Resurrecting an MTX500.
Posted: 07 Apr 2019 18:45
Way back in 1988 I had my MTX500 hooked up to a BBC Micro via RS232, using the BBC as a terminal to a program on the MTX when suddenly the Memotech stopped working... dead!
On and off ever since then I've tried to revive it. The first things I did were to replace the Z80 and Z80 CTC as they were easy to find back then... No luck.
About 10 years ago I swapped out the video processor... nothing.
With the tools I had I couldn't get any further.
Anyway, recently I picked up an oscilloscope and with that I managed to bring a BBC Model B+ 128K back to life. So, now with the new tool it's time to look at the old Memotech again.
Of course, the first thing to look at is what do the address and data buses do when you switch the machine on. Well, not a great deal it seems, at least one the memory data bus. One or two of the memory chips may be dodgy but... When I put the scope on 74LS157 chip D6 pin 9 (MA1) I got this...
That doesn't look right for an address line does it?! Especially when pin 10 of the same chip looks like this...
So my first plan of action is to replace D6 and see if the fault changes or stays the same. Of course, I'll fit a socket at the same time.
If nothing changes then as it's on the memory bus side and there's nothing else there than the memory it's probably a memory chip fault so I'll replace all the memory.
I do have a set of OKI chips but I don't really trust them. Will any 6164 chips do? I have almost a complete set of 120ms ones I bought in case I needed them for the BBC B+ but only needed one in the end. I can get another (along with a socket) easily from RetroClinic.
Any thoughts?
On and off ever since then I've tried to revive it. The first things I did were to replace the Z80 and Z80 CTC as they were easy to find back then... No luck.
About 10 years ago I swapped out the video processor... nothing.
With the tools I had I couldn't get any further.
Anyway, recently I picked up an oscilloscope and with that I managed to bring a BBC Model B+ 128K back to life. So, now with the new tool it's time to look at the old Memotech again.
Of course, the first thing to look at is what do the address and data buses do when you switch the machine on. Well, not a great deal it seems, at least one the memory data bus. One or two of the memory chips may be dodgy but... When I put the scope on 74LS157 chip D6 pin 9 (MA1) I got this...
That doesn't look right for an address line does it?! Especially when pin 10 of the same chip looks like this...
So my first plan of action is to replace D6 and see if the fault changes or stays the same. Of course, I'll fit a socket at the same time.
If nothing changes then as it's on the memory bus side and there's nothing else there than the memory it's probably a memory chip fault so I'll replace all the memory.
I do have a set of OKI chips but I don't really trust them. Will any 6164 chips do? I have almost a complete set of 120ms ones I bought in case I needed them for the BBC B+ but only needed one in the end. I can get another (along with a socket) easily from RetroClinic.
Any thoughts?