Check out the hires picture of this issue three board.
Same as Yours perhaps?
Certainly less alteration is needed for 2764 EPROM
EDIT as its an issue three board that rules out the Klutzed early ROM theory a special of some kind perhaps?
http://www.binarydinosaurs.co.uk/Museum ... /index.php
http://forum.tlienhard.com/phpbb3/viewt ... ?f=2&t=207
A bit of Google translate needed
A sentimental revisit to when I was 11 and had my first ZX81
My "new-to-me" ZX81 is not working with my modern TV (which cannot tune in to the signal). So, I'm looking for an old TV. Before I buy one though, I think I need to get clear on what TV I need, as I have the ULA chip referred to below (2C184E) which apparently doesn't work with many modern TVs.
So either I need to go back really far to a VERY old TV (I'm guessing that means an old B&W CRT one) or upgrade my ULA chip which, it appears, may let me use any TV with a UHF tuner and Coaxial socket.
Can you in fact happily swap those ULA chips (Does that cause another problem?) Anyone know where to get the later one, 2C210E, I'd need?
Can anyone advise, please?
RetroTechie wrote: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
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.gozzo wrote:But can't remember which variations seem to go with each other - maybe anything is possible!
- Posts: 3633
- Joined: Mon Sep 26, 2011 10:56 am
- Location: Looking forward to summer in Somerset, UK...
New ULA here
Other options here
You can put a later ULA in any ZX81. Indeed in theory you can put an older ULA in any ZX81.
Despite what you have read, some modern TVs do work with the older types of ULA that don't produce a "back porch" in their video signal. For example, I have a Sharp Aquos LC-19D1E-BK colour LCD that works fine with all my ZX81s (and other retro computers, although I do sometimes have to manually switch the colour standard from Auto to PAL as when in auto it selects NTSC or SECAM!).
Also the problem is not limited to LCD and other modern types. It also affects some colour CRT TVs. For colour CRT types, in general it is the more expensive types that cause the problem.
The problem is that there is no way to know in advance which TVs will work (well other than 625 line black and white sets which should always work).
The other option is to buy an add in circuit module that produces a modified video signal (which includes a "back porch").
Sellers details are in the link I posted above.
There are also many posts / threads on this forum about the subject...