Attic 81's - My collection and attempts to repair
Posted: Fri Nov 20, 2020 5:19 pm
If it's of interest, and if I might ask for help along the way, I thought I'd share the details of my ZX81 "collection" - 3 ZX81s that had sat in a very hot / very cold attic at my late Father's house for several decades.
I've recently started looking at them and trying to see what works. In researching this, I found this forum resource, which I'm very grateful for.
Here they are, as I found them :
ZX81 #1
=======
Issue 1
In case with original keyboard. Probably my original ZX81 that I bought in 1982, possibly not original case/keyboard.
ULA 2C184E 8151 in socket
ROM MOSTEK MK36809N-IRL 8206 in socket
CPU ZILOG Z8400A Z80A 8152 in socket
RAM 24 pin MOSTEK 8203 MK4801AN MK4118AN-41 soldered wonkily to board
Modulator 8205
heatsink is larger but thinner piece of alu from old clock face fitted by Grandfather
ZX81 #2
=======
Issue 1
ULA Ferranti 2C210E 8242 in socket
ROM Sinclair Research P21669 D2364C 649 (c) 1981 in socket
CPU Zilog Z8400A Z80A 8147 W in socket
RAM 18 pin NEC R1Z2IY-204 PD2114LC x2 soldered
Modulator 8208
ZX81 #3
=======
Issue 1
ULA Ferranti ULA2C184E 8209 in socket
ROM Motorola? ZCM38818P 8244 in socket
CPU NEC P24488-218 D780C-1 in socket
RAM Motorola? MCM21L14P30 8225 in sockets MCM21L14P45 8223 Ram was inserted upside down!
Modulator 8217
no heatsink
So, all boards had different varieties of chips. 2 x older ULAs and 1 with the backporch fix.
I tried all 3 of them in turn, and all had the same fault - white screen, no cursor.
I know that my original ZX81, the one with the home made heatsink, HAD been working when it was put away, but that was several decades ago. The other two I think I'd got from a boot sale and a junk shop respectively. They may not have been working when I got them. I know I paid £1 at the boot fair, I may also have paid £1 at the junk shop, both back in early 1990s.
I swapped some chips around to see what I could achieve, of course not knowing which ones worked and which didn't. Starting with ZX81 #1, I fitted this with the newer ULA, initially without success. But after leaving it for a few hours and coming back to try again, it worked first time - a cursor! Picture wasn't great, but I had a cursor. Power off and on and the cursor had gone again.
I found it would work occasionally but not often. And when it did work, tapping the board made it fail.
I took the socketed chips out, sprayed contact cleaner in the sockets and gently cleaned and bent the legs before putting them back.
After a lot more fiddling around, I got the 81 to work reliably. I was delighted! Though I found the first power on from cold would give a white screen, subsequent power ons would work OK. I replaced the two electrolytic caps, but this didn't help. I thought I could live with this quirk though.
But when I tried to load a 1K game from a laptop via the EAR cable, I got error code 4. Out of memory. I tried a few 1K games, all did the same thing.
PEEKing RAMTOP showed a stupidly low value - I didn't write it down and forget what it was now. But it wasn't right.
So I tried a different board, and settled on ZX81 #3, as it had socketed RAM.
This didn't work, and I more or less randomly tried swapping in different ICs without success. But when I removed the ROM I found the legs very dirty, so removed all ICs and cleaned as I'd already done on ZX81 #1. On reseating all the ICs, it worked!
RAMTOP showed a 1K ZX81, and I was able to load a few 1K games from the laptop. Success!
What I hadn't spotted until many days later when reviewing the photos for this post, was that the socketed RAM was upside down until I reseated all the ICs. I had naturally put them back the right way round without second thought.
This is what is currently working for me :
It's not been perfectly reliable though. I left it randomly plotting dots to slowly turn the screen black, and when I came back I found it had frozen. This may be the power supply I was using. It's a switch mode one from a Linksys router. Rated 9 volts, 0.6A. I'd previously put a 3.5mm jack on it for an Atari 2600 which it powers fine. Maybe it's noisy, maybe it's under rated, maybe the 81 doesn't like it?
I've also tried a 12 volt transformer PSU, which the 81 seems happier with (possibly) but I've not left this running it for long as I didn't want the 81 to overheat.
I've some 7809 regulators on order to make a better PSU for the 81. I'd welcome advice as to what might be the best PSU for long term use and reliability.
I've already had a splurge on ebay and ordered various parts
Some sockets, a longer mono 3.5mm lead to play progams into the ZX81 from the laptop more conveniently, drive belts for the cassette recorder I'm trying to repair, a heatsink for the ULA and a 32K SRAM that I'm hoping is as compatible as was claimed in the listing.
My general plan is to try to get at least two of the ZX81s working. One will be broadly original in it's case. It'll have 16K usable onboard, a backporch ULA and maybe a composite mod.
The other ZX81 will sit on a piece of plywood that's got a ZX81 keyboard stuck to it - something I did back in the 1980s. This 81 can have the 10% speed upgrade I've read about here, more RAM (32K?) maybe a modern ULA replacement, maybe CPU upgrade... basically a tinkering ZX81.
Lets see what I can get working reliably first....
I'd welcome thoughts, comments, suggestions, things to avoid.. anything really.
Thanks
Ian
I've recently started looking at them and trying to see what works. In researching this, I found this forum resource, which I'm very grateful for.
Here they are, as I found them :
ZX81 #1
=======
Issue 1
In case with original keyboard. Probably my original ZX81 that I bought in 1982, possibly not original case/keyboard.
ULA 2C184E 8151 in socket
ROM MOSTEK MK36809N-IRL 8206 in socket
CPU ZILOG Z8400A Z80A 8152 in socket
RAM 24 pin MOSTEK 8203 MK4801AN MK4118AN-41 soldered wonkily to board
Modulator 8205
heatsink is larger but thinner piece of alu from old clock face fitted by Grandfather
ZX81 #2
=======
Issue 1
ULA Ferranti 2C210E 8242 in socket
ROM Sinclair Research P21669 D2364C 649 (c) 1981 in socket
CPU Zilog Z8400A Z80A 8147 W in socket
RAM 18 pin NEC R1Z2IY-204 PD2114LC x2 soldered
Modulator 8208
ZX81 #3
=======
Issue 1
ULA Ferranti ULA2C184E 8209 in socket
ROM Motorola? ZCM38818P 8244 in socket
CPU NEC P24488-218 D780C-1 in socket
RAM Motorola? MCM21L14P30 8225 in sockets MCM21L14P45 8223 Ram was inserted upside down!
Modulator 8217
no heatsink
So, all boards had different varieties of chips. 2 x older ULAs and 1 with the backporch fix.
I tried all 3 of them in turn, and all had the same fault - white screen, no cursor.
I know that my original ZX81, the one with the home made heatsink, HAD been working when it was put away, but that was several decades ago. The other two I think I'd got from a boot sale and a junk shop respectively. They may not have been working when I got them. I know I paid £1 at the boot fair, I may also have paid £1 at the junk shop, both back in early 1990s.
I swapped some chips around to see what I could achieve, of course not knowing which ones worked and which didn't. Starting with ZX81 #1, I fitted this with the newer ULA, initially without success. But after leaving it for a few hours and coming back to try again, it worked first time - a cursor! Picture wasn't great, but I had a cursor. Power off and on and the cursor had gone again.
I found it would work occasionally but not often. And when it did work, tapping the board made it fail.
I took the socketed chips out, sprayed contact cleaner in the sockets and gently cleaned and bent the legs before putting them back.
After a lot more fiddling around, I got the 81 to work reliably. I was delighted! Though I found the first power on from cold would give a white screen, subsequent power ons would work OK. I replaced the two electrolytic caps, but this didn't help. I thought I could live with this quirk though.
But when I tried to load a 1K game from a laptop via the EAR cable, I got error code 4. Out of memory. I tried a few 1K games, all did the same thing.
PEEKing RAMTOP showed a stupidly low value - I didn't write it down and forget what it was now. But it wasn't right.
So I tried a different board, and settled on ZX81 #3, as it had socketed RAM.
This didn't work, and I more or less randomly tried swapping in different ICs without success. But when I removed the ROM I found the legs very dirty, so removed all ICs and cleaned as I'd already done on ZX81 #1. On reseating all the ICs, it worked!
RAMTOP showed a 1K ZX81, and I was able to load a few 1K games from the laptop. Success!
What I hadn't spotted until many days later when reviewing the photos for this post, was that the socketed RAM was upside down until I reseated all the ICs. I had naturally put them back the right way round without second thought.
This is what is currently working for me :
It's not been perfectly reliable though. I left it randomly plotting dots to slowly turn the screen black, and when I came back I found it had frozen. This may be the power supply I was using. It's a switch mode one from a Linksys router. Rated 9 volts, 0.6A. I'd previously put a 3.5mm jack on it for an Atari 2600 which it powers fine. Maybe it's noisy, maybe it's under rated, maybe the 81 doesn't like it?
I've also tried a 12 volt transformer PSU, which the 81 seems happier with (possibly) but I've not left this running it for long as I didn't want the 81 to overheat.
I've some 7809 regulators on order to make a better PSU for the 81. I'd welcome advice as to what might be the best PSU for long term use and reliability.
I've already had a splurge on ebay and ordered various parts
Some sockets, a longer mono 3.5mm lead to play progams into the ZX81 from the laptop more conveniently, drive belts for the cassette recorder I'm trying to repair, a heatsink for the ULA and a 32K SRAM that I'm hoping is as compatible as was claimed in the listing.
My general plan is to try to get at least two of the ZX81s working. One will be broadly original in it's case. It'll have 16K usable onboard, a backporch ULA and maybe a composite mod.
The other ZX81 will sit on a piece of plywood that's got a ZX81 keyboard stuck to it - something I did back in the 1980s. This 81 can have the 10% speed upgrade I've read about here, more RAM (32K?) maybe a modern ULA replacement, maybe CPU upgrade... basically a tinkering ZX81.
Lets see what I can get working reliably first....
I'd welcome thoughts, comments, suggestions, things to avoid.. anything really.
Thanks
Ian