Hello from NZ (MTX500 needing resuscitation)

[rogernz]

Hello from NZ.
I've just unpacked my MTX500 for the first time since 1987. I've owned it since new and remember how amazing it was in its day.

It was working fine when I packed it away but unfortunately it's not working now. I'm pretty sure it has the black screen and humming problem.

My first challenge is that the only monitor I have is an Eyoyo 8 inch monitor, model EM08C. I've seen others on YouTube use this successfully with retro computers. When I connect the MTX monitor output to the AV input of this monitor, the screen goes dark (but not "no signal") and I hear a humming from the monitor's speakers. Pressing CTRL-G doesn't produce a sound.

Can anyone confirm that if the MTX is working, I should see something on this display by plugging it into the MTX Monitor port and using the AV input on the monitor? The monitor also has a BNC input supports 1V p-p cvbs. Would this be a better input to use?

Also, given that the MTX was working when I last used it, are there any faults that are more likely than others? Any suggestions for a systematic fault finding approach? I have a multimeter and some basic electronics knowledge.

Thanks,
Roger

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[Dave]

Hi Roger,

Welcome to the forum!

That monitor should be OK, you say that it’s the only one that you have, but most TVs have a composite video input, usually with a yellow phono connector, perhaps you have one like that that you can try too? You could also try the TV output if your TV has an RF (coax) input. The picture would probably be poor, but it’s worth a try. It probably won’t work, but would eliminate your monitor as the issue. (Default TV channel is 36 but run the TV auto tune too.)

I assume that the red light comes on on the power supply?

Check the soldering of the audio and video wires in the back panel, they are very flimsy and often break.

You could try removing the RAM expansion card just to eliminate it as a source of the problem.
I would also try removing and refitting all of the socketed chips, CPU, CTC, ROMs, PAL and VDP.

You could also check the voltages on the board too, the easiest place to get to them all is by measuring on the pins on one of the 4116 VRAMs:
Pin 16 Ground
Pin 1 -5VDC
Pin 9 +5VDC
Pin 8 +12VDC

Unfortunately, it does sound like the common “black screen and hum” problem. That’s often caused by a bad RAM chip, but can be a whole load of things that stop the machine from booting.

Regards
Dave

[Martin A]

As Dave says black screen and hum, is a common problem.

Technically what it means is the MTX hasn't been able to run the start up proces far enough to get to the point where the VDP and sound chip are set up. Sadly any number of thing can cause that.

That's because the black screen is the default state of the VDP before the output is enabled and the tone is the power up state of the sound generator.

The good news is, the presence of the tone implies you have working 5 volts and a working 4MHz system clock because the sound chip is powered and dividing the system clock down to generate the tone.

Unfortunatly that's not a lot of help with the rest of the diagnostics.

[rogernz]

Thank you both for your help. Very useful and encouraging.

I've removed/replaced all socketed ICs.
Removed the RAM expansion card.
Good connectivity through the audio and video ports - no broken wires or poor connections
Power supply light comes on and all voltages are present on the board (checked at 4116 VRAMs)
Checked the reset circuit. That appears to be working. Even shorting pins 19, 20 on the J1 keyboard connector produces a reset response, but the dark screen and hum return.

I've followed the procedures in the service manual as much as can with a multimeter.
There seems to be some voltage at pin 1 of the two 74LS157's and pin 2 of the DRAM (I don't have any way to check the actual signal).

If I had access to an oscilloscope, would that help me diagnose further?

The comment in the service manual is a bit discouraging "Without a Ram Rig Test the chances of finding a faulty Ram with only an oscilloscope are very slim indeed."

Is it worth replacing the DRAM at this stage, or should I persist with fault finding? Are equivalents still available?

Thanks,
Roger

[Dave]

Do any of the RAM chips feel warmer/hotter than the others?
A hot one could indicate that it was faulty.

You could try piggy backing a good chip on the RAMs in turn, there is a very low chance that it might help you identify a failed RAM.

If (when) the above don’t help, then what to do?

As you say, without a ‘scope, you’re going to struggle to get more info from your fault finding, but even with one, it’s still not going to be easy. I think that the most likely failure is RAM, bit it could be a number of the support chips too, so no guarantees that changing the RAM will fix it, though that would be my best bet.

Replacements are still available, though you’d be better using 64k RAMs, then you’d be able to get rid of the RAM board. (You’d need to change the PAL, but that could be done later.)

The best way to remove the RAM is a one way trip, I.e., snip the legs off the chip first and remove each pin individually as it puts less stress on the board.

THERE ARE RISKS THOUGH . . . .

http://www.primrosebank.net/computers/ ... pgrade.htm

(The article above is very old, I have learned a lot since I wrote it, but it is still relevant and mostly accurate)


P.S., my memory isn’t what it was either
I forgot about this page which might help too . . .

http://www.primrosebank.net/computers/ ... repair.htm

It may possible to replace the PAL to disable the on board RAM and test your machine with just your RAM board in the same way

[Bill B]

It is perhaps worth mentioning this thread.

If you are not confident in your ability to build one of these yourself, you may be able to get assistance from a local electronics club or maker space.

[rogernz]

Thanks all for the information and advice. You've given me a lot to think about as I consider my next step.

Regards
Roger

[acsi]

Hi Roger,

Nice to see another MTX in NZ.

Depending on where you are I have a RAM tester (you'd need to desolder the existing in one piece of course). Unfortunately my desolder gun is on the fritz.

I also have several plug in cards like the MTX and MAGROM as well as a logic analyzer which could be used to see if there is any life.

Mike (Wellington, NZ)

[rogernz]

Hi Mike,
I'm in Auckland. Great to hear that there's a working MTX in NZ! I do have a desoldering device so I can undertake some repairs. Thanks for reaching out. It sounds like you've had lots of experience with your MTX. I'm considering what my next move is and will definitely check back for further advice.

Cheers,
Roger

[1024MAK]

Hello Roger,

A belated hello and welcome.

Any progress?

Mark