Cobra 148GTL-DX audio chip ?

[Cutlass327]

I recently found the plastic screw was broken off that holds the chip to the chassis. It seems I may have lost the the mica (?) insulator sheet that went under the nut. Where can these be found? Im guessing these are not electrically insulated on the heat sink tab, which is why they have the mica and plastic screws?

 

[Robb]

When a plastic screw is used, it is a fair bet that the chip is supposed to be isolated. I've seen this issue before! Should be a TA7222AP audio chip, a very commonly used audio chip in a great many radios.

If you do not have the original mica, you could cut two micas; but not the best way to do it.
But you will still need another nylon screw. They are available.

One solution would be to buy a small sheet of Bergquist "SIL-PAD" from a electronics supplier, such as Mouser of Digi-Key and cut it to shape and use that. Never found a supplier for the original mica for that IC; but I could be wrong about that.

The TA7222AP audio chip is still available from some eBay suppliers; but they aren't cheap as they once were.

 

[Cutlass327]

Do I really need a mica on the front part of the metal? I could see it if I was using a metal screw, but using a nylon screw I don't see an issue. I found some mica on Amazon, TO-3PL design. I figure if it is on the face, not between the chassis and the chip, shape shouldn't matter if it is just insulating the screw from heat.

 

[Cutlass327]

And just being curious, I found a few on eBay for $5-$10

https://www.ebay.com/itm/TA7222AP-5...528968&hash=item58a0407e48:g:P10AAOSwxudbadN~

 

[Robb]

The mica only needs to be between the IC tab and the chassis.

Would you put some heat sink compound on it?
Yes - please. Two small scoops - one for each side of the mica.

Attach nylon screw and do NOT over tighten.
That is why they break.

Ceramic insulators crack too easily and are toxic ('beryllium').
Better off with real mica or the sil-pad.

 

[nomadradio]

For years we would use an american-thread 4-40 plastic screw. A matching metal nut on the back side of the heat sink will be necessary.

Back in the day a local electronics distributor would have "Waldom" brand hardware on hand. Usually included plastic screws, but only in US sizes.

The M3 screw used by the factory and the matching nut are on Ebay if you turn over enough rocks.

Odds are the hole in the heat sink is not threaded. If it is you won't need a nut for the replacement metric screw. And if it IS threaded, you'll want to know that the thread pitch is 0.1 mm.

A junked radio would willingly donate a long, skinny TA7222 mica.

You can slip a common TO-220 rectangular mica under the chip's metal tab, one on each side of the screw. No need to trim it down. The tiny gap just above and below the screw hole won't cause a short from the tab to the heat sink so long as each mica is butted against the screw shaft. I suppose you could nibble a tiny notch in one or both where the edge meets the screw shaft, and close that gap.

More than one way to skin a cat.

73

 

[Cutlass327]

Ok. I had noticed the metal nut on the broken piece, that is what I heard rattle as I was moving the radio around. I was going to use a nylon nut from the kit when it comes, ordered from eBay the other day, but guessing the metal one is less prone to heat issues?

The mica is still behind the chip, I don't remember seeing any gaps between the chip and chassis, and there is still a decent amount of paste in there holding the mica in place.

 

[nomadradio]

In that case, the screw is all you need.

Any chance a local hardware store has 4-40 thread nylon screws?

And nuts.

73

 

[Cutlass327]

I'm planning to hit Lowe's tomorrow while I am out getting groceries. I'm going to look there - I ordered an assortment from eBay, didn't realize it was a slow boat from China... I'm not waiting a month or so!