Never was into Sinclair. But it's great keeping them working also for my nostalgia. I like seeing such devices, working or not. Thank you for sharing your fantastic hobby, Tommy.
A nice video with logical approach to fault tracing. Two things I would consider is replacing the 7805 voltage regulators with a switch mode alternative and putting low profile heat sinks on the ULA's. Anything to keep the heat down in the small zx81 case helps prolong the life of the chips and stops the keyboard ribbon cable degrading so fast. To keep authenticity you can keep the heat sink attached to the board as a sticker/poster board to list the mods that have been done.
You have 2 different ULA models. the 184 and the 210. They deliver different video signals. The 210 has the back-porch with makes it a lot better. This can be used for the simple comp-mod, that you did. The 184 will probably have problems without a comp-mod which implements the back porch.
@@horusfalcon Both I suspect. The 60hz mode was toggleable depending on the state of one of the pins (22, I think, though don't quote me on that) so later ULAs would be usable in either market
My ZX81 keyboard had a large break in it at the bending point. "Unfortunately" cutting the keyboard cable at the break and plugging in the remains resulted in a perfectly working keyboard, otherwise I would have constructed a replacement keyboard that fits on my 5 inch black&white TV radio alarm clock and put the computer mainboard inside and make an SX81 (as opposed to the Commodore SX64). The picture quality was pretty decent even through the RF modulator. The previous owner upgraded it to 8k RAM.
The standard ZX81 as supplied by Sinclair Research Ltd only has 1k bytes of RAM (Random Access Memory). This is in the form of either a single 8 bit by 1024 bit (1k byte) SRAM chip (IC4, 4118) or two 4 bit by 1024 bit SRAM chips (IC4a and IC4b, both 2114 type).
Hi.. I just discovered your channel and I have to say I absolutely love it. I have family that live in Norway, in Gressvik which is about 300 km from Oslo, I often visit when I can.. Love Norway.. Kepp up the fab work.. Love form uk Liverpool.. X
The IC 4A and 4B is for manufacturing flexibility, they can use either one large RAM chip or two smaller ones. There is a second unpopulated small RAM chip footprint underneath the large RAM chip installed on your board, you can probably see it on the back of the board.
Crazy how these were dirt cheap back then, but now they're expensive on ebay, more so than systems like the Vic-20 or TI-99.
Your videos keep getting better and better Tommy, Very educational as always. Keep up the great work. Cheers
Thanks a lot :)
I am simple man - I see ZX81 in thumbnail, I watch & like
Great :)
Never was into Sinclair. But it's great keeping them working also for my nostalgia. I like seeing such devices, working or not. Thank you for sharing your fantastic hobby, Tommy.
Thanks!
A nice video with logical approach to fault tracing. Two things I would consider is replacing the 7805 voltage regulators with a switch mode alternative and putting low profile heat sinks on the ULA's. Anything to keep the heat down in the small zx81 case helps prolong the life of the chips and stops the keyboard ribbon cable degrading so fast. To keep authenticity you can keep the heat sink attached to the board as a sticker/poster board to list the mods that have been done.
... hilfreiche Informationen zur Reparatur eines ZX81 - helpful information for repairing a ZX81. Danke/Thank You!
Thanks!
You can also replace the 7805 with a TSR-1 2450 for $5 and get rid of the heat sink too.
That's a REALLY COOL component, in the truest sense of the word :)
How nice to see the Springtime sunshine, at last. It's been dull and wet for too long here in the east of England.
Yes, great :)
You have 2 different ULA models. the 184 and the 210. They deliver different video signals. The 210 has the back-porch with makes it a lot better. This can be used for the simple comp-mod, that you did. The 184 will probably have problems without a comp-mod which implements the back porch.
Ah, great info :)
Is this a difference between PAL & NTSC models, or just differences in manufacturing over the years?
@@horusfalcon Both I suspect. The 60hz mode was toggleable depending on the state of one of the pins (22, I think, though don't quote me on that) so later ULAs would be usable in either market
@@TheTurnipKing Interesting. That would seem to imply someone saw a need for chips that could do both?
@@horusfalcon the 81 was being manufactured by Timex and was released in 82 in the US, both in limited numbers as the zx81 and TS1000
Thanks for the fun repair/fixup video, I love these!
Glad you like them!
My ZX81 keyboard had a large break in it at the bending point. "Unfortunately" cutting the keyboard cable at the break and plugging in the remains resulted in a perfectly working keyboard, otherwise I would have constructed a replacement keyboard that fits on my 5 inch black&white TV radio alarm clock and put the computer mainboard inside and make an SX81 (as opposed to the Commodore SX64). The picture quality was pretty decent even through the RF modulator.
The previous owner upgraded it to 8k RAM.
Ok, thanks for the info :)
The standard ZX81 as supplied by Sinclair Research Ltd only has 1k bytes of RAM (Random Access Memory). This is in the form of either a single 8 bit by 1024 bit (1k byte) SRAM chip (IC4, 4118) or two 4 bit by 1024 bit SRAM chips (IC4a and IC4b, both 2114 type).
OK, thanks for the info :)
Great to se them come to life :) they will be great in your collection :) great jobb Tommy keep up the good work :)
Thanks 👍
Hi.. I just discovered your channel and I have to say I absolutely love it. I have family that live in Norway, in Gressvik which is about 300 km from Oslo, I often visit when I can.. Love Norway.. Kepp up the fab work.. Love form uk Liverpool.. X
Awesome! Thank you!
The IC 4A and 4B is for manufacturing flexibility, they can use either one large RAM chip or two smaller ones. There is a second unpopulated small RAM chip footprint underneath the large RAM chip installed on your board, you can probably see it on the back of the board.
Ok, thanks :)
I've got two ZX81 and one worked up until recently, after being stored in perfect conditions and environment. Bad caps?
Probably not. More likely the ULA chip :)
Use Loctite instead of Super Glue. Much easier to use.
Great tip!
| Awesome! | :3
Thnx!