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I did a Rubber Jonnie and bought another computer
Posted: 03 Mar 2022 20:21
by 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
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.
Re: I did a Rubber Jonnie and bought another computer
Posted: 03 Mar 2022 20:35
by 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.
Re: I did a Rubber Jonnie and bought another computer
Posted: 03 Mar 2022 20:52
by 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.
Re: I did a Rubber Jonnie and bought another computer
Posted: 03 Mar 2022 20:55
by Icky
Is this a term now @PhilC when buying a retro computer :D
Re: I did a Rubber Jonnie and bought another computer
Posted: 03 Mar 2022 20:58
by PhilC
Icky wrote: 03 Mar 2022 20:55
Is this a term now @PhilC when buying a retro computer :D
It certainly seems that way
@Icky
Wait till ebay renames their vintage section to the RJ section :lol:
Re: I did a Rubber Jonnie and bought another computer
Posted: 03 Mar 2022 22:21
by rubber_jonnie
Icky wrote: 03 Mar 2022 20:55
Is this a term now @PhilC when buying a retro computer :D
I guess it is now 😁
Re: I did a Rubber Jonnie and bought another computer
Posted: 03 Mar 2022 22:22
by rubber_jonnie
PhilC wrote: 03 Mar 2022 20:58
Icky wrote: 03 Mar 2022 20:55
Is this a term now @PhilC when buying a retro computer :D
It certainly seems that way @Icky
Wait till ebay renames their vintage section to the RJ section :lol:
They better ask me first!
Re: I did a Rubber Jonnie and bought another computer
Posted: 03 Mar 2022 22:24
by rubber_jonnie
PhilC wrote: 03 Mar 2022 20:21
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.
Re: I did a Rubber Jonnie and bought another computer
Posted: 08 Mar 2022 15:40
by 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.
Re: I did a Rubber Jonnie and bought another computer
Posted: 08 Mar 2022 15:45
by 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.