ZX81 dead ULA/video problem?

[stians]

Hi guys and gals,

Thought I'd give a little follow-up on my previous post from here: viewtopic.php?f=7&t=1236#p13118 - gosh, has it been a year already?

I found the board again yesterday and started tinkering with it again. I suspect the ULA may be alive after all. In the video below you can see that I have socketed the RAM, as well as replaced a few resistors and caps with SMD equivalents. No difference in the behaviour were detected after that mod.

https://www.youtube.com/watch?v=3BNQRjSO7zU

I'm tempted to remove the UM1233 from the equation by doing the composite mod.

As for temperatures, the Z80 and ULA gets quite hot, but I can keep my finger on them for a few seconds. RAM and ROM gets comfortably warm.

I can still not detect a clock signal on the Z80's pin 26, but from what I read the ULA should be able to produce a image regardless of that. I'm hoping this is what I (kind of) am seeing here.

Any ideas, thoughts?

[Spirantho]

Hi,
Without a clock signal you will not get anything at all - so it's likely to be the ULA.
Are you able to desolder that ULA? It's one of the very early types. If you can't do it I can repair it for you but I'd need to charge £35 for it as it's part of my shop service (£30 + £5 for a nice new ULA - and I'll heatsink it for you too if you like).

[1024MAK]

I have an early ZX81 (that uses the first ULA version which was soldered to the board). It appeared dead (no video picture). There was no activatly from the Z80 CPU. On probing the Z80 clock input pin, I found no clock signal. So I tested the ULA clock output pin and again found no clock. So the ULA was found to be the cause.

Sounds just like your sick Zeddy.

As suggested by Spirantho, try changing the ULA.

BTW, the video picture is generated by the Z80 CPU and the ULA working together, if the ULA is faulty you will not get a picture.

Mark

[stians]

Spirantho: I can do the soldering myself, but I am very interested in a NOS ULA+heatsink if you have for sale.

[Spirantho]

Got some here:
http://www.mutant-caterpillar.co.uk/sho ... ts_id=2901
Any problems, just yell!

[Paul]

I still prefer Andy Rea's ULA.
No heat, better picture and nice gimmicks.

[Spirantho]

Andy's ULA replacement is awesome

However, it's just not ... original. To most people it doesn't really matter that it's not an original ULA but I personally like to keep things as they were when they came out. Otherwise it's not really a ZX81 any more, it's a modern replacement (after all, the ULA is the ZX81 really).

Horses for courses, really, but that's why I use original ULAs rather than modern FPGA replacements, even if they do feel exactly the same to the average user.