Sinclair ZX80 / ZX81 / Z88 Forums

Hi everyone,

So I'm new to the forums, let's introduce myself. I'm Jasper from the Netherlands. When I was 8 or 9 years old, we had a ZX81 at home. I learned a bit of programming on that machine. Of course, at a certain point I outgrew it and we moved on to a MSX1 and later a MSX2.

Although I still have the MSX machines from that time, I have lost the ZX81. I haved looked into getting a ZX81 before, but I always hesitated. The machine we had was upgraded to an aftermarket keyboard case, so the original keyboard does not bring much recognition to me. I never researched that out, I thought there would be infinitely many of those. Long story short, last week I stumbled on a ZX81 in a DKTronics case and I immediately recognised it. It was too tempting and I acquired the machine.

Today I am still active in the MSX community, and so I know a bit about retrocomputing. I can also read and write a bit of Z80 assembly if needed, but I leave the true programming to others. Yesterday my ZX81 arrived and I have a few questions already. First of all, is it safe to turn it on? Or should I perform some hardware checks first?

And secondly, what else do I need for my machine? I don't plan using it extensively, just for fun and nostalgia. And maybe explore a bit more the hardware side of things with my current knowledge. I used to have a 16K expansion. Should I get one now? Are they still out there? I have also read about the ZXPand+, which seems extremely hard to get. It even brings 32K, but is the extra memory really necessary?

Hello and welcome to our forum Jasper!



I hope you enjoy this forum

Unlike some other 1980s machines, there are no components inside a ZX81 that need to be replaced/renewed as a precaution. There are no Rifa paper capacitors, so no smelly smoke. The two or three (depending on which version board you have) electrolytic capacitors have an easy life and none of them are critical (*). So they can be replaced on failure.

* meaning that if they fail, they don't cause other components to be subjected to stress.

As you have a dk'tronics case, you don't need to worry about brittle keyboard membrane tails.

I do however recommend checking the board visually. Check for any damaged or broken components. Check that any vertically mounted resistors don't have their leads touching another. Compare the chips against a photo of the same version board to confirm that (1) the chips are indeed fitted with the correct orientation and (2) if the ROM chip is a 24 pin type in a 28 pin socket, that its in the correct position within the socket. Pins 1, 2, 27 and 28 of the socket should be empty.

From a safety perspective, check the condition of the mains cable for the PSU and the mains plug.

In terms of expansions / interfaces or upgrades, that depends on what you want to do.

The ZXpand+ may still be available. The original creator no longer makes them, but he has authorised someone else to produce them. They are available in limited quantities. See the listing on the Sell My Retro website here.

16K byte SRAM (static RAM) chips were not widely available even in the 1980s, but 32K byte SRAM chips are still available now. The original ZXpand and the later ZXpand+ use one 32K byte SRAM chip, hence why they have this amount of RAM. The advantage of the ZXpand+ is not just the RAM, but also the SD card storage system and the "AY" sound chip.

The RAM pack that you had in the 1980s would have used eight DRAM (dynamic RAM) chips (plus various other support chips) to get to 16K bytes. These DRAM chips (type 4116) have been obsolete for many years. These are still sold from time to time on market place sites. But the Sinclair types are not very reliable. Memotech types have better reliability. It's hard to say about other brands/types.

But there are other options. If you are a hardware type (can solder and are happy to modify your board) it's possible to fit a 32K byte SRAM chip internally. The basic modification however only actually uses 16K bytes of the chip.

Or you can get a RAM module, which is easier to fit.

And there are other storage solutions so you don't have to use a cassette deck.

Mark

Briqunullus >>> Really cool that you have had ZX81 and MSX 1 and MSX 2

I have had MSX1 Spectravideo 728 back in the 80s
It would be cool to get one again

Hello and where are you from?

Thanks for the replies. I have tested the machine now, but unfortunately it did not work. I'm not really into hardware, but I could check some voltages. There's 15V coming from the power adapter (shouldn't that be 9V?), the voltage regulator outputs 5V, but the Z80 only seems to receive 2V at pin 11. I will reach out to a friend to investigate further.

What I want to do with the machine? First of all some nostalgic fun. Run a few listings, and maybe some primitive Z80 code. Just to get a better understanding of the machine. Right now, it's just a black box for me. Maybe I pick up a ZXPand+ when it becomes available again. Though a regular 16K expansion and a CasDuino/TSXDuino kind of device for tape simulation will also do the job (and I can also use it on my MSX machines). Just what I find first.

Most important now is to get the machine running. If I have any specific questions I will post them on the forum.

Briqunullus wrote: ↑Fri Mar 14, 2025 3:40 pm There's 15V coming from the power adapter (shouldn't that be 9V?), the voltage regulator outputs 5V, but the Z80 only seems to receive 2V at pin 11.

15V from the PSU with it not connected to the computer is normal. The PSU is an unregulated type. Hence the output voltage is only at or near 9V when it's supplying its rated current. With no load (no computer connected) the output voltage will be between 12V and 16V approximately.

When connected to a ZX81, the PSU output voltage is typically around 10V to 12V.

The Z80 should have 5V (+/-5%) at pin 11. It should be a direct connection to the +5V output from the 7805 voltage regulator.

Without the correct supply voltage, the Z80 won't do much.

Check that any chips in sockets are correctly fitted (compare against photos online, don't rely on the socket polarisation mark being correct), no legs are bent and are firmly pushed home. Check or poor / dry solder joints.

If you do want further help or wish to discuss fault finding, may I suggest you start a new post in the Hardware section.

Mark