Composite mod issue: left side of screen missing - solved!

Discussions about Sinclair ZX80 and ZX81 Hardware
patters
Posts: 122
Joined: Thu Mar 11, 2021 12:34 pm

Re: Composite mod issue - left side of screen missing

Post by patters »

I'll try removing the 555. My first thought would be to wire up the second unit of the same composite mod to my Issue 3 ZX81 but unfortunately, despite working fine the other night when I took that photo of it on RF, it no longer shows the K, just a plain white screen. I suspect that means the RAM is caput. I have a 32K SRAM ready to replace it so I may have a stab at that, and if that fixes it I will try the composite mod. However, that machine has the back porch (210 ULA) so based on Mark's advice I should be ejecting the 555 from that one.

Thing is, I may need to request a refund for these mods so I'm reluctant to mess around soldering the second one if they're going back. The seller hasn't responded so that's a concern.

Regarding the yellowy colour banding, it looks worse in photo than in real life so I'm less concerned about that. I mentioned it to determine whether it's perhaps indicative of a less than good mod.
Last edited by patters on Sat Sep 17, 2022 11:56 pm, edited 1 time in total.
patters
Posts: 122
Joined: Thu Mar 11, 2021 12:34 pm

Re: Composite mod issue - left side of screen missing

Post by patters »

Without the 555 inserted image is very clear, complete (no left clipping) and in black and white (no yellow interference). The brightness is however quite low, as to be expected without the back porch in the signal. There's also some jailbarring.

So the problem appears to be the mod. Is this proof I should be returning them to the seller?

EDIT - I also took the 555 from the other mod and the result is the same. This isn't generating the back porch properly and it's really interfering with the signal. Without the 555 the image is so much clearer and sharper, though dark.
Last edited by patters on Fri Mar 26, 2021 7:11 pm, edited 1 time in total.
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Composite mod issue - left side of screen missing

Post by Moggy »

I have seen these boards before and I'm sure they were being peddled for Atari not Sinclair.

This picture is all I could find and no web site which is worrying.

Who supplied these boards? someone here might know of them and also daft as it sounds is the 555 correctly orientated ?(straw grasping I know)
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patters
Posts: 122
Joined: Thu Mar 11, 2021 12:34 pm

Re: Composite mod issue - left side of screen missing

Post by patters »

That's the one. There were a bunch of them specifically advertised as being for ZX81, twelve units for sale I think:
https://www.ebay.co.uk/itm/ZX81-Composi ... 4673425865
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Composite mod issue - left side of screen missing

Post by Moggy »

This where I saw them before , same seller by the looks of it.

https://picclick.co.uk/ZX81-Composite-O ... 39666.html
patters
Posts: 122
Joined: Thu Mar 11, 2021 12:34 pm

Re: Composite mod issue - left side of screen missing

Post by patters »

It seems to be a design that lots of people have made variants of. This one has an extra transistor and a hard disable switch for the back porch fix:
https://www.youmakerobots.com/home/73-zx2020.html

Here's another but without the trimmers:
https://rudedogretros.co.uk/product/sin ... rator-mod/

It's precisely because this seemed to be an established design that I didn't ask for advice before ordering.
Last edited by patters on Fri Mar 26, 2021 7:45 pm, edited 2 times in total.
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Composite mod issue - left side of screen missing

Post by Moggy »

Enlarging the picture on the site shows different wiring or is it my old eyes?
patters
Posts: 122
Joined: Thu Mar 11, 2021 12:34 pm

Re: Composite mod issue - left side of screen missing

Post by patters »

That's the pic I used to wire mine up. Just double checked.
GND in the corner of the PCB
ULA feed next
AV output
+5V is the red wire


My modulator can has the wires crossing over each other a bit because I wanted them long enough to allow fitting and removing the can without having to re-solder everything - specifically in case of this kind of palaver!
Last edited by patters on Fri Mar 26, 2021 7:31 pm, edited 1 time in total.
Moggy
Posts: 3231
Joined: Wed Jun 18, 2008 2:00 pm

Re: Composite mod issue - left side of screen missing

Post by Moggy »

No it's my old eyes. :(

Did they not supply a wiring diagram?
patters
Posts: 122
Joined: Thu Mar 11, 2021 12:34 pm

Re: Composite mod issue - left side of screen missing

Post by patters »

Nope bugger all info. And helpfully it says on the auction that they don't offer any support for the modding process. However this appears to be distance selling of something either defective or not fit for purpose, so hopefully PayPal will have my back.

This is it without the 555 inserted. So clear and sharp that you get the jailbarring. You can even see the differences between paper and border areas (in ZX Spectrum terms). But it is quite a dim grey, so not really acceptable like this.

Back porch disabled.jpg
Last edited by patters on Fri Mar 26, 2021 7:49 pm, edited 5 times in total.
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