I took a chance on a cheap "untested" but boxed Sinclair ZX81
Obviously I know when the seller claims it's "untested", they mean it didn't work when they last tried it. But in some cases, like with my Atari 2600 Jr, it can be something simple which the seller doesn't know about, or can't be bothered with
The Sinclair ZX81 was the follow-up from the ZX80, owners of the ZX80 could replace the ROM IC to almost upgrade to a ZX81.Like the ZX80, the ZX81 was available in kit form, or fully assembled, and it was designed to be cheap, and it shows from the keypad to the lack of sound and colour, but this didn't stop it selling by the boat load, particularly in the UK, where other systems were much, much more expensive
But with only 1KB of RAM available in it's default form, programmes were limited. But there were memory expansions available, which gave the system a total of 16KB, which made it a much more capable machine, but only if you could engineer a solution to the "RAM Pack Wobble" problem
In the US the ZX81 was known as the Timex Sinclair 1000, and shipped with 2KB of RAM
The ZX81 was succeeded by the Sinclair ZX Spectrum
I haven't seen a ZX81 for many years and I forgot just how tiny this computer is, it's 175mm long in it's longest dimension, and only 40 mm high. It weighs a colossal 350 grams. This machine will almost fit in the palm of your hand
Also note early ZX81's had no label with any kind of serial number present, I checked around online and it appears only later models would have them
What the significance of Barter R (P) is, I have no idea, it wasn't related to the seller in any way
Connectivity wise the ZX81 is limited to RF out (TV), cassette in and out on the left side, and a power socket. Note the 9V DC socket is the same as the EAR and MIC sockets
And an edge connector on the rear of the device, commonly used for memory expansions as the ZX81 only had 1Kb of RAM internally
A close-up of the keypad, note the distinctly British RUBOUT and NEW LINE keys
The box, or cardboard sleeve, is not in the best condition, but you can see the printing on the top and bottom
CB2 1SN isn't found by Google Maps, but 6 Kings Parade in Cambridge is
The polystyrene box is in good condition except for some masking tape, not sure why it's there, the polystyrene isn't cracked or broken, but the tape's been on there for so long, it won't come off easily
Also now it's S.+R. Barter (P). No, I still don't know
This looks like the original RF lead, it's not in the best condition, so I'll use another for now (I have loads as you would expect)
Before powering up the ZX81, I'll just pop it open and check everything is in order and reseat the IC's, It would appear the membrane tail has cracked, so the keypad will have issues
There are only four IC's on the motherboard, from the left we have the Ferranti ULA (Uncommitted Logic Array), 16K ROM (Yes, it's supposed to be smaller than the socket), NEC Z80 compatible processor, and the 1KB of RAM. The 5V regulator is at the bottom attached to a heat sink
Above the processor is the text ISSUE ONE, making this an issue one ZX81, and compared to it's peers, there isn't that many components on the board, most of the work is done by the ULA
My ZX81 didn't come with a power supply, so I used the PSU I bought for my Atari 2600
The ZX81 needs 9V DC at at least 700mA, and a 3.5mm jack plug with a positive tip, why Sinclair had to use the same kind of plug for the power as the tape recorder connections, and then put them right next to the tape recorder connections, I don't know
I wonder how many ZX81's were damaged by someone putting 9 volts down the EAR or MIC sockets?
Powering up the ZX81 and it boots to an inverted B (it should be a K), and only the top row of keys respond because of the broken membrane, however the characters are all garbled, so it looks like the ULA is faulty, original ULA's are hard to come by, but there are modern replacements available for around £20.
So I will need to replace the keypad and ULA to get a working machine, but the video output looks great, even through RF
Here are some photos of the ZX81 with other machines of the era
And it does just about fit in the palm of my (admittedly fat) hand
Update, my second, and fully working ZX81
This one came with the original PSU, which was tested and confirmed working, I got confused and thought it was a negative tip like the later Spectrum and Commodore C16, that's why it's showing a negative voltage
This one is very clean and tidy, and all the keys work
Nothing unusual about this one
The PSU in all it's glory, I still think it's a stupid idea to use the same plug on the PSU as the mic and ear sockets take. The side profile matches the side of the ZX81
The listing claimed it was fully working, and it is, every key works as well
The monochrome picture via RF looks amazing on this TV, however, there's no sound carrier, so you get loud white noise
The usual BASIC programme to check it works...
10 PRINT "IANWILLIAMHILL.CO.UK"
20 GOTO 10
RUN
Update, another machine for the collection
I managed to acquire another ZX81. This one kinda works, but has poor RF video output, however, this one came with accessories
The RF output is all over the place, and the picture breaks up a lot, but it does seem to work, all the keys respond how you would expect. I will probably use the ULA and membrane from this one to make the first ZX81 work as that has stable video, but a failing ULA
It's a bit grubby and has the remnants of sticky tape around the back, likely to cure the "wobble problem"
I got another PSU, this one outputs 15V with no load, this is a UK700, whereas the PSU above is a UK1200
I finally got the best user manual ever written
A ZX printer, I've no idea if it works, it will probably need a belt, and some paper
A boxed 16K RAM pack, this has the same tape pattern over it as the ZX81, it looks like security tape
Close up of the instruction sheet, with the BASIC programme to check the RAM
Software
Football Manager
Night gunner
Fun To Learn Series - English Literature 2
Long live the Sinclair ZX81!
?BAD DATA ERROR