ZX81 Hi-Res

[Andy Rea]

try connecting a 33pf capacitor across GND and RAM-CS ( i think its pin 20 but not sure) at the ram chip.

regards Andy

[britain4]

try connecting a 33pf capacitor across GND and RAM-CS ( i think its pin 20 but not sure) at the ram chip.

regards Andy

Thanks, I'll give that a shot too. I'll update with how it goes.

Andrew

[1024MAK]

/RAMCS goes to both pin 20 and pin 22 of the RAM socket.

Mark

[britain4]

try connecting a 33pf capacitor across GND and RAM-CS ( i think its pin 20 but not sure) at the ram chip.

regards Andy

I've just tried it, didn't make any difference I'm afraid - we might just have to see how it goes with a different chip!

Cheers

[RetroTechie]

Here's the chip

That's not "brand new", but 22 years old!

It's possible some IC('s) was damaged due to electrostatic discharge (ESD). This may easily happen during experiments, and damage often isn't visible. But a bit of bad luck, or one mishap too many, and you've got a non-functional part. Where "non-functional" can be anything from a single nuked cell in a memory chip, to totally dead. Note this may involve other IC('s) in the machine than the part being worked on.

Happened to me a while ago when working on that ULA replacement. Z80 cpu was the victim, an accidental touch of one of the pins at the wrong time, was the cause. This Z80 still appeared to be working normal, but in some demo's you'd see corruption on screen. Replacing this Z80 fixed the issue.

Just mentioning this here since symptoms look very similar to what happened in my case.

[1024MAK]

Worthwhile trying a different chip. New 32k byte SRAM DIP chips are still available from the big electronic suppliers (RS, Farnell etc). And yes, they will sell to individuals. To save handling charges for small orders, you can use a trade counter if you are near one.

Mark

[britain4]

Here's the chip

That's not "brand new", but 22 years old!

It's possible some IC('s) was damaged due to electrostatic discharge (ESD). This may easily happen during experiments, and damage often isn't visible. But a bit of bad luck, or one mishap too many, and you've got a non-functional part. Where "non-functional" can be anything from a single nuked cell in a memory chip, to totally dead. Note this may involve other IC('s) in the machine than the part being worked on.

Happened to me a while ago when working on that ULA replacement. Z80 cpu was the victim, an accidental touch of one of the pins at the wrong time, was the cause. This Z80 still appeared to be working normal, but in some demo's you'd see corruption on screen. Replacing this Z80 fixed the issue.

Just mentioning this here since symptoms look very similar to what happened in my case.

Well, the seller mentions they have been selling electronic components for 22 years, that's probably when the description was written!

I've got another (Cypress) on the way so we'll see if that's any better. Failing that I will try one from a proper supplier.

I've redone the mod now on another board, with a decent turned pin socket instead of the el-cheapo one I used before. The first board had seen a lot of appalling soldering from before I had the equipment/skills and I thought it was time for a fresh start haha.

It looks a lot neater (I used the exact same method as you posted 1024MAK) but as expected it is still giving the same result. At least now I can be 99.9% certain it is the chip as I've changed every other possible variable!

[Andy Rea]

try connecting a 33pf capacitor across GND and RAM-CS ( i think its pin 20 but not sure) at the ram chip.

regards Andy

I've just tried it, didn't make any difference I'm afraid - we might just have to see how it goes with a different chip!

Cheers

interesting i had the same problem years ago, and when putting a scope probe on the CS pin it would work, and removing the probe the artifacts would re-appear so putting the cap there reproduced the capacitive loading of the scope probe.

oh well was worth a shot. hope you sort it.

regards Andy

[britain4]

try connecting a 33pf capacitor across GND and RAM-CS ( i think its pin 20 but not sure) at the ram chip.

regards Andy

I've just tried it, didn't make any difference I'm afraid - we might just have to see how it goes with a different chip!

Cheers

interesting i had the same problem years ago, and when putting a scope probe on the CS pin it would work, and removing the probe the artifacts would re-appear so putting the cap there reproduced the capacitive loading of the scope probe.

oh well was worth a shot. hope you sort it.

regards Andy

That's interesting - worth a shot as you say but sadly no dice this time cheers anyway!

After a little scare with it not giving a K prompt (turned out to be a really shoddy solder joint on my wiring to the ZX8-CCB I somehow hadn't spotted) I'm just waiting on the RAM chips (I ordered one from the same listing as first time, then thought better of it and ordered a Cypress one from a different seller. Then I thought better of ebay sellers, bought 3 different ones from RS, then went back on ebay and saw a lot of 5 going for £1 so I had to have those too lol)

End result I have about 10 62256 chips on the way so if anyone needs a spare give me a shout...!

One thing I have taken from this is to anyone is thinking of doing the 16k mod with bending the chip legs out... just please don't haha

[britain4]

My new 62256s arrived from RS. Tried them in the Zed and... identical results as before! Exactly the same garbage around the hi-res elements.

I don't know what could be going on here - I've literally changed every component and the modding method and it still doesn't work right!

The capacitor does change the amount of garbage around the screen elements so maybe I will carry on playing around with that unless anyone has any other ideas