On Jun 05, 2016 07:09pm, Dave Drum wrote to Mark Lewis:

Dave,

I don't know if this will fix it but I have access to a couple of broken C64s
that may fit the bill.

Allen

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This text file describes what to do in order to fix Amiga 500(if one
of the CIA's is dead) with the parts taken from broken Commodore 64.

I SHALL STATE THAT THIS IS A HACK, ITS A HACK! ITS A HACK!!! HOWEVER
CONFIRMED, IT WORKS FOR ME.

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OPERATION:

So lets start. If your Amiga 500 has one of the two CIA's dead then you
can fix it for OS use and some rare NDOS games/demos. There are two such
chips(8520A), one labelled ODD CIA (A) at U7 and the second one EVEN CIA
(B) at U8, they are both socketed. The simpliest method to check which
one is dead, if Miggy does not start at all, is to remove the B one and
power it on. If it does start and shows up the so famous floppy screen,
then its obvious B died. But if it does not display anything then you put
B in place of A and try again, and if it does start then you leave it
there and go to strip the replacement chip from broken Commodore 64.

Note, if swapping chips does not work then you can stop doing anything
further as this may not be CIA's fault or (not likely to happen) both of
them are dead - and if so then you may skip as there is a need for at
least one 8520A!

In C 64 there are also two CIA's and they are at (locations according to
new pcb) U1 and U2 they have printed 6526B on them. So now you just need
to desolder one of them or both, clean up the pins and put in your Amiga
in place of B one. The pinouts are same so nothing can go wrong.

There are several mothods to strip the chip from pcb. I used ordinary
soldering iron and the simple sucking device to pick the tin. If you got
no such tools then you may use simple handy gas burner directly on the
pins from some distance. But before doing this set the nozzle not to cut
but to distribute the heat smoothly. Then try to toggle the burner form
one side to another and if you notice tin melting grab the ic - be
cautious its hot! Also, dont heat the chip too long!

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DOWNSIDES:

You can only use 6526B in place of EVEN CIA (B), why? I dont actually
know why doesnt Amiga boot when put in ODD CIA (A) as i have no plans
of these chips to see what are the differences between them. I however
suspect that this might be something with the registers or timers but
that is my own opinion, so dont take it as a real issue. Some day i try
to fiddle with some system monitor and peek-poke to see what is
different from under the OS.

As you read at the begining some NDOS games/demos might not work,
especially the ones who bang the CIA's directly or do some trickery
around them.

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OTHER IDEAS:

If you dare you can check if 8520A will work in Commodore 64 :) -
unfortunately, i wasnt able to check that since my C 64 has burnt
memory. I recommend to solder the socket first.

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megacz{at}usa.com

/\llen

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