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Gary C

Good end to a great week

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After rectifying my Einstein and sorting the sound out on my Amiga, I then decided to look at three other acquisitions. Two I have had for several months (a ZX81 and a VAX 4000 model 96) and one new one (A VAX 4000 VLC)

Firstup was the VLC. Powered up, system password changed and its working great. Not much more to say on that, other than its a great little machine. A VAX you can almost stick in your pocket. Buoyed by this, next was the 4000/96. This one was obtained some time ago, but would not power up reliably. Stripping down the system revealed that the Turbochannel option card has been installed without any standoff posts from the main motherboard. This means that when a card is plugged into it, the board shifts and half pulls itself out of its plug. With this reset, the PSU fires up.

Unfortunately this is as far as it went. The boot diagnostic LED's on the front (took me a few moments to find them, they are normally on the back on older machines) showing an unlisted code right from power on. For some reason I suspected the memory, and lo and behold, the SIMMS were in the wrong sockets. It has two banks of memory and each bank must have all four SIMMS installed to work, however the sockets for them are not sequential and I had three in bank one with one in bank 2. Quick swap and the console now comes up. Box has two RZ26 1GB drives, so a quick SHO DEV to make sure they are working and, oh dear, the system disk is making a terrible noise and wont work. The second drive works fine but with no system to boot, playtime is now on hold until I can sort either a new disk with a system on it (might just be able to find one in the pile of old VAXs that I rescued this one from) or I will have to install VMS onto the other drive.

So onto the ZX81. It was only the second machine I bought and has sat in a cupboard for about 18 months due to its dire state. Its an issue 1 unit but the keyboard tails were fragmented, the board had heavy corrosion and it refused to start.

Looking at the board, I decided to try and fix it. First all the sockets were removed and several corroded through pins found but only one track broken at a pad. Some new sockets and minor repairs + cleaning and now the CPU is running and the ULA is at least giving the basic video signal.
Address lines all looked good with healthy waveforms, but the data bus was really strange. There were signals, but they peaked at about 5v but would only go down to about 3.5v. Obviously something trying to hold the data bus high and fighting the others. With the RAM chips popped out, the signals now could be seen to vary between 0V & 5V so RAM suspect. Some new chips installed (after straightening the legs, why do people send chips through the post just in ESD foam !) and bingo, now running.
Keyboard next, and while the original could possibly be saved, there are some amazingly good reproductions and I wanted this machine to be useable rather than a museum piece. New membrane is slightly thicker than the original which would cause a slight crease where the ribbon enters the case, but the original case also has a slight lip so with a bit of adjustment, the new keyboard is in and looks perfect.
It came with a Memopak 16k that I had never been able to test, but what do you know ! it works fine. Just loaded up Invaders from a WAV file and its working.

Now, wheres ant attack

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Tags: vax 4000, zx81 Add / Edit Tags


  1. Gary C's Avatar
    oops, Ant Attack was for the Spectrum