My next challenge: Memotech MTX500.

Any 8bit repairs, images, cool things, upgrades etc.
stephen_usher
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Re: My next challenge: Memotech MTX500.

Post by stephen_usher » Sun May 12, 2019 8:37 am

I found a hack to allow 4164 RAM chips to replace 4116 RAM and replaced the faulty VRAM chip and, success!
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I now just have to wait for a replacement 74LS273 as the dead flip-flop stops part of the keyboard scan from working. Then I've got to see if I can stop a lot of the keys on the keyboard from "chattering". Douse the whole lot in switch contact cleaner? Probably.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

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Re: My next challenge: Memotech MTX500.

Post by PhilC » Sun May 12, 2019 9:47 am

Well done Stephen. Very happy to see you bought it back to life.
Must get myself one at some point.

stephen_usher
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Re: My next challenge: Memotech MTX500.

Post by stephen_usher » Sun May 12, 2019 11:01 am

PhilC wrote:
Sun May 12, 2019 9:47 am
Well done Stephen. Very happy to see you bought it back to life.
Must get myself one at some point.
As with most things, they're going for silly money on eBay, and there's no guarantee that they even work!
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

stephen_usher
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Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
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Re: My next challenge: Memotech MTX500.

Post by stephen_usher » Sun May 12, 2019 11:07 am

P.S. It's so nice to have this working again after 31 years!

Here's the same machine back in 1985. :-)
img002.jpg
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Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

stephen_usher
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Joined: Mon Nov 13, 2017 7:19 pm
Location: Oxford, UK.
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Re: My next challenge: Memotech MTX500.

Post by stephen_usher » Thu May 23, 2019 8:16 pm

Well, it seems that it wasn't completely fixed, merely 90% there.

As long as a program didn't try to use the upper 16K of RAM, which pages in over the 2nd 16K block, everything works fine. Try to use the memory paging and let's say that you have an undefined result.

When the machine is first switched on from cold it's fine for about the first minute and then things go wrong.

The problem is that every chip in the main memory and ROM access (including all the page switching logic) has been swapped out and replaced by brand new components and the issue still exists.

The 'scope seems to show a stable +5V rail, but there is evidence that the TIP2955 has failed (hot wire wound resistor) so all of the +5V line is being supplied by the regulator, which isn't supposed to be able give the full 0.9A required, merely 500mA. It has been suggested that this *may* be the issue.

Does anyone here have any thoughts?

(If you want to look at the circuit diagrams they're here: http://primrosebank.net/computers/mtx/d ... hPages.pdf)
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

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Re: My next challenge: Memotech MTX500.

Post by exxos » Thu May 23, 2019 8:45 pm

Odd things like that I would see first whats getting warm on the thing.. and try freezer spray on anything which is getting warm and see if that increase your "run time" anything.

Though of course bad caps can be a issue.. but like I found on the ST.. there could just be design issues from day 1 which never cause a issue until recent years. Such things are 10K pull pull-ups, you may need to swap them out for 4.7K's for example.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

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Re: My next challenge: Memotech MTX500.

Post by stephen_usher » Thu May 23, 2019 9:44 pm

exxos wrote:
Thu May 23, 2019 8:45 pm
Odd things like that I would see first whats getting warm on the thing.. and try freezer spray on anything which is getting warm and see if that increase your "run time" anything.
That was the first thing I tried. Cooling one of the chips seemed to make a little difference, swapped it for another brand new chip and there was no difference.
Though of course bad caps can be a issue.. but like I found on the ST.. there could just be design issues from day 1 which never cause a issue until recent years. Such things are 10K pull pull-ups, you may need to swap them out for 4.7K's for example.
There are very few electrolytics in the design, they're mainly ceramics and poly, and none of those look to be on the data or address buses. There are no pull-up resistors either. Everything's driven either by the memory (via buffers and multiplexers) or Z80. Without anything driving them the address lines and data lines float to around 2V.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

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Re: My next challenge: Memotech MTX500.

Post by exxos » Thu May 23, 2019 9:53 pm

stephen_usher wrote:
Thu May 23, 2019 9:44 pm
Without anything driving them the address lines and data lines float to around 2V.
There is some 10K pull ups on the diagram.. but nothing should float even if nothing is using those lines.. Assuming the Z80 tristates and switching to 0V ( like the 68000 does) those should have pull ups on address and databus. Otherwise its depending on the LS chips floating to 5V to drive a logic HI.
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
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Re: My next challenge: Memotech MTX500.

Post by stephen_usher » Thu May 23, 2019 10:21 pm

I could only see the pull-ups for the keyboard lines.
Intro retro computers since before they were retro...
ZX81->Spectrum->Memotech MTX->Sinclair QL->520STM->BBC Micro->TT030->PCs & Sun Workstations.
Added code to the MiNT kernel (still there the last time I checked) + put together MiNTOS.
Collection now with added Macs, Amigas, Suns and Acorns.

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Re: My next challenge: Memotech MTX500.

Post by exxos » Thu May 23, 2019 10:33 pm

I'm not familiar with the thing at all, Just spotted 10K on some chip and thought I mention about pullups causing issues, but doesn't seem to be any much anywhere on the thing.

Is there any proper RAM test software to see what faults are actually appearing and where in RAM ?

That TMS chip seems to be linked to RAM, so I guess like you mentioned, it could be faulty if you changed everything else..
4MB STFM 1.44 FD- VELOCE+ 020 STE - 4MB STE 32MHz - STFM 16MHz - STM - MEGA ST - Falcon 030 CT60 - Atari 2600 - Atari 7800 - Gigafile - SD Floppy Emulator - PeST - HxC - CosmosEx - Ultrasatan - various clutter

https://www.exxoshost.co.uk/atari/ All my hardware guides - mods - games - STOS
https://www.exxoshost.co.uk/atari/last/storenew/ - All my hardware mods for sale - Please help support by making a purchase.

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