ZX81 Blank screen???

[lusciousthelock]

Hiya Folks,

I've recently been restoring a ZX81 that was an elder brothers and been in storage since about 1987! The initial picture quality was really poor, so I upgraded this with a composite video board. With no improvement and garbage on the screen I realised I needed a new ULA. The Keyboard was playing up next so I bought a new one and since I was ordering a ZXpand at the same time, I bought the updated version of that too. Two weeks of excellent reliable service was spoiled today when I plugged in the 16k ram pack. After typing in a long program, I had a bit of ram pack wobble and lost the lot. Not only that I couldnt get the computer to come back to life, just a blank screen. Unplugging the ram pack resulted in my happy ZX81 returning to life. I was explaining to my young daughter (she's 9) about creating random numbers and getting the ZX81 to roll the dice for a board game they were playing when the screen went blank again. Nothing brings it back. I know the power supply is good, but I cant tell whats going on with my 81'?

Any ideas? The screen is blank, no garbage, nothing. Power supply tests fine. Could this be faulty memory?

[gozzo]

Could be pretty much ANYTHING! Check the +5v out of the regulator first(4.75 to 5.25 is ok, the '9v-ish' going in is not too important), then, if they are socketed, try replacing the chips, one at a time, starting with the ULA, then CPU, then RAM, then ROM.....modulator failure is unlikely but possible, as all 3 of my Acorn Electrons (sorry for mentioning a non-Sinclair beast!) appear to have had them replaced (curious..??) other than that, poking an oscilloscope around could be useful to see if any activity...A 'wobbly' RAMpack certainly COULD kill a chip or two...

[sirmorris]

You said you plugged in a ram pack - what sort was it?

C

[lusciousthelock]

The ram pack I plugged in was an original genuine sinclair one straight out of its original box. The fact it died so suddenly was the strange thing. I was literally typing as I had been for a couple of hours trying a few bits of old basic out and then nothing! Completely dead. It wasnt hot as far as I could tell and when it eventually died the ram pack hadn't been plugged in for an hour. I guess the power supply was fine as it always made a quiet hum when plugged in and continued after the screen went out??? Anybody reccomend a good replacement unit? At the time, the program was filling out all of the 1k ram and I was starting to lose bits with the overspill. Could this have overloaded the ram if it was a bit dodgy with age?

I have another ZX81 which is in need of repair, so when the parts arrive I'll try and swap some bits over and see what's what.

[lusciousthelock]

Swapping bits between the two ZX81's gave no results. I swapped ULA's, the Z80 and the Rom in different combinations. One seems completely dead whilst the latest one fitted with Pokemon's composite board and new ULA only produces a white screen with nothing showing (Odd because I soldered J7 to produce inverse video). I cant swap ram between the two machines as one is a two chip socketed type and the other is a single soldered chip. I've ordered 2x 2k chips. The latest ZX81's heat sink seems to get hot fast- clue??? So next thing to do is to find an alternative power supply. I cant see anything popped or out of place on either board.

[PokeMon]

One seems completely dead whilst the latest one fitted with Pokemon's composite board and new ULA only produces a white screen with nothing showing (Odd because I soldered J7 to produce inverse video).

So if you remove J7 than you should have a black screen, is that right ?
Did you try to adjust the pixel trimmer as the voltage level may vary from ULA to ULA ?
Did you try to get a picture over the RF modulator ?

Do you have a oscilloscope available to check ULA's output ?
This would be easiest way to check the 3 voltage levels or to check if the ZX81 is running at all (simply measuring this signal on A15).

See here: viewtopic.php?f=7&t=1236&p=13143&hilit=A15+ULA#p13143

[lusciousthelock]

Haha success! The Ram on the first ZX81 was faulty, swapping out for some from yet another dusty old ZX81 in my garage, so that's one cured! The second ZX81 was cured by, yes adjusting those pots on the ZX8-CCB, which required quite a few turn's. I thought those tiny pots would require a few carfeul movements in each direction for fine tuning but I must have turned one of them atleast 3 or 4 complete rotations to get my stable picture. This second ZX81 however, seems to run pretty hot? A lot hotter than the other one for some reason? I better fit a bigger heatsink in the future.

It turns out that I now own 4 ZX81's which was a surprise to me as I dont remember them, only my brother's old one. Isn't it weird that despite owning two issue 1's and two issue 2's that each circuit board seems completely different with differnt Ram chip configurations? Was Sinclair a bit stingey and only used the junk he had in his tool shed at the time?

[sirmorris]

Hehe something like that

Congratulations on your functioning new toys!

[Paul]

two issue 1's and two issue 2's

If you own a issue 2 board, could you please post a photo?
Never seen one. I only have issue 1 and issue 3.
Kind regards
Paul

[lusciousthelock]

Sorry meant issue 3 with the red motherboard