Query on a Issue 1 ZX81

[gozzo]

oo-er, wonder what that extra hand-wired chip is for? as that's a USA ZX81, they are usually 2k?, and the RAM chip appears to be a normal 1k 4118, so...maybe there is actually 2 4118's piggybacked, with that extra chip for the select signals??!!

[PokeMon]

Sounds good, the only thing I'd do to this one is to install the CCB module inside the modulator to make it usable.

B

I wouldn't slaught the modulator on the Issue One.
I would modify the second one and leave this as a collector value or I would only solder the 3 points on the board (+5V, GND, ULA #16) which is fully reversable.
The output could be via a chinch cable (like an extension cable with a female connector) soldered to the video board, fixed internally with sticky double sided tape and let the cable go out through the connector for example. This way modules can be connected and the RF output is working.

[gozzo]

No need to 'butcher' the modulator, just disconnect the internal connection to the centre contact of the phono socket, and connect the output lead of the CCB to it through one of the side holes..easily reversible...

[balford]

I don't know, unless a machine is absolutely pristine and boxed, I'd rather modify it as nothing bugs me more than machines not being used

Here's my own Issue 1, with gutted modulator but I still have the old innards so if I ever wanted to put it back to original I could. With the lid on it's hard to tell it's been modified



B

[1024MAK]

As an aside, a while back I found a picture of an Issue 1 board, with component positions as per issue 1 locations, but without the hand-routed trace layout:



Perhaps this is the reason there's no issue 2?

B

There are some differences between the issue 1 USA and the UK issue 1 boards:-
C8 slightly repositioned and different track layout between C8 and the edge of the board.
Silk screen for R32 / R31, also C10 / C12, and R29.

Mark

[gozzo]

Balford - you can still modify and fit a CCB or similar device and use it on modern TV's or whatever WITHOUT removing the innards of the modulator(just disconnect it), it is totally unnecessary, I personally would never do it.

[balford]

I completely understand - different strokes for different folks

B

[PokeMon]

Very nice picture from your Zeddy board, Brendan.
Excellent work.

[Moggy]

ZX81 Variations

There are many variations of ZX81 boards.

There were (as far as we know) only two board (PCB) issues that were manufactured: issue 1 and issue 3. Issue 1 boards generally have a green solder resist layer on the bottom. Issue 3 boards generally have a red solder resist layer, however early issue 3 boards use a green solder mask. However, there may be other variations.

In order to try to keep costs low, Sinclair used various supplies of compatible or equivalent components. You you will see various brands of Z80 CPUs for example.

Sometimes none of the chips are socketed. Sometimes the ULA, the Z80 CPU and the ROM are socketed. It is less common to find the RAM chip(s) socketed.

Three different RAM chip types were used. Timex 1000 machines had a single 24-pin 2k byte RAM chip. ZX81s had either two 18-pin 1k x 4 bit SRAM chips (making a total of 1k x 8 bit = 1k byte) or a single 24-pin 1k byte SRAM chip. These were supplied by a number of different manufacturers.

ULAs
There are three ULA chips, two types are common.
The earlier 2C158E type have a four digit date code on the top (eg 8111 meaning year 81, week 11), the type number is on the bottom, but these ULAs are normally soldered directly to the board.
The most common type is the 2C184E. These have the type number on top and are often in a socket. Both this ULA and the earlier 2C158E ULA generate a TV picture which lacks the “back porch” part of the video signal. This causes many colour TV's to show a very dark image with white parts shown as dark grey.
The best and last ULA version is the 2C210E. This ULA does generate the “back porch” part of the video signal, and so is the most compatible with colour TVs.

ROMs and EPROMs.
Although Sinclair used a number of manufacturers for the mask ROM, and sometimes used EPROM chips, there are only two different versions of the code. The first (“old” ROM) code has a significant bug in the mathematical section of the code (the SQR problem). So Sinclair modified ZX81s that were still being made / had been made using mask ROMs that had the faulty code. These ZX81s were fitted with a small daughter board containing two 74 series LS TTL chips. This board fits on the Z80 CPU and effectively changes one byte of the ROM code so reducing the impact of the bug.
The second version of the code (the “improved” or “new” ROM) corrects this bug. However, the mathematical routines are still not as accurate as they could be.

Dating a board
For most of the production of ZX81s, Sinclair did not use serial numbers.
The best way to date a Zeddy is to compare the date codes on the chips and the modulator. If the modulator has not been changed, this is the best source for the date.

More info in these forum threads search.php?keywords=%2BULA++2C158E

A large amount of the above information compiled from the friendly folk on the Sinclair ZX World forum

So just keep in mind that what makes your first Zeddy rare is the components used and the extra board that "fixes" the buggy ROM code. Issue one ZX81s are common.

Good luck with you new purchase

Mark

Excellent post the best most succinct and accurate description of a Zeddy I have seen yet.
perhaps Rich, it could occupy a pinned heading of its own for newcomers seeking information, to look at

[balford]

Very nice picture from your Zeddy board, Brendan.
Excellent work.

Cheers Karl, I've added some more photos in my new thread over in Misc so as not to derail this one too much