The Sinclair QL Forum

Hello,

Three years ago I revived my 1984 QL on a recent LCD TV ; this last month, I tried to resurect even older things.

First, my 1981 ZX81 : I had to repair the keyboard wires (the membrane is still OK - all keys - but the connections with the circuit board were partially cut where they were bent. It works, but only on an old style black&white TV. I don't understand why when plugged into the UHF of my recent LCD TV, it gives a too dim picture ? Anyway, I even could load programs from a cassette and with a recorder >30 years old ! Maybe, I will now save them as MP3 files on my recent PCs ?

After, I resurected an even older computer that I had built as an Elektor KIT : this was in 1979 my second µP system, their so called "Game Computer". It is powered by a Signetics 2650 µP linked with a Signetics 2636 PVI (a video coprocessor for managing sprites, backgrounds, joysticks, etc. - videogame oriented). This Elektor system has a 2Kb rom monitor for (hex) machine code input-save-load-run, 5Kb ram for (your) games, and a lot of chips around that... I successfully could load games from a cassette and with a recorder >30 years old, but I first had to "burn" a new 2716 Eprom (2Kb) because after all these years, I found two faulty bytes in my original rom. I also took the opportunity to correct a third byte that was a known bug. Then I tried to resurect my first µP system, also an Elektor KIT built in 1978, based on the SC-MP, but the monitor rom appeared to be faulty too, and alas, I cannot repair this one : my Eprom programmer accepts only 2Kb and up, and the monitor of this system is on three 512b roms.

No problem, this first "antediluvian" system was actually a lot less interesting than the "game computer" : it could only blink some leds and do some beeping.

But the "game computer" I will now use again, especially as I have nowadays access to a lot of documents that in 1979 (before the WEB) were not so easily available.

And the 2650 processor - to my opinion - has an architecture and intruction set almost as interesting as the 68xxx much more powerful chips - when indeed you program only in assembler and on a rather small memory map).

Lot of (nostalgic) fun ahaid, I guess ?

Bye, Paul

Here is an old picture of my late SC-MP system : Bye, Paul

The ZX81 keyboard connection is peculiar - someone has replaced the original molex keyboard connectors on the motherboard with a bit of flat ribbon which has to be bent over the motherboard to attach to the solder points on the bottom of the board.

You can purchase replacement molex connectors and fit these to the motherboard and then the keyboard membrane will just push into the connectors (as it would have originally done so on the ZX81).

The dim picture on a modern TV is due to the ULA in your ZX81 missing what is known as the back porch signal which modern TVs require in order to latch onto the output signal from the ZX81. There are three options to fix this:

a) Purchase a replacement ULA for the ZX81 (these are thankfully now fairly readily available).
b) Purchase and fit a ZXVid module which fits inside the TV modulator on the ZX81, and provides a stable composite video output signal
c) Purchase and fit a ZX8-CCB module - this is similar to the ZXVid, but can require more fine tuning (the ZXVid has never required any changes) - it is designed to fit loose inside the ZX81 case, but can just about fit inside the TV modulator.

All of these provide the missing back porch signal.

Those old kits look like something from a sci fi film set

RWAP wrote:The ZX81 keyboard connection is peculiar - someone has replaced the original molex keyboard connectors on the motherboard with a bit of flat ribbon which has to be bent over the motherboard to attach to the solder points on the bottom of the board.

You can purchase replacement molex connectors and fit these to the motherboard and then the keyboard membrane will just push into the connectors (as it would have originally done so on the ZX81).

I think what he's done here was shortened the tails on the membrane due to them being damaged further up, and then de-soldered the molex connectors and put them on the end of a piece of ribbon cable so they can plug into the now shorter membrane tails.

I've done this myself in the past before replacement membranes became available.

Cheers.

Phill.

Those old kits look like something from a sci fi film set

Sci-fi ? That would mean "anticipation" more than 30 years old

When I was a kid in the fiftees (not a kit ) I was a fan of the american sci-fi cartoons published by Artima (in France, I guess), and I remember well a story of a kid building a kit computer and using it to calculate a winning base-ball tactic. When in 1981 I bought my ZX81, I thought "well, these times have now come" ; this was more than 30 years ago ! Could we imagine then all that we live today ?

I was born end may 1948 ; the first true computer (the english EDSAC, google for it) stuttered its first computation end may 1949, I must have been a toddler by then. And my first contact with a computer (feeding punched cards of fortran through a "ticket counter" ? for a big air conditionned computer ) was in May 1969.

Old timer Paul

Hehe, I love hearing about the old stuff. It wasn't a proper computer in those days unless it had a hatch where you could climb inside