Thread: Zx81
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Old 26th Mar 2020, 6:19 pm   #199
Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 9,183
Default Re: Zx81

I don't really like to be the *Voice Of Doom* but I think it might be a bit early for John to try to remove a 40-pin IC intact from an antique double sided PCB, especially when there is absolutely no need to attempt that.

As with the keyboard decoder IC which was replaced in his PET, by far the better approach at this stage would be to cut all of the ULA pins high up beside the body of the chip, remove the body of the chip, desolder each pin individually and then use the desolder tool(s) to clear the holes ready for the replacement ULA (or the socket for it) to drop into.

Needless to say this should only be done when the replacement ULA is already physically in front of you - you don't want to chop up the old chip and then find that oops, there is a problem in the supply chain and you aren't going to get one after all.

John, if you do decide to try to get the original IC out in one piece as some sort of challenge, the one thing you absolutely MUST NOT do is remove 90% of the solder and then try to lever the chip straight up off the PCB with a screwdriver or similar wedge between the body of the chip and the PCB. If you do, some of the top side PCB pads and probably some of the through hole plating will come off with it. The sharp end of the screwdriver / lever may also cause damage to any PCB tracks under the ULA.

If you get to that stage the correct way to release the chip is to use a blunt wooden object like the square end of a small brush handle to push the chip from side to side, back and forth, until the little traces of solder which are keeping the chip in place crack and let go.

Let me put it this way: The PCB is irreplaceable, damage that and the whole machine may be lost. The chip is almost certainly duff, so it's better to destroy the chip and remove it in pieces than it is to destroy the PCB.
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