM   #4
scottjoplin is offline scottjoplin  Wales
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Yes, it does look more like that, dried and shrunk
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Old 18th June 2018, 12:37 AM   #5
MaccAu is offline MaccAu  Australia
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Thank you Scott and AvTecdh
Its wakefield 120 thermal compound from a 2014 , i kept it sealed and in a plastic bag in a draw? It was nice and sticky when i mixed it up.

No use trying new compound? okay to leave it then?
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Old 18th June 2018, 04:50 AM   #6
Ian Finch is offline Ian Finch  Australia
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It depends on what you mean by ''looks a bit cracked". Thick slabs of low grade mica can look cracked, as in the sheets it is composed of, may have delaminated eons ago. Cracked, as in flakes of mica have pulled away from the surface during handling, may be acceptable but this reduces the contact area and heat conduction considerably where only the paste is providing the heat transfer. Cracked, as in someone was careless and bent the mica such that it is buckled and delaminated or partly broken along fold lines, renders it useless. In that case, buy new mica or use silicone rubber with no paste at all.

Some years ago, I noticed that the quality of the mica used by some Japanese manufactures was absolutely superb. It was incredibly thin, clear as PET film and had fantastic performance. In contrast, the replacement mica I bought was thick, already delaminated (translucent and flaky) and raised the operating temperature of the transistors by around 20C IIRC. Finally, I bought some of this super quality mica from an Ebay seller in Taiwan a few years back. It's amazing quality but good luck locating it now that the MT200 type is EOL.
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Old 18th June 2018, 05:35 AM   #7
MaccAu is offline MaccAu  Australia
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Hi Ian,
That's good if its not the thermal grease as i have a bit of it left, rather a lot.

The layer is thin thermal grease on the heatsink and thin thermal grease on the sanken.

I have stored it in a plastic bag and a box, and noticed that some was it was a bit flakey looking and some of it cracked. I tried to choose the best of them

I got the wrong silcone pad from Mouser, not for a 200w IC.

I get mine from Wagner Electronics, dont use ebay as i fear the quality. Wished i knew your supplier though, he sounded good.

The Wagner Wes YC90 MT200 mica washers - about 1mm thick i think maybe less
YC90 Mica Washer, Semiconductors, Electronic Components | Wagner Online Store

The V9 had silcone pads, you being in Australia like me, where would you go for the silcone pads

It might be best i re do it with fresh mica or silcone rubber?
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Old 18th June 2018, 09:18 AM   #8
Ian Finch is offline Ian Finch  Australia
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As we don't make micas or many electronic components at all, here in Oz, I don't think it's unpatriotic to say I wouldn't buy micas here either, particularly an uncommon size that wasn't supported here in the past and originates from the same sources. Having said that, these Sanken transistors shouldn't be a problem even using the standard type mica supplied by Wagner, warts and all. 1mm thick? I hope not - you could split at least 5 from that thickness and still have too much - standard thickness is 0.1mm, best is only 0.07mm. However, the tolerance allows up to 0.3mm thickness and this means you may still be getting rubbish unless your supplier cares what they sell. This one is typical and similar to Wagner's quality. Also check their average quality silicone pads: 25pcs Mica Sheets MT-200 24x39mm Insulator SANKEN Transistor Heat Sink Pad | eBay

Some folk have rubbished Kapton tape but, it's actually great for MT200 types used in home audio applications when you can't get decent thin mica. Rod Elliott promotes and sells 0.025mm (25μm) thickness x 25 wide tape from his ESP website but you can also buy this Dupont product in greater quantity on Ebay, relatively cheaply in 30m rolls of virtually any width - more than enough for a lifetime of DIY. All you need is a pair of sharp, top quality (as in thick, rigid blades) scissors and a 3.2-4mm wad punch with a piece of laminex or similar hard plastic as a 1-shot anvil, in order to cut plenty of good "micas" for MT200, MT100, TO264, T0247, TO3-P, TO220, TO126, even T03 and T066 if you like shiny metal things.

Silicone rubber pads I once bought from Altronics but when my stock ran out, I needed fair quantities and I bought 250 for the local price of 20 and the performance was comparable (that is, the temperature rise of the the transistor cases was similar to original). I wouldn't be over-concerned about finding the highest quality alumina filled silicone rubber because you can buy these anywhere and unless you party really loud, a new class A amp isn't going to need a lot of cooling.

Thermal paste is essential for all but conformal silicone rubber type washers but it must be absolutely clean, with no lint, specks of dust, dried out lumps or other dirt particles which limit flow under the clamping pressure, preventing closest possible contact between surfaces. Unfortunately, in reworking old paste, you can't ensure this without proper blending equipment. Ideally, there shouldn't be any paste remaing on the surfaces, except in low spots, over the entire contact faces. Otherwise, every micron of paste thickness just increases the net thermal resistance (it's nowhere near as good a conductor as many seem to think). I once thought the economy of large tubes and jars of the stuff were a good idea too but open jars just invite trouble - now I often use the small syringe packed type, in good brands.
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Old 20th June 2018, 11:40 PM   #9
MaccAu is offline MaccAu  Australia
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Thanks for the help and guidance Ian and explaining, i wasnt happy with the micas, had a look at them they are pretty thin, though thermal compound the clear oil is gone but appears sticky still, yes it was a small jag, couldve gone with 2 4g tubes instead
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