I did a Rubber Jonnie and bought another computer

[PhilC]

And today arrived a rev 1 zx81. I already had a dead rev 3 that showed absolutely no signs of life.

So I plugged in the power and video after fitting the composite board and was greater with a nice white screen. Unfortunately lacking a K cursor.

20220303_200609.jpg (252.04 KiB) Viewed 2509 times

So I've had the scope out to try and work out what was going on. The long and the short of it was.....

Replaced dead cpu
Replaced dead ram
Replaced dead rom

Everything seems to be chugging along nicely now, lots of activity on all the new chips but still no K.

So it looks like we now have a dead VLA chip.

Glad I never plugged that PSU in that came with the ZX81 as I'm willing to bet it would kill it all again.

Time to go ZX81 ULA shopping.

[stephen_usher]

That's a very complicated composite video mod. I just use a 100uF capacitor and be done with it. Works well.

Surely if you have a white screen then the CPU must be generating the video otherwise you wouldn't even get a sync pulse.

[PhilC]

@stephen_usher everything looks good from the hour I had to spend on it. The data lines, address lines, ram CE etc all give a good indication of it working but still no K.

I need to read up a bit more about it I'm thinking about getting a VLa81 for it or one of the pcbs as I've got the xilinx chip it uses.

[Icky]

Is this a term now @PhilC when buying a retro computer

[PhilC]

Is this a term now @PhilC when buying a retro computer

It certainly seems that way @Icky

Wait till ebay renames their vintage section to the RJ section

[rubber_jonnie]

Is this a term now @PhilC when buying a retro computer

I guess it is now

[rubber_jonnie]

Is this a term now @PhilC when buying a retro computer

It certainly seems that way @Icky

Wait till ebay renames their vintage section to the RJ section

They better ask me first!

[rubber_jonnie]

And today arrived a rev 1 zx81. I already had a dead rev 3 that showed absolutely no signs of life.

So I plugged in the power and video after fitting the composite board and was greater with a nice white screen. Unfortunately lacking a K cursor.

20220303_200609.jpg

So I've had the scope out to try and work out what was going on. The long and the short of it was.....

Replaced dead cpu
Replaced dead ram
Replaced dead rom

Everything seems to be chugging along nicely now, lots of activity on all the new chips but still no K.

So it looks like we now have a dead VLA chip.

Glad I never plugged that PSU in that came with the ZX81 as I'm willing to bet it would kill it all again.

Time to go ZX81 ULA shopping.

I got a vla81 from here: https://vdrivezx.com/vla81/ works a treat.

[PhilC]

And my new VLA81 arrived and fits in the board quite nicely.

But the ZX81 still refuses to boot. It is a step nearer now which is good. I'm getting a good /halt and a few other bits. The rom is getting very hot, so I'll have to try and work out how to get a new rom sorted for it.

[rubber_jonnie]

What is it doing right now then? What are the symptoms, if you replaced the ROM, RAM and CPU then there's not a lot left.

Mine needed a CPU and VLA81.