MTX-512 Board Revision 4000-06

About original Memotech hardware.
SchmitS
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MTX-512 Board Revision 4000-06

Post by SchmitS »

Hello
At that time I'm working on an upgrade of the DC supplies of my MTX. It will be supplied by the new one in the FDX.
(See post on Facebook)

During the investigation I saw that the revision of my board is 4000-05 and in the "Service Manual" pages 49 and 50 a " Timing Chain Modification" is suggested and implemented on 4000-06.
Unfortunately my MTX suffers from the detailed symptoms.

My questions are very simple, are these upgrades very helpful and was they really implemented on the next versions ?

Thank you in advance for your advices.

Best regards
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Dave
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Re: MTX-512 Board Revision 4000-06

Post by Dave »

The "next versions" are only the 4000-06 board revision, they are very uncommon and I have never seen on in real life.

I have not had to do the timing chain mod on any of my 4000-04 or 4000-05 boards, but if you are suffering the problems described in the Service Manual that I guess that implementing the mods is worth doing - obviously Memotech saw benefits to it.

regards
Dave
Martin A
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Re: MTX-512 Board Revision 4000-06

Post by Martin A »

I've done the timing chain mod on my MTX with an -05 board, as part of a long and involved attempt fo fix what turned out to be 4 different problems. Whether it was really needed or not we'll never know, the "timing fault" turned out to be something else. It doesn't appear to have harmed anything, which is reassuring!

Having the '04 chip socketed did make some of the other diagnostics easier.

Write up and photos on Dave's site here: http://primrosebank.net/computers/mtx/r ... air_ma.htm
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1024MAK
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Re: MTX-512 Board Revision 4000-06

Post by 1024MAK »

The replacement of chip 9D where the 74S04 is replaced with a 74HC04 should not normally be needed.

If the 4MHz clock signal to the rest of the board is working okay, there is no need to carry out this work.

So if the machine runs, then it’s fine.

I think Memotech provided this information, as 74HC04 chips were becoming more readily available at reasonable prices, whereas the 74S04 even at the time, was old technology.

In the next diagram, this modification (R14 & C6) is simply to allow a greater range of different manufacturers DRAM chips to be used. It makes a very slight change to the timing of the DRAM chip control circuits.

Again, if your machine is working, there is no need to do this work.

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer in Somerset later in the year :D

Not as many MTXs as Dave! :lol:
SchmitS
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Re: MTX-512 Board Revision 4000-06

Post by SchmitS »

Hello
Sorry for this late reply, I was abroad this week-end.
Thank you Dave, Martin and Mark for your recommendations and advices.
My MTX is disassembled to remove the components of the supplies and in same time I'm able to make the suggested upgrades if they have a positive result. 74HC04 is very cheap and I will place it on a socket.

The biggest trouble is the start-up with the FDX, it needs many power on/off to reach the boot from floppy.
The "Reset" keys have not effect to relaunch. If it hangs, when it's warm, I have to wait a cool down before to restart.

When I saw the color of the 10mF capacitors and the poor cooling heatsink for the supplies components the first step, for me, is to relocate
new supplies in the FDX. I also discover that the 4700mF for the 12Vdc was damaged.

When I finished I will publish the state, I just hope for the better ;)

Best regards
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Dave
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Re: MTX-512 Board Revision 4000-06

Post by Dave »

When it did not boot the FDX, were you monitoring the MTX video output too?

I.e., are you sure that the MTX was not starting up in normal, MTX mode, rather than failing to start at all?

(If there is a problem with the FDX interface card or the FDX boot ROM, the MTX will start up, outputting video to its 40 column TV & video outputs, but the FDX 80 column screen will be blank)
SchmitS
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Re: MTX-512 Board Revision 4000-06

Post by SchmitS »

Hello Dave

I always monitor the 2 video outputs.
At power-up if it doesn't start most of the time the both are blank or sometimes only the BASIC 40 columns is launched.
When the RAM test is completed the boot starts correctly, but sometimes the RAM test failed.

The RESET Keys has no effect to reboot the BASIC 40 columns sometimes takes the hand.

When CPM runs, everything turns perfectly, drives, screens, ...

But after a while, it could be minutes or hours, the keyboard is locked and nothing happens anymore.

The RESET keys have no more effect, I have to wait a cool down for a new power on.

I seems to be a temperature trouble and/or bad contacts.

Before to investigate more I will first migrate the MTX supplies from the retrofitted one in the FDX and also replace the ribbon cable between MTX and FDX.

Thank for your help ... if you have more ideas you are welcome.
I will never give up.

Best regards
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Dave
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Re: MTX-512 Board Revision 4000-06

Post by Dave »

Of course, it could still be a power problem, but there are a couple of worries in those symptoms . . .

The RAM test failing is not a good sign, but unfortunately, they are all soldered in, so swapping them out is a little tricky (as I have found :-) ) so best to eliminate other issues as far as possible first.

The lack of a working reset is also a worry, but hopefully, relatively easy to fault find. It might be hard to tell, but is the system actually not resetting, or failing to start after the reset? When you get the machine up again, check that the /reset line is being driven low by the reset circuit be measuring the voltage on the /reset pin - either on Z80 pin 26 or another convenient place such as pin3 of IC 9C or the /reset connection on the edge connector pin 18A (on the component side of the PCB)

Have you tried using the MTX without the FDX and the RS232/interface board connected? Does it have the same start-up issues?
SchmitS
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Re: MTX-512 Board Revision 4000-06

Post by SchmitS »

Dave

The RESET signal is coming down, but has no effect to restart with or without the FDX.
MTX alone, only ribbon cable removed, has also the same trouble occurrences.
It doesn't always start at first power-on.

RAM "failure" gives no result back, it just stoppes.

I'm more confident that some sources of troubles will be solved with the ne supplies and connectors.
I hope the more treacherous cause(s) will be isolated.

As per my 39 years experience in industry, always think simple at one thing, on right time, to solve random issue.

Components are on order ... to be continued.
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1024MAK
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Re: MTX-512 Board Revision 4000-06

Post by 1024MAK »

When it stops / crashes it’s worthwhile checking the signals on the various Z80 control lines, and checking the A14 and A15 address lines. Also test the voltages on the three main power rails, +5V, +12V and -5V, just in case this is a power issue. This may help to give us an indication of what’s going on, or rather, not going on.

Also, can you tell if it takes longer before it fails if you leave the lid / keyboard section off?

Mark
:!: Standby alert :!:
“There are four lights!”
Step up to red alert. Sir, are you absolutely sure? It does mean changing the bulb :!:
Looking forward to summer in Somerset later in the year :D

Not as many MTXs as Dave! :lol:
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