Hello Everyone from Sunny Porthcawl

[lesoleil70]

Hi

Been a ZX spectrum fan since a child, started fixing up and collecting 70's and early 80's computers about two years ago.

Recently got my third ZX81 and to my disappointment it needs a new ULA.

My world of ZX81 has already taken me on the following small journeys / restores.

1st Pickp - Ebay. New Membrane required. Now working 100%
2nd Pickup - part of boot sale bundle. Working 100%, composite modded this one.
3rd recent pickup, looks like early model, has the Z80 with the board soldered on top, this is the one that needs a new ULA.


I'm on the hunt for a ULA if anyone can help me out. I know you can get one from Mutant Catterpillar, just a but pricey for my needs just want to get this running.

Nice to meet you all, looking forward to particpating here. I can already see a few usernames I recognise from WOS and Denial.

[Paul]

What date code and type is your broken ULA?
I guess you would prefer to have your replacement as accurate as possible?
At least I wouldn't want to replace a 2C184 by a 2C210.
Maybe I can help with the correct replacement.
kind regards Paul

[Moggy]

Welcome aboard lesoleil70

The board you describe on top of the CPU was used to correct a division rounding error in the early ROM known in some quarters as the square root bug. The board didn't totally cure the problem and there is some info in this thread..

viewtopic.php?f=7&t=762&p=6932&hilit=sqr+bug#p6932


@Paul.

On a zeddy this early I would say with certainty that the ULA is a 2c158.

Unfortunately the last ULA I had of this type I gave to Balford so no help here sorry to say.

[Fwapp]

I've seen a few "square root fix/kludges" in ZX81's with 184 ULAs. I thought they might have replacements but the date code matched the modulator/other chips on the board.

The really early ZX81's also seemed to have softer, spongy feet , not the hard rubber feet.

[Paul]

@Paul.

On a zeddy this early I would say with certainty that the ULA is a 2c158.

Hi moggy. I know. I have all three types as spares.
As I am no collector I am ready to help.
If he doesn't care I would advise to use a 2c210.
kind regards Paul

[Moggy]

@Paul.

On a zeddy this early I would say with certainty that the ULA is a 2c158.

Hi moggy. I know. I have all three types as spares.
As I am no collector I am ready to help.
If he doesn't care I would advise to use a 2c210.
kind regards Paul

Agreed Paul the later ULA is the way to go unfortunately I have no spares at this moment in time or I would happily send one FOC.

The soft rubber feet I seem to recall being fitted to the power packs or the printers not sure which.

[Fwapp]

I've no ULA's either (well, I do have a pile of dead ones) which is a shame. It'd be good if the modern replacement that was around few years back was a available again.

Otherwise it's a case of keeping an eye out on the likes of eBay, they do turn up occasionally.

On the feet, yes it's the soft ones like on the printer/psu that I've found on early ZX81's, as have a few other guys I've asked to check their's. This type of zeddy is normally a 154 ULA with an earlier datecode (8113 and below)

I've noticed the feet on eBay Zeddys as well - the type of zeddy that comes with a ZX80 PSU.

[lesoleil70]

Thanks everyone for the warm and helpful welcome to this forum.

The dead ULA is 2C184E
I put up a few pics of the Kludge Z80 on my twitter account.

https://twitter.com/lesoleil70/status/7 ... 6527697921

It would be great to get this one running with any compatible ULA available to me. I'm happy to adjust brightness and I'll be using a CRT with this one.
, thanks in advance if there's a ULA available to get this one running again, looking positive.

[1024MAK]

Hello and welcome to our forum

Any good working ZX81 ULA will work in any ZX81 board.

The difference between the two early versions and the later (and last) version is that only the last version has the circuitry to generate the back porch part of the video signal.

Mark

[balford]

Hello

I don't suppose you bought that machine with the kludge and dead ULA from blooda on eBay? I have a funny story if so

Ian at Mutant Caterpillar Games will have some 2C210E's in stock if you're stuck.

Moggy, I didn't realise that was the last 2C158E you had. Now I feel doubly humbled

B