Making the ZX81 run cooler.

[Jamtex]

I will have a ZX81 (with a ZXpand) in an exhibition, it will be running 8 hours a day, 7 days a week for 6 months. So I want to do everything I can to make it run cooler so it will last that long.

So any suggestions on what I can do to bulletproof it? Would adding a larger heatsink to the 7805 help? Adding heatsinks to the ULA and Z80? Cutting a hole in the machine and adding a fan? The top part of the machine above the ZX81 logo will not be visable and be hidden by a panel.

Any suggestions greatly recieved.

[sirmorris]

Hi! And welcome to the community!

The very best thing you can do to make the machine cooler is replace the regulator.

Try one of these (or its equivalent)

It's a drop-in replacement. I have a zeddy with one of these in place and it doesn't break a sweat. It's a direct replacement, anyone with soldering skill can remove the old one and substitute this.

HTH

C

[Andy Rea]

As Well as the regulator, the CPU (if it's a NMOS one) will get quite warm along with the ULA, the CPU can be replaced with a CMOS part

[sirmorris]

+1 for the CPU. And if the ULA is in danger of burning itself out then, well, there's a solution for that too

[1024MAK]

Expensive solution:-

• Fit a switching regulator in place of the 7805
• Fit a CMOS Z80 in place of the original NMOS CPU
• Fit a modern SRAM in place of the 1k SRAM (also saves the use of a external power hungry RAM pack)(needs some minor mods)
• Use Andy's replacement "ULA" in place of the Sinclair ULA
• Fit a modern CMOS EPROM in place of the Sinclair ROM (needs some minor mods)

Cheap solution:-

• Buy a couple of spare Zeddies!

Slightly better cheap solution:-

• If you are not using an external RAM pack, use a 7.5V dc or 9V dc regulated switch mode PSU instead of the standard Sinclair PSU, the internal regulator will then run much cooler

Mark

[RetroTechie]

In order of importance:

• If you can get your hands on a CPLD-based replacement ULA (like Andy's), go for it! If not: expect original ULA's to fail over that period of time. Stick a cooling plate on top (a piece of alu, cooling paste in the center, hot glue or silicone glue on the ends) before running extended periods of time.
• Replace regulator with a switching one. Recom and Tracopower have some nice drop-in 7805 replacements, there are others. Regulator cooling plate can go in that case. Equally effective: replace 7805 with a low-drop 5V regulator, and put in regulated 5.5~6V DC. If you have to keep the 7805 around: put in regulated 6.5~7V DC (no less, no more), and go easy on peripherals. Consider a larger cooling plate inside (like: width of the case), or an external +5V supply.
• Replace Z80 with a CMOS version. Speed rating isn't important as for CMOS versions the power consumption scales roughly with clock frequency (and even the fastest rated Z80's work in a ZX81 board).
• If you need 16K: mod it internally using a low-power 32K*8 bit SRAM (easiest is using half of that). I'm pretty sure no original 16K RAM will survive that kind of work hours, RAM packs are even worse than original ULA's (RAM pack wobble aside ). With a low-power SRAM, 16KB or 1KB doesn't make any difference for power consumption.
• Do the composite video mod, making sure to cut power to the modulator.
• Replace ROM with a CMOS EPROM (27C64). Problably won't help much, but 27C64's are in near-infinite supply.
• Of minor importance but may still help to avoid problems: use turned pin (gold plated) IC sockets. Soldering directly into the board is even better, but that makes replacing parts time-consuming. One or two spare ZX81 boards (or spare ZX81's) wouldn't hurt.

With all of the above, no need for extra cooling even in the cramped original case. I've got a 16KB ZX81 here that's under 1W (about 0.8W) total (including regulator), and that one still has original ROM inside. Typical power input is ~65 mA @12V, at that power level nothing gets hot.

Depending on what visitors can touch/use: various mechanical protections (think cable pulling, moving the case / monitor, etc). Tie/screw/glue down everything you don't want to come loose!

[Jamtex]

Thanks for the suggestions. The unit will be mounted to a shelf in a unit and a wooden front will cover everything behind the ZX81 logo will be hidden out of sight. The only add on I will be using is a ZXpand. The power I would probably route via the back of the machine so I can mount it like this with the power cable visable or accessable.

The replacement voltage regulator seems to be the best way forward and it seems to be cheap.

As I try to use machines in the original way as possible I do want to use the RF modulator (as part of the ZX81 experience is the noise on the picture), I do have a TV that does have manual tuning so shouldn't have any issues.

So I will also use thermal paste and a DIP heatsink for the ULA. I will buy another ZX81 as a back up and will look at replacing the Z80 with a CMOS Z80 but I do have arcade machines that run a similar sort of day, that use the NEC Z80 too and never had an issue with them failing

Would the replacement ULA work with the RF modulator?

[PokeMon]

Why not simply use the emulator ?
It runs as cool as your PC and no ULA will ever burn out.
Has more power consumption - but who cares about power.

So now I hope to get a +1 from Charlie, too.

[Jamtex]

Because emulators are a cop out. If you go to a museum you want to see original items not faux copies of items. LCDs TVs and emulators are horrible.

In the exhibition I help run, 98% of the 150 odd games (and soon to be 180!) we have run on original hardware with the screens they originally came with so CRTs and even monochrome CRT TVs too. So I want people to play 3D Monster Maze on a ZX81 on a CRT TV using a ZX81 keyboard.

[PokeMon]

In that case it would be best to cool the internal ULA with a mounted cooler on top - have them in electronic shops for DIL40.
And I would follow the hint with a stabilized and regulated power supply with 7.0 or 7.5 volts and able to drive the needed currency (minimum 600-800 mA).
Maybe you could mount a bigger cooling plate to the regulator which fits inside if you use an external memory kit (for beeing "original").

I think these two or three steps are sufficient for your needs. Will cool down the power regulator and the mounted cooler will cool down the ULA.
The CPU doesn't get very hot, same for internal memory or internal rom. Replacement of these components is not really necessary.

By the way - my proposal with the emulator was meant more funny.