Sinclair ZX80 / ZX81 / Z88 Forums

Right...

As I mentioned earlier, I dug my shiny Issue 1 zeddy out and set it up with an old telly.
I was having a frustrating time tuning it in so I did a quick trawl and found the transistor - resistor composite mod.
Almost identical mod to the speccy one so I carried it out and hey presto a half decent image.

The keyboard is fully working... almost smug at this point

'K' cursor at the ready I typed in 10 Print "Hello World";
and.... it printed hello world!! no retention of the program... I let it go cold and tried again... nothing.

I've been round the board tidying up suspicious solder joints.

I'm guessing I've part blown the ULA

Is this a known curable fault or do I need a new zeddy??? I'm guessing ULAs are scarce....

Any help gratefully anticipated

Hi Marcus,

When you say the program was not retained....

Do you mean that you entered:

10 PRINT "HELLO WORLD"
RUN
LIST

and there was no program there?

Or did you forget to enter the command LIST (which means list the program)?

Hi

When you say the program was not retained....

It executes immediately, as though there was no line number entered

Strange...

What about if you enter Does that get stored?

It does not sound like a ULA fault. More likely suspects are:

• RAM (have you got an external RAM pack to test with?)
• ROM

Mark

Hi Mark

'1 REM TEST ' no it just flags 0/0

RAM (have you got an external RAM pack to test with?)

Yes, I've just tried my MEMOPAK 16k and its working... phew

The little chap is starting to remember the lines as entered

There are a few 'bits' flashing randomly around the screen that disappear when I gently press the ULA

Wow this gets hot rapidly

Yes, the ULAs do run hot.

With the power disconnected, if the ULA is in a socket, carefully using a blunt flat knife, or a suitable flat bladed screwdriver, gently lift one end of the ULA. But only about 2 to 3mm. Then do the same at the other end of the chip. Repeat so that the chip is gradually eased out. Don't let the knife or screwdriver press into the PCB or the tracks could be damaged. As the chip becomes more free, hold a finger or thumb on top to prevent it moving too far when the socket finally lets go. If you don't do this, the chip will move suddenly and then some of the pins will bend and may break off when you try to straighten them.

Once out, clean the pins. Check that the socket contacts all look the same. Then carefully put the ULA back in. It's also worthwhile checking the solder joints on the underside of the circuit board. If you see any poorly made joints or dry joints, then the joint needs to be resoldered.

Mark

If the ULA is socketed - remove it and push it back in (whilst the ZX81 is off of course!) in case there is a build up on the legs...

I have 6116 RAM chips on sellmyretro.com if you want to replace the internal RAM, but there is no need if you are going to use the external RAM pack - it might just need the RAM re-soldering

The ULA is soldered to the PCB

I've just re-soldered all joints on the underside. No change to the random screen bits flashing.
Now I'm about to change the 2 electrolytic caps

I changed the caps and added a heatsink to the ULA
It now seems to be operating without the screen artifacts

I'll swap the 7805 for a switch mode unit so I can stop burning myself on the glowing heatsink.
Then I can give it a soak test.

Thanks for the pointers