Strange ZX81.....

[Prime]

Ok just got a recently purchased ebay ZX81

Issue 3 board, 2C210E ULA, 2x2114 RAM, the unusual thing is the ROM, it's an Eprom and not a mask programmed rom.

I suspect it's a 2764, as it is marked 64K ZX81, and RTI8248 (date code), but has the TI logo on it also.
The eprom has patch wires connecting pins 2 and 22 to GND and pin 27 to a track on the mainboard (/ROMCS' ).
Pins 2, 22 and 27 are not inserted in the socket.

I somewhat suspect that this is a later machine being an issue 3, the box it came in has no end pieces just a cardboard tube round the polys.
It also came with an issue 3 rampack which is currently non-working, does anyone happen to have the schematic for it ? this interestingly
enough has a small ULA in it too, I gues to do the refresh and /RAS /CAS multiplexing, hopefully that's not what has died !

Cheers.

Phill.

[Andy Rea]

With that ram pack i'd start by checking the voltage rails, old electrolytic capacitors have a habbit of 'drying' out and loose the ability to work as a capacitor.

i'd imagine that the circuit bottom right as you look at the component side is similar to the original

ftp://ftp.worldofspectrum.org/pub/sincl ... matics.png

looks like that mini ula is replacing the refresh counter and most of the multiplexing...

Regards Andy

[Prime]

With that ram pack i'd start by checking the voltage rails, old electrolytic capacitors have a habbit of 'drying' out and loose the ability to work as a capacitor.

Yep done that all present and correct +5, +12, -5 IIRC one of the most common fault on spectrums is the similar voltage generator circuit failing, usually TR4 + TR5 so was first thing I checked
might also re-solder the expansion connector and check continuity as it does also look a bit dodgy

i'd imagine that the circuit bottom right as you look at the component side is similar to the original

ftp://ftp.worldofspectrum.org/pub/sincl ... matics.png

looks like that mini ula is replacing the refresh counter and most of the multiplexing...

Well the LS157 is still there, but I would imagine the ULA is replacing the rest, should be possible to cook up a GAL/PLD/LS replacement in any case.

Cheers,

Phill.

[sirmorris]

If you're going that far why not just replace the RAM with a 32k static chip?

C

[Prime]

If you're going that far why not just replace the RAM with a 32k static chip?

In practical terms yes, especially as I already have the static->zx81 ram socket boards made

But from the point of view that I hate having non functioning equipment hen, plus this is the only official Sinclair unit I have so would be nice to have it working.....

Cheers.

Phill.

[iturbez]

In practical terms yes, especially as I already have the static->zx81 ram socket boards made

Can you show us this board, please?

[Prime]

Dammit looks like the ULA is faulty, I'm not getting a /RAS signal out of it oh well.

Looks like it's back to ebay then

Cheers.

Phill.