Sinclair ZX80 / ZX81 / Z88 Forums

I bought an issue three zeddy which sadly has arrived DOA. It looks like it has an EPROM? I think I saw one like this in a recent post.


I am wondering if the EPROM is likely to be the weak spot?

If I disconnect the wire just above the RAM chips can I simply unplug it and put a normal ROM in there?

Thanks!

Lardo

As the ROM socket is incompatible with 27xxx series EPROMs, “mod” wires are required. If there are no track cuts or other wires on the circuit board, then yes, it should be possible to remove the EPROM and it’s wires. Then a normal ROM chip should work.

EPROMs were never intended to last much more than ten years. But then, neither were ZX81’s
Some EPROMs may forget some of their programmed data after all these years, but other than that, they are no different to any other chips in a ZX81 in terms of reliability (well, apart from ULA’s being the most unreliable part). The RAM chip(s) and the Z80 then racing to being the second most unreliable parts IMHO.

Mark

Thanks. There are no additional wires. Track cuts may be less easy to spot...

I have poked this with a logic probe following the same pins as suggested when trying to get my rebuilt zeddy going and nothing is pulsing. Everything is pretty much high or low or in some cases nothing at all.

The ULA power pin is at about 4.98 volts.

Does that point at any obvious culprits?

Before seeing the EPROM my original plan was to remove the ULA and then Z80, put sockets in and see if other chips worked.
The EPROM situation complicates this as I won’t know if the socket is actually working if I plug in a normal ROM having removed the EPROM. Argh.

There is another issue 3 with an EPROM in this thread...

Mark