Sunday, 6 May 2018

Binatone TV Master Mk 8 TV Game Composite Video Conversion

I have done several TV Game composite video conversions recently, and they have all been pretty much the same, locate the composite video signal feeding into the modulator. Disconnect or remove that and fit a composite video buffer board to drive the TV.
I've done black and white ones, and colour ones, this one is monochrome, with greyscale, but that should be the same, shouldn't it?
Well, no, this one turned out to be quite a problem. I always start by testing these systems on an old CRT TV, to make sure they work. You can see from the picture above that this has a grey background, white text and black text.
I fitted the usual composite video board, but when I tried it, I only got black and white, no grey.
I went over this and checked it out. The composite video board was doing it's job correctly. The input signal was the same as the output. I also checked the signal without the buffer, and back on the old style TV.
It seems the levels were all a bit wrong. Old TVs were very forgiving with poorly formed composite signals, modern ones much less so. The 'back porch' signal, the lower flat section at the start of each line, should be setting the black level. That was almost as low as the sync pulse before it. Anything following at that sort of level should be displayed as black, the highest peaks would be white, and anything inbetween in grey. (the back porch is the thing missing in the ZX80 and early ZX81s that give a very washed out picture on a modern TV set without some changes - more on that in an earlier blog). Here, none of the signal goes near the black level, and the rest is small white peaks on grey.
I thought it might be the chip at fault (there's not much else on there), so I tried swapping the chip with another (from a colour TV game), and the chip worked fine in that system, and that system's chip produced a bad output on this one. The main difference in the colour version is it had an Aztec PAL colour encoder board to generate the colour burst, the rest is the same. (yes, the pin is meant to be missing, it's used on the AY-3-8610 version of the chip to enable the light-gun games which the cheaper AY-3-8600 doesn't have)
I tried various options, including various ways of capacitively coupling the output, and even a convoluted Op Amp circuit to try to boost the signal in the relevant areas, but still got essentially a black or white signal. No grey.
Checking the datasheet for the AY-3-8600, showed the recommended circuit. I suppose it comes out of the way the signal are generated, but it's really quite neat. You get separate outputs for the different parts required, one pin will be high when it should be drawing a bat, one when it should be drawing the ball etc. You can combine these how you like, or could probably drive TTL RGB quite nicely with some minimal circuitry. It's also interesting to see there is no connection to 'left video', which presumably means that will be black, as the other outputs will be off at that point.
Checking the board showed a different approach. They had omitted the buffering transistor, and the values used on the resistors combining the signals were different, higher values in a different ratio.
Running out of ideas, I decided to try swapping those resistors and matching the reference design. I didn't have exact matches to hand, so made up 2.4K using two 1.2K resisters and 1.6K with 1.5K + 100R. Not the neatest solution but the values were right.
I also added a buffer transistor on the back of the board, following the reference design, rather than using the buffer board.
With those changes. the signal was now showing a better range with white peaks, black peaks going down to the back porch set black level, and a grey generally in the middle.
On the screen, shades of grey as it was meant to be. Not sure why the values on the original board were so wrong, but the reference design works fine.
As a side note, many of these Binatone systems have warnings of the dire consequences of not using their mains adapters. It's unclear what the redacted model number was, but it is stamped 02/5011 below, as seen on the last working sticker when I opened one up on a previous unit and found a 7808 regulator in there.
This time, I see nothing, even though the box says it is the right adapter for the TV Master Mk.8. Centre tapped half wave linear power supply. I suppose the resistor across the smoothing capacitor would stop the voltage getting too high off load maybe? At least it has a fuse (3A though) and a thermal fuse.

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