Sinclair ZX80 / ZX81 / Z88 Forums

Manufacturers normally use mask ROM chips for mass production, as for a large production run, a mask ROM is cheaper per unit cost compared to using EPROM chips.

However, if there are supply shortages, manufacturers sometimes use EPROMs in place of mask ROM chips. It’s unusual for a detailed handwritten label to be used though. Commonly either there is no written/typed/printed information, just a plain sticker, or a printed label/sticker. Occasionally, no sticker or label at all!

Note that a sticker/label (even if blank) is supposed to be used on EPROM chips to prevent UV light getting on to the chip through the quartz window.

Independent repair / service agents may use a EPROM if a replacement mask ROM is not available (or costs too much).

Individuals that modify their machines will obviously use a EPROM.

It’s always rather hard to guess who performed some modifications. Some will have been done in the factory. Some may have been done if the machine was serviced/repaired. Unless said modification adds functionality, it’s uncommon for an individual owner to carry these out. Unless they have electronics as a hobby (or profession) as well.

Mark

jesperp wrote: ↑Sat Jun 11, 2022 11:23 pm Anyway, my intention is to bring the unit alive again.
Until now all DC supplies to the IC’s is checked and the crystal is generating a pulse to the CPU. But that where the dynamics ends - no beep when started, and no output to monitor or TV.

Not finding a video output on either tv or monitor, on these machines is quite common, as a very old tv is often needed.

However when you mention that it does not even make a beep, the matter is worse.

On ZX81 and it’s clones, there is no video chip, so any output is generated with software that makes precise bit banging to the tv to create video.

This requires that all ICs must be working: Z80, ULA, RAM and ROM.

Finding Z80, RAM and ROM is possible. The ULA however is impossible, and no replacement exist. You cannot use a ULA from a ZX81, as they are completely different.

mrtinb wrote: ↑Sun Jun 12, 2022 3:54 am

jesperp wrote: ↑Sat Jun 11, 2022 11:23 pm Anyway, my intention is to bring the unit alive again.
Until now all DC supplies to the IC’s is checked and the crystal is generating a pulse to the CPU. But that where the dynamics ends - no beep when started, and no output to monitor or TV.

Not finding a video output on either tv or monitor, on these machines is quite common, as a very old tv is often needed.

However when you mention that it does not even make a beep, the matter is worse.

On ZX81 and it’s clones, there is no video chip, so any output is generated with software that makes precise bit banging to the tv to create video.

This requires that all ICs must be working: Z80, ULA, RAM and ROM.

Finding Z80, RAM and ROM is possible. The ULA however is impossible, and no replacement exist. You cannot use a ULA from a ZX81, as they are completely different.

Thanks Martin
This will lead me on the right track in my efforts.
When got my hands on a second device, and used this as sparepart machine (or visa verse), i wil be back.

Thanks Mark
Your comments to labeling the ROM gives sense.
Service situation or a relatively qualified electronic upgrade, could explain why the ROM label is year 1985.

Have a nice sunday!

My "Your Computer" chipset:

CPU: NEC Japan D780C-1 8344XD
ULA: C4005 8440
ROM: AMI 8436HJ 10830-21103-01

Apparently, the 4 block RAM (2114) is "missing" on the computer.
Note: No (Your Computer) label on the front, nor any seriel placard on the back center.

CPU: NEC Japan D780C-1 8344XD - this is the NEC company’s ‘unauthorised’ clone of a Z80A. To software it looks exactly the same as a real Z80A. And to hardware, it’s pin and signal compatible. Sinclair also used D780C-1 chips in both the ZX81 and the ZX Spectrum computers.

Apparently, the 4 block RAM (2114) is "missing" on the computer.

A 2114 SRAM chip is 1024 locations each of which is 4 bits wide.
So for 1k bytes of RAM, you need two chips.
Four 2114 chips when used with a Z80 microprocessor system provides 2k bytes of RAM.

On your board, they have used a Toshiba TMM2016P SRAM chip. This provides 2k bytes of RAM in a single chip.

The designer and manufacturer have done the same as Sinclair with the ZX81, and provided the option to use either range of SRAM chips.

Mark

Hi 1024MAK

This forum is floating with knowledge.
Already used several hours scanning the treads, and when posting something new - a nice well-founded answer appears almost instantly.
I am a little bit amazed how many chip-versions and board configurations, this "toy-store" computer is produced/sold with.
Especially taken into account the relatively few years the computer was produced/sold. 1983-1985?

Hope you will get a nice summer in the UK.

This forum is quite brilliant.

A lot of knowledge about the ZX81, and less about the Lambda 8300.