Sparkly pixels

[Lardo Boffin]

Hello all

I recently bought a ZX81 which had the following issue when outputting RF:-

IMG_5453.JPG

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IMG_5461.JPG

(6.57 MiB) Downloaded 443 times

A number of the pixels flash on and off randomly. There are a few of notes:-

1) The flashing only occurs on the part of the screen actively being displayed. In the second image (my work in progress game) only the first 11 columns are used. There is no flashing outside of this. In the first image I get flashing all over the screen.

2) When first switched on from cold there is no flashing. After 10 to 20 seconds it starts and then gets worse.
After about a minute it stops completely. I have left the zeddy running for about an hour without it coming back.

3) I get the same problem connected to a CRT TV as well as a LCD TV.

4) I have a ZXpand connected (which works perfectly, tested on other zeddys).

This strikes me as being like a mechanical process where maybe something is heating up, causing the issue and then gettting warm enough to no longer cause it?

I have fitted a ZX8-CCB in case that helped but it is still the same.

Any thoughts or suggestions? I don't have any sophisticated analysis hardware.

Thanks!

Lardo

[PokeMon]

2) When first switched on from cold there is no flashing. After 10 to 20 seconds it starts and then gets worse.
After about a minute it stops completely. I have left the zeddy running for about an hour without it coming back.

Well - why not just wait a minute till it stops completely ?
This tends to a thermal problem inside the ULA - the most heated part.
You could verify this by cooling IC's with a cooling spray one for one to see any effect on screen to be sure it is not another IC.

[Lardo Boffin]

2) When first switched on from cold there is no flashing. After 10 to 20 seconds it starts and then gets worse.
After about a minute it stops completely. I have left the zeddy running for about an hour without it coming back.

Well - why not just wait a minute till it stops completely ?

I guess that's kind of the plan but if anyone had seen this before and knew that continuing to use it would cause further problems or damage I would stop and get it sorted.

[Lardo Boffin]

After a while the picture gets unuseable:-

IMG_5462.JPG

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Different screen but I thought I would eliminate the TV factor!

I have swapped the ULA from one that is working and I get the same issue so I assume its not the ULA. Any thoughts on what to try next? Z80? Recapping? New memory?

Thanks for any advice!

Lardo

[Lardo Boffin]

I took the plunge and changed the Z80 as well and it made no difference.

One thing I have just noticed is that it only does this with the ZXPand is plugged in - the ZXPand is fine itself (on two other zeddys).

Could the extra power draw be affecting anything? I don't have anything else to plug into the back to test this theory with.

Lardo

[PokeMon]

Yes the ZXpand draws more power from your Zeddy.
So I think you have no pixel effects without ZXpand ?
Then you should use another PSU.

[1024MAK]

Okay, so it's not the ULA, or the Z80, and it can't be the internal 1k RAM, as you were using external RAM.

Are you saying that the symptoms only occur when you are using a ZXpand?

Please run the machine with no expansions plugged in for a while to confirm.

Do you have a RAM pack? Does it do it with this?

Have you tried another Sinclair PSU?

Suspects:-

• 7805 Voltage regulator or heatsink not fitted tightly.
• ROM
• Poor socket connection or dry joint

Mark

[Moggy]

Could be a dirty edge connector or the xpand not fully seated.
I say this because the first spand I bought I fitted into a Memotech case which had the Velcro that Memotech supplied fitted to the pack and the rear of the zeddy. This meant that the spand stood off from the case slightly and during boot up I got speckles and artefacts.

Removal of the Velcro meant the spand pushed fully home and problem solved.

[Lardo Boffin]

Yes the ZXpand draws more power from your Zeddy.
So I think you have no pixel effects without ZXpand ?
Then you should use another PSU.

I don't get the pixels without the ZXPand plugged in - I have left it for a good 10 mins. Typically when plugged in it happens within 20 to 30 seconds

I have just tried another PSU - a new one I got for my toastrack and it does the same.

[Lardo Boffin]

Okay, so it's not the ULA, or the Z80, and it can't be the internal 1k RAM, as you were using external RAM.

Are you saying that the symptoms only occur when you are using a ZXpand?

Please run the machine with no expansions plugged in for a while to confirm.

Do you have a RAM pack? Does it do it with this?

Have you tried another Sinclair PSU?

Suspects:-

• 7805 Voltage regulator or heatsink not fitted tightly.
• ROM
• Poor socket connection or dry joint

Mark

Yes - it only happens when the ZXPand is plugged in. But i don't have much else to plug in to test.

I do have a 16k memopak which I have just plugged in (and it works!) and that does not cause the issue. I have left it for around 10 mins with a bunch of text on the screen. I don't have any games I can load via cassette (until Virus / Pandemic arrive anyway!) so it isn't exactly being stress tested.

I have tried another PSU - same result.

I will check that the heatsink is tight and do a visual check on the regulator. I will also reseat the ROM but I don't have a spare to try with.

Thanks to all for the suggestions!

Lardo