*** M E M O R U M ***

Today's fun is installing a TMS2RGB board to my original Memotech MTX500, which I had to fix three years ago after being dead since 1988.

So, the first task was to clean up the bodge wires on the bottom of the board, which was a mod to allow the use of normal EEPROMs rather than the original mask ROMs. After that it was off with the modulator! (This is part of a cunning plan... to be continued...)


The TMS2RGB board sits on the underneath of the TMS99xx VDP socket. Unfortunately, whoever fitted the socket at Memotech wanted the pins to be as flush with the PCB as possible and had cut them down, meaning fitting the new board would be tricky...


To connect the TMS2RGB board to the rest of the world it needs a ribbon cable soldered onto the board and taken elsewhere where a plug needs to be fitted so that an external cable can be hooked up. I got this micro-8 way DIN socket and cable. All I needed to do after soldering the board on is fit to the ribbon cable and fix the socket in an appropriate place.

Of course, with the TMS2RGB being on the bottom of the motherboard, routing the cable might be a problem.

Anyway. After refreshing the socket pin solder, sucking up the excess so that the pins stood proud of the PCB at least a little bit I managed to get the board on. (That was a bit of a struggle.)


At this point I hooked up the machine to power and video and turned on... no smoke... Blue screen with "Ready" and a flashing cursor. The machine still worked!

Back to the fabrication. Solder the wires onto the socket and then route them around to the board and solder them on. Then attach the socket to the top of the board with superglue and solder. (I would have preferred just solder but it wasn't strong enough.)

Can you guess what my cunning plan is yet?...

Oh yes, I'm using the opening which was used for the modulator for the new RGB socket!


After putting it all back together the modification to the Memotech is complete. Now I have to make the other end of the cable before I can properly test it. However, signals are coming out of the wires at the end of the cable, which is encouraging.

That's brilliant Stephen, as I received a TMS-RGB a few weeks ago and was planning to install it soon. Your pictures and explanation helps settle a few questions and unknowns on the process for me.

OK, well it worked for a little while... Then the display went screwy.

Now it seems to short out the RGB2HDMI and the Memotech doesn't start with that connected. Similarly the OSSC. I think something's gone pop on the TMS-RGB board as with the cable disconnected the machine starts normally.

Looks like something on the blue channel has gone VERY bad.

The blue signal should not look like this...


25V peak-to peak?!!!!!!!!

I think it's killed my RGB2HDMI's blue input and may have done damage to the OSSC.

(The other channels are fine, with good signals at ~ 2V peak-to-peak and with the correct looking output.)

Found the problem after dismantling the MTX...

The moral of the story is... beware how you route your cables!


I'd let the ribbon cable rest over the top of the legs of components coming through the board. Unfortunately, just at this point there's a pad on the lower case which, when the bolt to secure the motherboard and give thermal transfer from the heatsink was tightened it compressed the motherboard onto the cable and the lead from one of the diodes connected to the 16V A/C input from the transformer punched through the insulation and into the blue lead. If it had been 0.5 millimetres further over it would have hit the GND and there would have been a connection between ground and 16V A/C, which would not have been pretty.

As it is I seem to have got away with a damaged ADC daughter board on the RGB2HDMI (I hope that it's just the daughterboard).

The output from the TMS-RGB seems to have vertical banding that I didn't notice before but it otherwise works with the OSSC.