Sinclair ZX80 / ZX81 / Z88 Forums

On a ZX81, either lower the value of the RAMTOP system variable (unless your system has more than 16K bytes of RAM) then store your data above this point or if your system has RAM in the 0x2000-3FFF area, use this. Everything from 0x4009 through to the address stored in RAMTOP will or may be replace...

David G wrote: ↑Thu Apr 24, 2025 9:27 am tight fitting metal lid to keep RF in. Would this cause the ZX81 to overheat?

Aluminium is a good conductor of heat. The large surface area of the aluminium case may be enough cooling for this arrangement.

Mark

Post above edited.

Depends on the PSU manufacturer. Many have the thermal fuse soldered on the circuit board separately. It’s a bit tricky to change as it’s under the transformer which is also soldered to the board. Normally they are in an insulated sleeve. And yes, spares are still available. I forget now what temper...

What someone who knows what they are doing does and what you offer to sell to others are rather different things. So, for example, I’m used to working on live (as in mains voltage or above) equipment. So I know the dangers and take appropriate precautions when working on live equipment. But when sel...

In a word, yes.

Any and all mains voltage wiring, terminals or other conductors must be suitably insulated or in an enclosed so that a person can't touch it.

Mark

The UK PSUs are: UK700 - 240V AC mains input. Output rated at a nominal 9V DC, maximum current 700mA (0.7A). When not connected to a load (the computer), depending on the local mains voltage, the output can be as high as 16V DC. UK1200 - 240V AC mains input. Output rated at a nominal 9V DC, maximum ...

Hello and welcome to our forum AndiD ! :D :D :D I hope you enjoy this forum :D If you have any questions, please ask. Ah Gee-Wiz BASIC, that takes me back to the mid 1980s programming a Victor 9000 to test the printers while on work experience (a nice break from school). Then many years later, find...

David G wrote: ↑Fri Apr 18, 2025 9:51 am * TI LM393P 8-pin chip
* voltage regulator 78M05C

The LM393 is a comparator with an open collector output.

The 78M05 is a 500mA version of the 7805 5V voltage regulator.

I have to say, the unit does look impressive

Mark

Four bit DRAM chips became more popular as the price of DRAM fell. They turn up on 8 bit computer main boards later on as well (for example the 256K bit 64K x 4 bit type used in Amstrad made +2A/+2B/+3 and Acorn BBC Master).

Mark