The Memotech MTX Series
Memotech MTX RAM Fault Diagnosis / Repair
Courtesy of Martin Allcorn
System Description : MTX 512 Serial No.?????, 4000-??
computer board with 32k DRAM chips.
Problem Description : Unbricking a brick
There have been a couple of failed attempts at upgrading an
MTX500 (32kb) to an MTX512 (64kb) by removing the 32k DRAMs and
replacing them with 64kb ones. Both Andy (see
here) and myself (see
here) have tried, and failed, with this upgrade, but, other
than poor workmanship, there does not seem to be any reason why an
upgrade like this can't be done.
Martin Allcorn has recently (July 2014) managed to recover a
faulty MTX500 and upgrade the memory to 64kb by doing just that.
It is encouraging that Martin has been successful in
replacing the RAM and should give me the impetus to revisit my
failed upgrade and see if I can recover the situation, Martin
has kindly documented his MTX problems and fixes, it is posted here in the
hope that others will find it useful - particularly if you have
bodged a memory upgrade like me :-(
MTX500 to 512
memory upgrade and repairs
the saga of the Bricked MTX
Back in the 80's early 90's, I had an MTX500 that was
upgraded to 64k and RS232 so I could run CP/M and the FDX. When
the FDX died in the mid 90's, the MTX went to a family member
for their children to play on. Eventually it went out of use and
ended up in a loft.
After my Grandfather died, I inherited his MTX500, but that
had no software. A few years back the upgraded MTX and one box
of software was located and returned to me. However, when I went
to try the software out, the expanded system would only work
with the expansions removed.
At that point I decided to upgrade one of the 2 500's to 64k
on the motherboard. I de-soldered all the RAM and the LS157
multiplexers. The idea was to use 2 32k Static RAMs and some HCT
logic on a small daughterboard to make up the 64k RAM.
For whatever reason that didn't work, it looked ugly too. So
I went to plan B and removed the PAL and ROMs and put all the
memory on an external card. I worked around Memotech's paged ROM
system by using a 128k flash chip divided into 8, 16k pages,
with the OS ROM programmed into the lower half of each page.
The RAM was initially the 2, 32k static RAMs. That's the red
function keyed MTX on the
development page. The red keys were to remind me which MTX
needed the expansion card plugged in to work! The final version
of that card has 4 RAM (2x 32k and 2 x 512k) and 1 ROM sockets
and gave the system 784k RAM and 128k ROM. Obviously the card
sticking up from the side like that wasn't a long term solution,
but did lead indirectly to the development of
Initial Fault Diagnosis and Repair
At this point the MTX display started to distort and wobble
after being on for a while, the same symptoms it had with the
original 32k expansion board fitted. With a CMOS Z80 and CTC
fitted things got better so I thought it may be a power related
issue. I replaced the 5v and 12v regulators and both 4700uf
capacitors which improved the situation, but didn't quite fix
The next step was a much simplified 64k only RAM card, but to
return to using the original EPROMs, this particular MTX was
factory modified to use 2564 EPROMs instead of mask ROMs, so
there were some cut tracks and yellow wires to accommodate the
Fault no. 1. occurred at
this point, I managed to put the PAL back in the wrong way
around and thoroughly cooked it. Since building the standard
logic based expansion card, I'd acquired a more modern
programmer as was able to program an ATF16v8 PLD to replace the
The true source of the power problem was revealed when I
happened to touch the 10W resistor R62, it was HOT, indicating
the real source of the power problem was the TIP 2955, which I'd
not replaced when I did the regulators. It also explained why
the CMOS CPU swap helped, as the reduced current consumed by
that device reduced the rate at which R62 overheated.
Having replaced the TIP, all of the power related problems
went away so fault no. 2 was
here for the diagnosis and repair if a similar TIP 2955
Dave generously provided some 6164 DRAMs, so that I could
install those on the board along with some new LS157
multiplexers and get rid of the expansion card. With a suitably
programmed PLD replacing the dead PAL and the motherboard links
set for 64k Drams, I put the ROMs back in their sockets, and got
. . . . . . .
just the black screen and hum of a
A Z80 doesn't NEED RAM to work with so I wrote a custom ROM,
based on some of the early MAGROM code, that initialised the VDP,
turned the sound off and then displayed the results of some very
basic tests of the RAM. With this fitted onto the expansion
card, and using the new internal RAM it revealed that the
visible 48k of the 64k of RAM was working and seemed reliable.
It's not possible to re-program the old TI 21v EPROMs with
modern programmers, so I re-wired the ROM sockets to use modern
EPROMs and put the 2864 EEPROM with the test program into the OS
ROM socket and got the expected result. So I programmed 3 2864s
with the full MTX ROM set and plugged them in, I had a working
64k MTX512. YAY!
. . . . . . . . . . . for about 3 minutes until
There then followed a couple of weeks experimenting with the
Part of the RAS/CAS
timing for the dynamic RAM goes through the PAL and part through
the on-board LS logic. As the PLD I was using in place of the
PAL has a much faster response time typically 3ns (and 15ns
worst case) instead of the original device's 25/35ns, I thought
there may be a RAM timing issue.
The RAS delay
is fixed as it is MREQ
delayed by being passed through 2 LS04 inverters. In generating
MREQ goes through
the PAL, 2 inverters and then a resistor-capacitor delay at
R14/C6 before the multiplexers are switched and then a further
delay with R15/C5 and 2 more inversions before
CAS is generated.
I was hoping that increasing the resistor/capacitor delay at
R14/C6 I could restore the original timings. The theory being
increasing that delay by 20ns would restore the timings to their
I tried numerous variations, based on some formulae found on
the net, and some additional values Dave came up with when he
ran the timing chain through
Sometimes it would work for an hour or more, on other it would
need 3 or 4 resets to even start. But I never found a stable
solution. So eventually gave up and put the original value parts
back, as there had to be another problem.
As part of the
MTX+ project Dave and I have built
diagnostics cards to enable the status of the system to be
seen in the event of problems. So I put together a small adaptor
board to enable me to connect the MTX+ diagnostics board to the
MTX500. I also made a 2Hz clock to fit in place of the 74HC04
chip at 9D on the board enabling me to watch on the indicator
lights each clock cycle as the system booted. ("Ready" would
take around 2 days to come up at that rate).
[For details of the 2Hz clock, see Martin's
post on Memorum]
That revealed fault no. 3
: address line A7 was intermittently stuck high. A check
of the circuit diagrams in the manual revealed that there are
only a few places where A7 comes into contact with board
components. The CPU and memory, and the I/O port decode logic.
I removed the socketed parts, (the Z80 and ROMs) and the RAM
multiplexers, then tested with a multi-meter. A short was
present, an indication that the I/O select could be the culprit.
In the I/O logic, A7 is connected to pin 13 of the 74LS27,
the pin next to the 5v power supply. There were no visible
external shorts, so I replaced the chip. The stuck A7 problem
vanished. Fault no. 3 was
Since that chip is located on the MTX motherboard right next
to the PAL it's possible that the chip had been damaged when I
cooked the PAL in the socket next door.
Having fixed that, the diagnostics board was able reveal
no. 4 : once the system was warm, A12 would start to
stick high too. A12 is only connected to the Z80 and memory.
After much searching, I found that one of the tracks that had
been cut in the factory to fit the original EPROMs was bridging
when warm, there was probably a minute fleck of foil that I'd
disturbed when I made the extra changes to fit the modern EPROMs.
However, having widened the cut the problem went away
fault no. 4 was
fixed. Since then there have been
no further problems of any kind. (and I've done considerable
amounts of MAGROM “testing” to make sure!)
A selection of photos from the process can be
found on the Smörgåsbord
The Moral of the story:
It IS possible to upgrade an MTX500 to 64k, it
should only need new RAM chips and a new PAL/GAL/PLD and the
motherboard links being correctly set.
Depending on the RAM it might need a timing chain
modification, but with the chips Dave supplied that wasn't
Cooking the PAL in the process is not to be advised.