ZX81 essential maintenance?

[hjalfi]

My new ZX81 with 16kB Memopak arrives today. Huzzah!

Before I apply power, is there any work I should do on the board? For example: I know that you shouldn't power up an original Spectrum without recapping, because capacitor failure in the power regulator can cause the wrong DRAM voltage, resulting in your RAM asplode. But I know the ZX81 works differently (SRAM vs DRAM so no weird voltages), and the capacitors aren't as critical.

Further down the line I want to replace the linear regulator with a switching one, for reliability and to reduce thermal stress; which will probably involve replacing the keyboard membrane too because no doubt the ribbon will disintegrate when I open up the machine. But I want to do that later. Is the machine safe to power up as is?

[1024MAK]

There are only two electrolytic capacitors on the ZX81 PCB. Both are non-critical in that failure will not cause any further damage. So it is safe to power on a ZX81 without opening the case.

One is on the +9V rail. The other is part of the reset circuit.

So very few people replace them unless they actually fail.

The main problems with the ZX81 are the membrane tails breaking, and the contacts on the 3.5mm jack sockets (power in, ear, mic) loosing their springiness.

It is however worthwhile testing the Sinclair "9V" power supply. Note that this is an unregulated type, so the voltage when the output is not connected can be as high as 16V.

Mark

[hjalfi]

Good to know.

I plan to admire the outside of the 9V supply and then put it on a shelf somewhere. Old computers are interesting; old and rather dubious unregulated mains power supplies with rotting electrolytic capacitors are not. I've got a modern 9V switch mode for this.

(When I tried Ihe power supply for my Spectrum, it started at about 5V and slowly dropped to 3V before I hurriedly unplugged it.)

I'll give it a try and report back, then --- I have a genuine portable CRT TV with RF input, so I shouldn't have LCD issues.

[1024MAK]

Keep in mind that some non-Sinclair 16K RAM expansions rely on a Sinclair "9V" power supply being used, as these actually output about 11V when connected to a ZX81.

The Memotech RAM pack uses this supply to feed the +12V power pin on the 4116 DRAM chips via an additional smoothing capacitor.

So if you use a replacement power supply that actually outputs 9V, the Memotech RAM pack may not work.

There are three different ULA chips that may be in a ZX81. The first two versions don't produce the part of the video signal known as the back porch. The last ULA version does generate this part of the video signal. The lack of the back porch part of the video signal causes some colour TVs to incorrectly auto calibrate the contrast levels, resulting in a very dark picture.

Hopefully either your ZX81 has the later ULA chip, or your TV is happy with the non-standard video signal.

Good luck

Mark

[hjalfi]

Hmm.

Okay, I'll try it and see. Would 12V be safe? As I have lots of power supplies which generate 12V.

(However, I don't think I want to hook up a 12V power supply until I've had a chance to replace the 5V linear regulator, because it would get seriously hot.)

(Speaking of which, I have some stick-on heatsinks intended for a Raspberry Pi, and if there's room, sticking a few on the ULA would probably do no harm at all...)

[Lardo Boffin]

(Speaking of which, I have some stick-on heatsinks intended for a Raspberry Pi, and if there's room, sticking a few on the ULA would probably do no harm at all...)

Unless Moggy finds out.

viewtopic.php?f=7&t=2201&start=20#p22552

[1024MAK]

Hmm.

Okay, I'll try it and see. Would 12V be safe? As I have lots of power supplies which generate 12V.

(However, I don't think I want to hook up a 12V power supply until I've had a chance to replace the 5V linear regulator, because it would get seriously hot.)

(Speaking of which, I have some stick-on heatsinks intended for a Raspberry Pi, and if there's room, sticking a few on the ULA would probably do no harm at all...)

12V is safe. However, the heatsink on the 7805 will get a bit hotter than if using a Sinclair power supply unit. This will not however be a problem if you are only powering it for say no more than one hour.

Of course if you replace the 7805 with a switching DC/DC converter / regulator, then this problem disappears

The ULA does not need a heatsink. If you fit a switching DC/DC converter / regulator, the inside of the ZX81 will not get anywhere near as hot.

Mark

[Moggy]

Sage advice as always Mark.

[McKlaud]

I can only add to all items pointed out by Mark, that Sinclair's RAMpack (16K) was built with 4116 RAM chips and there is a circuit which generates all required voltages. Capacitors there are critical one if you use the pack continuously. The same issue like with any newer ZX Spectrum 16K/48/+ lower RAM chips.

[1024MAK]

Yes, the Sinclair RAM pack uses a similar circuit to that in the ZX Spectrum to generate the +12V and -5V supply voltages for the 4116 (or equivalent) DRAM chips.

But most third party 16K RAM packs (that use 4116 DRAM chips) use the unregulated +9V for the +12V rail (with an extra large electrolytic capacitor to improve the smoothing). To generate the -5V supply, various designs of DC/DC inverters are used.

Mark