ZX81 blank screen

[msknight]

I may be barking up a wrong tree here, but the only picture I've seen is a motherboard with a 550 bios and a kludge processor.

The other bios and processors you're swapping out... are they also kludge processors and 550 roms?

The reason I ask is because... and someone else will know the answer to this... a kludge processor might not work with a later ROM and vice versa. Something like this might be messing up the testing and swapping, if it is the case.

[1024MAK]

You should only use the 'bugged' ROM with a machine that has the daughter board. The daughter board overrides data from the ROM at various addresses. So it may mess up the execution of the ROM program in later ROM chips.

Mark

[jarweb]

I wasn't sure if the two chips needed to be swapped together or what would happen in they weren't. What symptoms would I get if I used the chip with the daughter board on a non-550 ROM ?

The other board doesn't have the 550 rom and altered processor but, as I've said, I've swapped all three at the same time with the same results.

[voorsk]

I suspect I'm the new custodian of this zeddy.

so far, I've disconnected the 16k ram, and put in a known-good 1k, and I've added a zigg.net composite mod. output is blank white, sometimes a moving black and white pattern.

the power connection is verry dodgy, so my next plan was to test voltages and possibly fit a traco power regulator. might need to replace the power connector, as i can't get it to tighten up.

i'd better read through this thread and see what's already been tried!

edit: I'm up to date! I guess the important bit is that all 3 known-good ICs didn't fix the problem. When I received it, it had a modulator fitted, so I guess that didn't work, either.

It also sounds like the ROM might have more issues than needing help with it's maths. What kind of EPROM would be good for creating a replacement?

edit 2: 27C64's seem like a good bet, judging by other posts. No-one seems to mention access times, tho. Is 300ns okay, or is that too slow?

[Moggy]

Just my two bobs worth.

I have recently converted one zeddy to a ZX80 and fitted Big Bang EPROMS to two others.

All three machines gave the white-screen-with-odd blobs here and there which quite frankly stumped me.

Long story short it turned out that in my case the ROM sockets, although looking good were somehow klutzed and my solution was to solder the ROMs directly to the board which cured all three computers.

I'm not suggesting for one minute that this will fix your situation and is a drastic fix just something to think about when all else has been covered, I'm someone who is not afraid to experiment as they are not really rare items not matter what ebay listers proclaim and can only say that this worked for me.

As for EPROMs I'm old school and use 2764's which only need two lifted pins and two link wires, the programming voltage is higher than a lot of modern burners can handle so 27C64s are a better bet both of which are readily available from Ebay, I use one supplier in particular from Bingley, not that far from you, whose listing I can pass on to you if you are interested.

Access times never had a problem with regardless of chip used.

[voorsk]

I bit the bullet and opened up my other issue 1 board, which definitely works, and I don't think I damaged the keyboard membrane while doing so. Fingers crossed...

• It has the same ULA version, and a standard Z80 without the math mod, so I swapped over the 550 ROM to test that on it's own. No K.
• As it was out, I then put the working 649 ROM into the dodgy ZX. No K on the good board.
• Next, I tried the working Z80 in the dodgy ZX, in case the math mod was messing things up. No K.
• Then, I tried the working ULA in the dodgy ZX, so all ICs are good. No K.

So, ICs are ruled out, sort-of. They weren't helping, but something else is amiss. Time to check voltages, and then maybe replace the electrolytics.

edit: 7805 is producing 12V and 5V, all ICs are receiving 5V.

[Moggy]

I doubt very much much the caps have anything to do with it, one's a filter and the other a reset cap and you're are positive that all the actives are OK having checked them all, the ROM socket however.

The 7805 should only produce only 5v or near enough not a mixture of 5v and 12v.

[voorsk]

The 7805 should only produce only 5v or near enough not a mixture of 5v and 12v.

I didn't twig - that's the input. My UK700 9V PSU is putting out 12V!

edit: Now using a 9V power adapter, which I usually use with my speccys, with the inverter switched for the plug from the UK700. That won't fix anything, but it'll take a load off of the 7805.

oh, and the caps made no difference, but at least they can be crossed off the list.

[Moggy]

The power supply is an unregulated one so off load anything up to 14v is normal and once under load it gets pulled down to something more reasonable like 10-11 v, the reg turns this into 5v for the chips and the rest into heat dissipated by the reg's heat sink.

Fitting to a switching jobby removes about 90% of the heat found inside a cased zeddy and really does make a difference.


EDIT.

I do believe some memory modules need this 10-11v to function and using a power supply that provides less than this can cause them not to work.

[voorsk]

aah, work over, time to play again.

I decided to test the ICs. As the ROM seemed to be a definite gonner, there was no point keeping the elaborate hat on the Z80 (which is a shame, as that's what attracted me to it), so off it came.

I put the working ROM and ULA back into the working board, along with the hatless Z80. Turning it on produced a scrolling pattern of dots, then after a reset, a scrolling pattern of shapes, then another pattern after another reset. Looks like the Z80 is also toast.

So, working Z80 back in, tested it went to a K (it did - phew!), and then I put the suspect ULA into it and turned it on. The ULA works!

I'm gonna leave the dodgy board for a bit, because I'm not 100% certain if my composite mod works, and the proper mod is still wending it's merry way from Germany