Sinclair ZX80 / ZX81 / Z88 Forums

Is there a definitive list anywhere of the different issues and versions of the ZX81?

I opened up my one for the first time today, as I fitted a new membrane. I've taken others apart before, but not this one. It says "Issue One" on the board, and has "1980" on it. It's got a green PCB (aren't some of them red?) and it has four chips inside (don't some have five?).

I tested it to see if it had the old ROM with the square root error, and it doesn't.

What variations are there out there? I'd be interested to hear what you've all got.

Lee

I have an "Issue One" board in front of me - it too is in green. But it definitely has 5 chips (the ULA, ROM, Z80, and two R12038 chips - one is in the section marked IC4 on the circuit board).

I have 6(or 7..lost count!) zx81's plus 3 TS1000's , some are issue 1 some are issue 3, some have green resist some have red, and some have 5 chips some have 4!! But can't remember which variations seem to go with each other - maybe anything is possible! Have never yet seen an issue 2..does it exist? Only thing I do know is that the earlier ULA 2C184E is a problem!

Issue 1 boards are with the curvy, hand-drawn traces. I have one which has green soldermask, not sure if other colors soldermask were used for this issue board.
Issue 2 boards are nowhere to be found in the wild, so this would be either non-existent or extremely rare prototype / pre-production version (first incarnation / test samples of issue 3 board?).
Issue 3 boards are with the CAD-produced straight traces. I have several with red soldermask, but wouldn't surprise me if other colors exist. AFAIK both issue 1 and 3 boards are common.

ULA's come in 2 versions: 2C184E which lacks the back porch in its produced video signal. This causes many modern TV's to show a dim image with white parts shown as dark grey. The later ULA 2C210E fixes this issue.

The RAM comes as 2 small 18-pin (1Kx4 bit) SRAM chips, or a single 24-pin SRAM (1Kx8 or 2Kx8). So a duo of 1Kx4 RAMs will only be found in 1KB machines (=UK models, IIRC US-sold models came with 2KB RAM standard).

For Z80 and ROMs I've seen several manufacturers (well at least Nec & original Zilog for the Z80, maybe other 2nd sources were used too?), and many different ones for the RAM chips.

Apart from ULA version (=a pure hardware / video issue), ROM contents and RAM size, all of these differences are invisible from programmer's point of view. So

gozzo wrote:But can't remember which variations seem to go with each other - maybe anything is possible!

Wouldn't surprise me if many different sets of parts were used & changed often in production depending on price / availability etc. Remember that ZX81 was primarily designed to be low-cost... Also when you grab a ZX81 it might be hard to tell whether board is original, repaired / replaced at some point, ULA's swapped etc.

I have also seen mention and a picture somewhere - can't remember where - of another ULA variant - 2C158E ???!!! I haven't seen one personally, maybe these were the first versions? This ULA was shown in conjuction with a small add-on PCB on top of it --- ????

One of my zx81's has a UV EPROM chip instead of a mask-programmed ROM - this looks original, as it has "ZX81" actually printed on it next to the type number (2564) plus several pins bent out and soldered to wires going to various pcb pads...???? Anyone else seen one of these??

gozzo wrote:I have also seen mention and a picture somewhere - can't remember where - of another ULA variant - 2C158E ???!!! I haven't seen one personally, maybe these were the first versions? This ULA was shown in conjuction with a small add-on PCB on top of it --- ????

I can confirm that the 2C158E ULA exists, I have recently replaced one against Andy's ULA so it's now in my toolbox. But my ZX81 did not have the addon Board. The addon Board doesn't correct the ULA but the masked ROM that had an error. I also have an original ZX81 eprom in a different machine. That was used when there were production problems with the masked ROM.

gozzo wrote:One of my zx81's has a UV EPROM chip instead of a mask-programmed ROM - this looks original, as it has "ZX81" actually printed on it next to the type number (2564) plus several pins bent out and soldered to wires going to various pcb pads...???? Anyone else seen one of these??

I read in Wikipedia article that some early ZX81's were shipped with this SQR bug in the ROM even while that bug was known (!), and some were returned because of that. I can imagine that EPROMs were used to fix those (and perhaps new production going out @ the time), while updated ROMs were on order. Of course that's only speculation on my part.

Since an 8KB EPROM (28 pin) is not pin-compatible with an 8KB ROM (24 pin) those bent pins + wires might have been factory re-work so printed labels makes sense. Would seem like relatively much assembly work vs. place part & solder, so that would have been fixed with updated ROM chips quickly I think... Btw. expect that EPROM to go bad one of these days (grab any EPROM datasheet, and look for "data retention").

The 2c158 ULA does exist as I have three at this moment in time and others in the past.


This has been mentioned before but to save looking here's the early ULA and ROM fix for those who haven't seen it.

I have to say though that it didn't totally cure the SQRT problem, as I discovered when playing some of Andre's games.
He makes use of SQRT calculations in some of His work and when played with the hardware ROM fix CPU some of the games don't play as they should do, plus it doesn't pass the logic tests that the "Shoulders of Giants " ROM does. fit a standard ROM and CPU and all's well.
Also early issue three boards were green later ones red as were the American Timex ones.

Moggy.

PS Many moons ago a Guy who was selling some zeddie stuff on Fleabay claimed the hardware add-on was an after market fix that You had to have done Yourself. I personally never saw it advertised in any mag so I always considered it to be wrong unless Anyone knows different.

RetroTechie wrote:
Since an 8KB EPROM (28 pin) is not pin-compatible with an 8KB RO ( SQR bug in the ROM even while that bug was known (!), and some were returned because of that. I can imagine tha24 pin) those bent pins + wires might have been factory re-work so printed labels makes sense. Would seem like relatively much assembly work vs. place part & solder, so that would have been fixed with updated ROM chips quickly I think... Btw. expect that EPROM to go bad one of these days (grab any EPROM datasheet, and look for "data retention").

Although 28 pin I always thought the 2564 was a direct drop in replacement for the zeddies 28 pin socket therefore compatible.
The reworked wires may serve some other purpose.the only reworking I have seen or done is with 2764 EPROMS

Moggy.