MTX500 4000-05 video daughterboard

About original Memotech hardware.
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gunrock
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MTX500 4000-05 video daughterboard

Post by gunrock »

Hi,

Just trying to get colour back on my composite out, as the video board was pinned to b/w. May have cocked it up!

So I have no continuity at link b and I do have continuity at link a, which is desired. However I also have continuity from one side of link b and link a.

A picture might help of the rear of the board:
20220820_200547.jpg
20220820_200547.jpg (181.1 KiB) Viewed 2683 times
Should there be continuity between those left hand points of link a and b?

My Memotech doesn't boot anymore :(, no beep or black screen, just no signal. FYI: Keyboard is detached.
Steve G
Danish Memotech MTX 512, MFX and loving it
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Dave
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Re: MTX500 4000-05 video daughterboard

Post by Dave »

Hi,

not much time, but .. . . . .

have you checked the circuit diagram? The links are connected on one side, in your photo, you can just about make out the PCB track intentionally connecting the two pins

Image

regards
Dave
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Dave
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Re: MTX500 4000-05 video daughterboard

Post by Dave »

Probably an obvious point, but . . . . .

The cable that connects the video board to the rear panel is really delicate. It’s worth checking that the wires or crimps at the board end have not broken, or the wires on the back panel have not broken when you were working on the machine.

Oh, and even more obvious, the 4 pin connector was refitted the right way around? :)
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gunrock
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Re: MTX500 4000-05 video daughterboard

Post by gunrock »

Thanks Dave, all good advice. Sorry for the lack of response. Did some investigation but didn't have time to write up.

A brief history
So this MTX-500 I got last year from an Ebay seller (untested, without power supply) and other than a quick 'power up and bash the keys a bit' test, I haven't touched it. It has it's composite output pinned to monochrome and I wanted to fix that. Now when I tested it, I tested it on a small 7" LCD that I have and that told me the input was 60hz, which I thought was odd at the time as it has a TMS9929 in there, so can't be a US machine.

I gave it a good clean - think it had had a cup of tea spilt in it some time ago as there was a brown sticky residue on the underside of the keyboard PCB that was a little hard to shift.

And so...
I decided to change the link on the video daughterboard from b to a and I must say, it's quite a densely packed little board and my de-soldering skills are somewhat behind my paltry soldering skills. :cry:

However, got it done and tested continuity with my Multimeter and it looked good. Placed it back inside the MTX500 and as it was evening I used one of my work-from-home screens to test. So this probably was a bad idea, as it is a Sony Playstation 3D display and the only analogue inputs are the Y/Pr/Cr inputs. So, by utilising the Y input, I can get an MTX to display a ropey b/w image, which is good enough for testing. But no dice in this case, nada. To make matters worse, this display also powers down after a few seconds if there is no video signal, so I couldn't even do the CTRL-G bell test.

Thus I posted an SOS here (why do i do these things at night? :lol: )

The next morning
I had another MTX512 that was also a 4000-05 board and that outputs a monochrome image too. So I tested that again on my Sony display and yes, I get a b/w image . So I took the daughterboard out and put it into the MTX500 - no image. At this point I thought the MTX500 was generally faulty.

So, I took the daughterboard from the MTX500 and put it into the MTX512... no image. Ok, so now I'm thinking I have a dodgy MTX500 and I've ruined it's daughterboard. I put the original daughterboard of the MTX512 into the MTX512 and tested again, got a b/w image.

Some time later, as commitments allowed, I hooked up the MTX500 to the TV in my bedroom (which is 32-inch Bush/Argos job with a side-av in). I get a colour image!! So both the MTX500 and daughterboard work! :?
MTX500_colour.jpg
MTX500_colour.jpg (230.77 KiB) Viewed 2660 times
I take it back to the Sony display and no, same thing -"No signal detected" and it goes to standby.

I start thinking it's the presence colour in the signal that is making it not work, perhaps the colour carrier is affecting the monitors ability to sync or similar.... ? Then I remember that my Danish MTX512 outputs colour and that used to show a b/w image if I fed it through the Y input of the component-in on the Sony display. Only one thing left to do: hook up my Danish MTX to the Sony display and the answer is, yes it does show an image.

Conclusion, TL;DR
Something with the MTX500 machine outputs a somewhat out of spec signal (compared to my other two MTX machines) that the Sony display can't handle. Given that I'm abusing the component-in to do this, I guess that's reasonable.

Anyway, I have a Retrotink Mini in the post so this kind of funny business will be a thing of the past. Sorry for sending up the Bat signal unnecessarily!! :oops:
Steve G
Danish Memotech MTX 512, MFX and loving it
stephen_usher
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Re: MTX500 4000-05 video daughterboard

Post by stephen_usher »

Different PAL versions? Maybe the Sony doesn't support the version of PAL that the Memotech is outputting. There are 4, maybe 5 versions from what I can remember.
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gunrock
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Re: MTX500 4000-05 video daughterboard

Post by gunrock »

Hi Stephen,

Could be? It's a UK keyboard with a "£" on it and most of the esoteric PAL variants were Latin American, so probably not one of those.

If I had an oscilloscope (and the expertise to use it :lol: ) we could probably compare the MTX512 with this MTX500 and look for differences in the output signal.
Steve G
Danish Memotech MTX 512, MFX and loving it
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gunrock
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Re: MTX500 4000-05 video daughterboard

Post by gunrock »

Actually Stephen, you may have a point, this image from an auction on Ebay suggests there might be other PAL formats supported.
Mtx_box.jpg
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Steve G
Danish Memotech MTX 512, MFX and loving it
stephen_usher
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Re: MTX500 4000-05 video daughterboard

Post by stephen_usher »

Indeed, that's on my MTX500 box too. :-)
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gunrock
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Re: MTX500 4000-05 video daughterboard

Post by gunrock »

Well interestingly, my Retrotink 2x Mini turned up and whilst it works well with the UK MTX512 (pinned to b/w), this MTX500 produces a signal that it can't lock to.

The image tries to for a half-second, then disappears, then it attempts to sync again with wobbly results. This goes on three or four times then the display goes black. When the image is on the screen, CTRL-G produces an audible tone, in fact I wrote a basic program blind and it ran fine.

The Retrotink is supposed to handle PAL, NTSC and SECAM signals, so it shouldn't be an incompatibility there.

I might try and replace the VDP with a known working one, it's a long shot but its easy to try. I need to look at the schematics and Dave's troubleshooting pages to see if maybe a faulty cap could be affecting the video circuit.
Steve G
Danish Memotech MTX 512, MFX and loving it
Martin A
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Re: MTX500 4000-05 video daughterboard

Post by Martin A »

If the signal is there, but the display is not locking on to sync, then it might be worth checking the timing crystal is in spec.
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