Resurrecting an MTX500.

About original Memotech hardware.
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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 10 Apr 2019 19:36

stephen_usher wrote:
10 Apr 2019 17:46
1024MAK wrote:
10 Apr 2019 15:33
The edge-connector details are in the Memotech BASIC Tutor Reference and Operators Manual / Memotech Operator's Manual (on one of the pages that has the schematic diagrams) and also in the MTX 500/512 Service Manual and Technical Information manual (PDF page 23, paper page 20).
Does that give all the contact pitch and board thickness details? (I can't have a look at the moment as I'm at work.)
No. But it is a standard PCB edge-connector with a 2.54mm / 0.1 inch pitch. Same as a ISA or ZX Spectrum type (except a location slot in a different place and different number of contacts).

Mark

Martin A
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Re: Resurrecting an MTX500.

Post by Martin A » 10 Apr 2019 20:25

stephen_usher wrote:
09 Apr 2019 23:58
Address lines A0-A6 show activity, A7-15 do not. Could this be just the ROM start-up code getting so far within the first 256 bytes before crashing and restarting? But then I should see data on the data bus and I don't. (Well, I often see one edge and sometimes see multiple data on some data lines but never all of them.)

The data lines are sometimes continuously pulled high, sometimes pulled low but most of the time floating just under 2 volts (with a lot of noise) Almost as if both the memory and CPU are waiting for the other to put data on the bus or acknowledge it.
The rom boot code jumps pretty much straight into the memory test, it should start tesing the ram at 04000H within aa dozen or so instructions from starting up.

Once that's done the top 16k is cleared and the paged roms are tested for auto run.

At that point the VDP and PSG are set up. That's what makes the blank screen and "tone" such a pain to diagnose. All you really know is the system didn't get to the point where the VDP is set up.

It's probably worth scoping the ROM chip selects to see if the roms are selected. IF the system happened to get switched into RAM only mode it could be processing all sorst of garbage from uninitialied, or even non existant, memory.

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 10 Apr 2019 20:56

They are surprisingly difficult to get hold of, at least the open ended ones with repositionable keys.

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Dave
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Re: Resurrecting an MTX500.

Post by Dave » 10 Apr 2019 21:06

You can just remove the contacts from position 5 and make a key of your own.
Putting in a small piece of PCB works well

If you use a 30x2 connector and plug onto the external edge connector, you don’t actually need a key

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Dave

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 10 Apr 2019 22:18

After doing some more 'scoping it looks like something is pulling the data lines to strange voltages, which changes with every reset using the two keys.

Sometimes the machine "runs" with the data lines active. D5 always looks the best often D0 and D2 are good too. The others are variable. They can be pulled high. low or at strange voltages. Sometimes you can see possible data rippling at the top and sometimes it seems almost like an inverted signal, but that's silly.

I've moved the RAM chips around and the behaviour doesn't move with the chips.

At one point I tried using freeze spray and thought I'd found the problem chip and the one running the keyboard as when that was chilled the machine wouldn't reset into a "running" state until it warmed up, but after trying it a second time this wasn't the case.

It does seem as if something directly affected by the /RESET line comes back in a strange state pulling the data lines to strange voltages (I've had consistent steps of around 1V, 3V 4V and 5V repeated so that the 'scope triggers them continuously.)

It can't be the CTC as the same behaviour happens with that removed. Similarly the video chip.

I've some video of a strange reset which I'll upload to YouTube and post.

P.S. Here it is: https://youtu.be/5VVGaIJvrm8

stephen_usher
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Re: Resurrecting an MTX500.

Post by stephen_usher » 11 Apr 2019 12:01

I've now got a replacement 74LS244 so I think that I'll chop out 2E as that disconnects the most logic from the data bus. As far as I can see the only other chips on the data bus are the 74LS244 (2A and 1D) running the keyboard, the ROMs and the 74LS373 handling the parallel port input (8B).

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Re: Resurrecting an MTX500.

Post by stephen_usher » 11 Apr 2019 23:00

I managed to safely desolder all three 74LS244 chips so I fitted sockets and tried the machine with none in place... No change. Swapped them around, replaced one and then another with the new one... No change.

So, I've now replaced/removed every chip on the data bus except 8B (74LS373) which acts as the input for the parallel port.

Otherwise the only other constant chips are the ROMs.

Do you know where I might be able to get a set (A, B and C) for a 4000-05 motherboard?

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1024MAK
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Re: Resurrecting an MTX500.

Post by 1024MAK » 12 Apr 2019 00:12

If you adapt the PCB (cut some tracks and add some modification wires) it is possible to use EPROMs.

But first, try removing all the ROM chips and see what happens to the data bus. If all the data bus lines float to logic high, the Z80 will execute RST 38h repeatedly. If all the data bus lines float to logic low, it will execute NOPs repeatedly. Well, while in the address range of the ROM...

Mark

Martin A
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Re: Resurrecting an MTX500.

Post by Martin A » 12 Apr 2019 00:31

Unless someone has some pulls from a dead MTX, the only solution I'm aware of for broken roms, is to replace the with EPROM or EEPROM.

But that's a one way trip as it means cutting tracks details of how Dave fixed one of his are here:

http://primrosebank.net/computers/mtx/r ... eplace.htm

I've had to do something similar with one of mine, but in that case it was already modified in the factory for 2564 EPROM, I did the minor tweaks to use 28C64 EEPROM instead. There's a picture of it on that link on Dave's site, with it's bright yellow labels.

For testing, an external ROM board with a 32k EPROM containing all 3 OS roms and a GAL for decoding shouldn't be too hard to design. If you want I'll have a bash at putting one together.

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Dave
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Re: Resurrecting an MTX500.

Post by Dave » 12 Apr 2019 00:33

stephen_usher wrote:
11 Apr 2019 23:00

Do you know where I might be able to get a set (A, B and C) for a 4000-05 motherboard?
http://www.primrosebank.net/computers/m ... 12roms.htm. ROM Images

http://primrosebank.net/computers/mtx/r ... eplace.htm. ROM Upgrade

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