ROMs

Discussions about Sinclair ZX80 and ZX81 Hardware
citizenfish
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Re: ROMs

Post by citizenfish »

1024MAK wrote: Wed Jul 07, 2021 6:56 pm How bad? Pad broken? Through-hole plating ripped out?

Mark
Through hole came out with the chip and broke a trace that came with it
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1024MAK
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Re: ROMs

Post by 1024MAK »

I assume you intend to fit a socket.

If there are/were PCB tracks on the top (component side) for the holes where you damaged them, then cut off any tracks that have come loose and which are floating around.
Use some wire strands from a piece of multistranded wire/cable or some 5A fuse wire or similar and solder one end onto the end of the broken PCB track. Then take the free end and carefully thread it through the relevant PCB hole for the ROM chip. Do this for each damaged pad/through-hole. Use a very small amount of hard plastic glue or superglue to hold the wires in place. But don’t use too much, and do use glue near the holes. If the distance that the wire goes is short, there is no need to use glue.

If there are PCB pads where there were no PCB tracks on the top (component side), ignore the above section.

Once you have wires replacing all the top side damaged tracks, use a multimeter on the continuity range (bleep mode) to test that your wiring is correct to the schematic circuit diagram. Keeping in mind that the routing used on the board may have used the through-hole connection to get from A to C via B. If this is okay, carefully put the socket in place while jiggling the wires so that both the socket pin and the wire fit through the hole. Solder only one pin for now and use a undamaged PCB pad. Now use a multimeter to test again.

When happy with the socket and the testing, the next step is to very carefully wrap each wire strand around it’s respective socket pin. Wrap the wire around tightly (but don’t break it) for two turns. Then lead the wire to the bottom side track where it is supposed to connect to (if relevant) and cut it to length and solder it to this PCB track (again if relevant). If it does not need to go to a bottom side PCB track, cut it off close to the socket pin. Now solder the wire where it wraps around the socket pin.

A similar method is used on the bottom side where a PCB track/pad is damaged on the bottom side, but is not used on the top side. Cut off any tracks that have come loose and which are floating around. Use a wire strand and solder one end onto the end of the broken PCB track. Then take it to the socket pin and wrap it around the pin as described above.

Next, again test using the continuity range, this time, test EVERY combination of component connections for each pin. That is, for address line A0’, test all these combinations:
IC2 (ROM) pin 8 to R26 and to IC1 (ULA) pin 5
R26 (1k) to IC2 (ROM) pin 8 and to IC1 (ULA) pin 5
With the resistors you will have to try both sides (both leads) if it is not clear from the PCB layout.

Do the same for all the other pins. It would be wise to print out the schematic and then mark each circuit as you test it at each stage, use a different coloured pen for each stage, and mark each component pin as you test.

A schematic is available here ;)

Mark
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citizenfish
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Joined: Sun Apr 11, 2021 5:31 pm

Re: ROMs

Post by citizenfish »

Thanks Mark that is brilliant!
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