Resurrecting an MTX500.

[stephen_usher]

Of course, if I'm going to have to replace the PAL I might as well change the jumpers and make the machine into an MTX512 at the same time.

[stephen_usher]

I've ordered a programmer and separately some 16V8B GALs from Mouser. Those will take a few days to arrive.

I'm thinking now of options for ROM replacement given that the EEPROMs mentioned in the article on PrimroseBank don't seem to be available any more.

Surely board connected into ROM B socket with a couple of flying leads to ROM A and ROM C would give all the lines needed to run a 32K ROM? Put the three ROM select lines through an inverter and use the ROM B and ROM C select output as two address lines. Then put all three inverted ROM select lines through a three-way or gate and invert the output to be the ROM select.

[Dave]

https://www.mouser.co.uk/ProductDetail/ ... gbep2as%3D

[stephen_usher]

https://www.mouser.co.uk/ProductDetail/ ... gbep2as%3D

Thanks... With the correct part number I found them in the RS catalogue, so ordered two packs of two.

[Martin A]

Surely board connected into ROM B socket with a couple of flying leads to ROM A and ROM C would give all the lines needed to run a 32K ROM? Put the three ROM select lines through an inverter and use the ROM B and ROM C select output as two address lines. Then put all three inverted ROM select lines through a three-way or gate and invert the output to be the ROM select.

I did some experiments with an internal rom board before settling on the external design.

All you need logic wise for just 3 roms is a single 3 input AND gate, Chip select for ROM B goes to A13 on the rom, chip select for ROM A goes to A14. The 3 chip selects are then anded to make the 32k rom chip select, the output of the AND will go low when any of the 3 rom selects go low.

There's no need for inverters on the upper address lines if you get creative with the rom placements:

The first 8k of the rom is left empty, can't be selected as the MTX board never pulls 2 rom selects low at the same time.
ROM A goes in the 8-16k area as With ROM A CS low, A14 is low (as CS A is low) and A13 high (as CS B is high).
ROM B goes in the 16-24k area as With ROM B CS low, A14 is high (as CS A is high) and A13 low (as CS B is low).
ROM C goes in the 24-32k area as With ROM C CS low, A14 is high (as CS A is high) and A13 high (as CS B is high).

The problem with the test configuration was height. The Header pins, plus the board, plus a socket plus the chip in the socket didn't allow enough clearance for the keyboard. In the photo the empty socket is a stand in for the AND chip, and that too is in danger of being hit by the keyboard PCB, PCB itself would have needed to be shortened with the last 4 rows of holed removed. There's 2 pin headers to pick up the extra chip selects, but that's for stability, the other pin would be unconnected.

With shorter header pins, it might be doable, but it wasn't with the ones I had to hand so I went for the external solution. I didn't want to solder the memory chip to the daughterboard, but that might be another solution, as that cuts the height by 5mm or so,.

Putting a 32k (e)eprom directly into the socket with some artfully bent pins, with the AND chip dead bugged on top is another possibility. (Sinclair Spectrum model 1 nostalgia anyone ??). But realistically that too is a 1 way trip for the rom.

rom daughter board.jpg (70.55 KiB) Viewed 11725 times

[stephen_usher]

Well, I've started the mods (removed sockets and cut tracks) for the use of three eeproms now, as described on primrosebank.

I've ordered some more sockets as two of them fell apart when I de-soldered them.

I should have everything except the GAL by the weekend, then I'll have to wait for that to arrive from Mouser. (EEPROMs and programmer should arrive tomorrow (along with parts to fix the PAL board) and the sockets and wire wrap wire on Friday.)

P.S. If anyone wants the old ROMs, in case they still work, I'm sure I can find a way to post them to you for free.

[Dave]

If anyone needs them to repair a machine, it might be worth trying them.

On the other hand, if nobody needs them, I could try reading them using my adapter to try and prove whether they are faulty or not

[stephen_usher]

I've made the motherboard mods for the EEPROMs, written the data to the EEPROMs and fitted them...

No difference.

The GAL arrives on Monday.

I can spend the weekend getting the PAL board operational again. I've got the transistors for Q1 and Q5, which were close to where the crocodile clip bounced, plus some CD4013s and CD4016s. I hope it's not the 1889.

[Martin A]

If it IS the 1889, have a word with Andy.

When he put this page together he had spares of that.

http://www.nyangau.org/memotech/parts.htm

[stephen_usher]

Thanks. I did try ordering one from Retroleum a couple of days ago and got the automated e-mails confirming the order but nothing afterwards. When I previously bought some memory from there it was quite quick.

I may cancel and order one from Mutant Catapillar Games. He's definitely reliable.

P.S. I've tried the Diag ROM in the ROM A socket and no difference. (I did find that one of the LS157 chips had become faulty, even though it's pretty new. It was holding A12 high. I replaced it with one of my new spares.)