Sinclair ZX80 / ZX81 Forums

[Liveinabin]

Hallo all. This is my first post on your lovely forum after getting a Zx81 for Christmas (25 years after my original).

Unfortunately, it doesn't work (although we're keeping it as the eBay seller is an idiot and would probably bin it if we return it).
All we get is a black screen (on CRTs - White on an LCD) with no 'K'.
Opened it up, snipped and reconnected the keyboard cable, it was cracked. Everything looks ok but I'm no electronics whiz.
Tried Save"something" with headphones in the EAR port - nothing.

One odd thing, I checked the power supply output with a multimeter. Showing 13.7v. Should be 9 right?

Other than that, no ideas.
It's a UK model, issue one. Very nice condition apart from the fact it doesn't work

[bobs]

I think the power supply doesn't actually supply 9v, but more usually. The voltage regulator takes care of converting that down to 5v, but it just gets a little hotter the more voltage that comes in.

The problem could be an old ULA - old/first issue - which doesn't display very well on modern TVs. I think if you open the case the old and new ULAs have different numbers - sure somebody here knows which is which.

[Moggy]

There are at least three issues of ula that I know of, the earliest being 2C158E next is 2C184E
and lastly the one that seems to give best tv picture 2C210E usually found on issue three boards.
The fact that You are getting just a black screen suggests a blown ula.

[Liveinabin]

Thanks for the replies! It's a 2C184E. Blown ULA huh? Well that's not such a chore, I can pick up a new ULA on eBay. However, I'm anxious to avoid whatever may have blown said ULA in the first place. Still a little worried about that power supply, should I be?

[makeit2day]

You can check the voltage at the 7805 regulator to see if you have 5 volts at its output terminal.
If you do then the power supply is OK. The power supply should supply a positive output at the tip
of the plug. Good luck.

[Liveinabin]

Pardon my idiocy but, say, I'm sitting here with Zx81 opened and powered up - multimeter in hand. Where do I stick the prongs to test for this 5v?

[iturbez]

Maybe this could help...


----------
| o |
----------
| 7805 |
----------
| | | --> Vout (5Vdc)
| | -----> GND
| ----------> Vin (9 - 12 Vdc)

Good Luck!

[Liveinabin]

Thanks. That's excellent.

Well, the first good news so far. The 7805 reads 4.89V so I guess the power is fine. So I guess the next step is to buy a new ULA and see what happens.

Thanks all. And sorry to be scrounging for help in my first thread. I promise give more useful input when I have a working machine

[Liveinabin]

Blimey. That was quick. New ULA arrived in the post today and look..



I can't believe it! Thanks SO much for your help guys. Now to start amassing hardware and tapes for it (why did I ever give away my copy of Murgatroyds Revenge?).

[iturbez]

Congratulations!
Nice job!