MTX512 - Screen Trouble

About original Memotech hardware.
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SchmitS
Posts: 16
Joined: 27 Nov 2020 09:30
Location: Luxembourg

MTX512 - Screen Trouble

Post by SchmitS »

Hello

When I turn on my MTX there is a character displayed instead of a space on a few lines of the screen.
Before going further into research, RAM or decoder, have any of you already encountered this problem ?

Thank you in advance for your good advice.

Best regards

MTX512 - Trouble.jpg
MTX512 - Trouble.jpg (1.55 MiB) Viewed 41884 times
Martin A
Posts: 765
Joined: 09 Nov 2013 21:03

Re: MTX512 - Screen Trouble

Post by Martin A »

There's a write up of VRAM test and repair here http://primrosebank.net/computers/mtx/r ... repair.htm

The reverse quote has an ASCII code in hex of #60. a space has a code of #20.

That #40 difference suggests that there is a rogue bit appearing on D6.

At the time TI designed the VDP, they numbered thier address and data busses the opposite way to modern converntion, so what we would call D6, they would have referred to as D1. RD1 is connected to the VRAM chip at 7G, so that would be the one I would test first.
SchmitS
Posts: 16
Joined: 27 Nov 2020 09:30
Location: Luxembourg

Re: MTX512 - Screen Trouble

Post by SchmitS »

Thank you for your replies

I found the same result that bit 6 was corrupted at 1 on some adresses.
I was hesitating between CPU or VDP RAMs. This means that BASIC uses VRAM as a buffer for keyboard inputs.

Now I have to find new one to replace them.

Have a nice day.

6=36h >> v=76h
MTX512 - wrong caracter.jpg
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space=20h >> @=40h
MTX512 - Cursor.jpg
MTX512 - Cursor.jpg (1.34 MiB) Viewed 41865 times
Basic input
MTX512 - Basic Line.jpg
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Martin A
Posts: 765
Joined: 09 Nov 2013 21:03

Re: MTX512 - Screen Trouble

Post by Martin A »

As far as I've been able to work out, yes, the OS does use the input line on the screen during some part of the command processing. The manual says there is a keyboard buffer at #FB4B i'm not sure though at what stage that gets filled.

If the CPU ram misbehaves, then you'll likely just have a non starting MTX.
SchmitS
Posts: 16
Joined: 27 Nov 2020 09:30
Location: Luxembourg

Re: MTX512 - Screen Trouble

Post by SchmitS »

Hello

It works again, I exchanged VRAM 4116 by 4164.
-5V is no more needed.
I also repaired 2 defected keys.
MTX512 - VRAM.jpg
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MTX512 - Board.jpg
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MTX512 - Alpha.jpg
MTX512 - Alpha.jpg (1.47 MiB) Viewed 41645 times
MTX512 - Graphic.jpg
MTX512 - Graphic.jpg (1.22 MiB) Viewed 41645 times
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1024MAK
Posts: 756
Joined: 24 Dec 2012 03:01
Location: Looking forward to summer, in Somerset, UK

Re: MTX512 - Screen Trouble

Post by 1024MAK »

Nice 8-)

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer in Somerset later in the year :D

Not as many MTXs as Dave! :lol:
Martin A
Posts: 765
Joined: 09 Nov 2013 21:03

Re: MTX512 - Screen Trouble

Post by Martin A »

Shame it's being force to show Toado though !
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