Distorted image

Discussions about Sinclair ZX80 and ZX81 Hardware
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tholin
Posts: 5
Joined: Thu Sep 21, 2017 7:16 am

Distorted image

Post by tholin » Thu Sep 21, 2017 7:25 am

Hi.

I recently bought a ZX81 on eBay. It turns on ok, but the output image is distorted. I clueless about electronics in general and the gear inside the ZX81. I've attached a couple of the image on screen. The first one is without the ram expansion plugged in, the second on with. The TV set is an old Daewoo 14" color CRT model. Does anyone know what the issue might be?

Thomas

EAB181E0-79AB-4376-B52E-B873F03C87DC.jpeg
EAB181E0-79AB-4376-B52E-B873F03C87DC.jpeg (5.18 MiB) Viewed 242 times
11DA71FD-BE21-438A-BA75-E4374694003F.jpeg
11DA71FD-BE21-438A-BA75-E4374694003F.jpeg (5.91 MiB) Viewed 236 times

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sirmorris
Posts: 2671
Joined: Thu May 08, 2008 4:45 pm
Location: oxon, uk

Re: Distorted image

Post by sirmorris » Thu Sep 21, 2017 8:33 am

Hi Thomas.

I think the RAM may be an issue. I would suggest leaving it off until the picture is sorted. It looks like there is some output but it's not clear from the angle of the picture what it is. Can you please take a closer look at what is on the screen?

If you have tweaked the fine tuning on the TV then you may need to adjust the modulator. This is a last resort however. To do it you need to disassemble the zeddy and therefore risk breaking the keyboard membrane, so try other things first. Another TV if possible perhaps?
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

tholin
Posts: 5
Joined: Thu Sep 21, 2017 7:16 am

Re: Distorted image

Post by tholin » Thu Sep 21, 2017 8:43 am

Thanks for your reply. :)

I can see a distorted text cursor at the bottom of the screen when the RAM module isn't plugged in. Reading the characters I type in is possible, but difficult. Whenever the RAM module is plugged in, moving it slightly up or down changes the output significantly.

tholin
Posts: 5
Joined: Thu Sep 21, 2017 7:16 am

Re: Distorted image

Post by tholin » Thu Sep 21, 2017 8:50 am

I tried to take a clearer photo. You should be able to see what's going on in this one. I pressed the "goto" key, and you can barely see what the text on screen is.
CB287671-1B48-4595-8400-82DA67137174.jpeg
CB287671-1B48-4595-8400-82DA67137174.jpeg (7.11 MiB) Viewed 217 times
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290A7F90-F851-41F5-AABB-256AFBE0B0CD.jpeg
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User avatar
sirmorris
Posts: 2671
Joined: Thu May 08, 2008 4:45 pm
Location: oxon, uk

Re: Distorted image

Post by sirmorris » Thu Sep 21, 2017 10:15 am

It looks to be either tuning or timing. Hopefully the former.
--=== ZXpand+ ... A more moderner all-in-one upgrade solution for ZX81 ===--
* SD/MMC storage * 32K Hires-capable RAM * AY sound chip * Joystick port * Serial *
* BASIC enhancements - plug & play * Reset button * Auto-boot facility *

tholin
Posts: 5
Joined: Thu Sep 21, 2017 7:16 am

Re: Distorted image

Post by tholin » Thu Sep 21, 2017 1:08 pm

Well now I don't have the cursor anymore, just the white screen. That can't be good..

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