exxos's Atari, Amiga & retro forum

Way back in 1988 my Memotech MTX500 just stopped working whilst it was connected to a BBC Micro via a serial cable.

Over the years I've tried multiple times to repair it, initially in the 80s replacing the Z80 and Z80CTC and about 10 years ago the TMS graphics chip, all to no avail.

Now I have an oscilloscope and have experience not only with the Acorn A4000 but now bringing a BBC Model B+ 128K back to life it's time to have another try at the Memotech.

Of course, the first thing to check is what the address and data buses are doing when you switch on.

Well, the address lines on the processor side look OK, but just look at the mess the address line 1 on the memory side of the 74LS157 multiplexer (pin 9) looks like! Compare that to pin 10's voltage: I'm guessing that it could be a RAM chip, but the cheapest and easiest replacement is the 74LS157 so I've ordered enough to replace any/all of that type of chip in the machine. I'm also going to socket them. If that doesn't make any difference then I'll have to change all the memory chips (4164s).

Always fancied an MTX512.

You could try the piggyback method with the ram chips. When I had dodgy 4164s on my 800XLs I would piggyback a known good chip over each soldered in ram chip until it ran.

I've tried this but the 2.5 volt base for the A1 line never goes away. So trying to replace the multiplexer first is the quickest and easiest test. Otherwise I have to replace each RAM one by one, or just bite the bullet and replace them all and put them in sockets.

Yeah it doesn't always work unfortunately.

I'd socket the ram chips anyway, computers I want to keep I've always done that, knowing how unreliable old ram can be.

If your ram is branded MT, I'd probably bin the lot anyway from having bad experiences with that brand before.

The original is all OKI. They're actually 4132s which are really 4164s where either the upper half or the lower half failed the original testing. The ones on my machine were the 'H' designation so there's a jumper setting the system to use the high addresses in the chips.

The MTX512 (64KB RAM) used fully functional 4164s, a different jumper setting and a different PAL configuration.

Thankfully the original manual contains a full circuit diagram and later versions even the source for the PAL for both the MTX500 and MTX512.

That's cool that they included that. Will definitely get one at some point.

Hope you get it fixed soon.

So, after removing all the memory chips, MA1 (memory address line 1) is still bad. The only item on that line now is the 74LS174 so it looks like it's definitely that chip.

The replacement, along with some sockets should arrive tomorrow. I'll install the 7 sockets I have for the memory tonight and fit the other along with the 74LS chip.

Always thought that the MTX was a cool machine, would still love to get my hands on one.

Well, the memory's replaced as is the 74LS157 but still no joy. At least MA1 looks correct now.

stephen_usher wrote: 09 Apr 2019 18:36 Well, the memory's replaced as is the 74LS157 but still no joy. At least MA1 looks correct now.

It's still a step forward. Is the memory definitely good?

Sounds like it's time to get the scope out :D