My first MSX! Loving it, but I have questions about the video

Rookie (20)

07-06-2020, 17:23

Hallo all. I just bought my first MSX - a Sony HitBit HB-75B (I'm in UK). What a thing of beauty it is, inside and out.
I grew up with Sinclairs and Commodores and have always wanted to try out one of these mysterious Japanese machines The build quality is absolutely insane!

Anyway. It's in lovely condition and I've spent the weekend meticulously cleaning it (partly to fix a couple of dead keys) and it's sparkling. Only one downside - I'm stuck with RF for the video because the RGB SCART really isn't wanting to play ball.
On SCART, I get a slightly washed out (and too cyan?) picture that cuts to black every second or so (at most). When it is on, the picture is sharp, albeit covered in jail bars. Looks like this:

It's off rather more than it's on.

Hardware wise, I can't see much wrong at a casual glance. Solder points are good and clean, I ran continuity on the 21 pins and they're OK. The TV can do RGB SCART well (in 50 or 60hz). Sadly I don't have an oscilloscope so I didn't want to touch the variable resistors inside.

It's not the end of the world if I can't fix it - my TV does an OK job with RF (pictured below) but the SCART socket was one of the reasons I went for the HitBit in the first place :( My standby would be to build a composite cable and run it into my Retrotink, but that's hardly RGB.

Any thoughts? I'd value absolutely any input. Cheers!

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By VegaVegas

Resident (57)

07-06-2020, 18:08

Maybe RGB needs amplifying?? Or maybe some bad caps or things like that

By Grauw

Ascended (9578)

07-06-2020, 18:58

It’s a very nice MSX indeed! I have a HB-75P myself.

The TMS9929A video chip does not output RGB natively, but rather Y, R-Y and B-Y (also known as YPbPr). The RGB is constructed from that with some circuitry. Take a look at pages 32 and 53 of the Sony HB-55P/75P/75B service manual. There you will see that the VR1, VR2 and VR3 pot-meters control the B, R and G output levels so you should be able to adjust these to make the colours not washed out.

As for the screen going blank, seeing as you are using an LCD monitor, it seems to lose sync. Since the sync signal is taken from the composite signal, my first thought would be to check the composite video output of this machine. Is that working ok? The composite video output signal can be taken from either the AV DIN (pinout on page 55) or the SCART connector if you have one of those AV breakout adapters.

The VR4 pot-meter controls something about the composite signal, so you may have some luck adjusting that. Maybe a previous owner messed with the potmeters so they are not in their proper positions anymore.

Alternatively the blank signal on SCART pin 16 could also be at fault. It tells the TV whether to use the RGB signal, and could be causing the TV to switch signal sources briefly. It should output somewhere between 1V and 3V (nominally 2.5V).

By VegaVegas

Resident (57)

07-06-2020, 19:18

Ah, I agree you should use some old CRT TV to determine things first. Also some diagrams of your cable will be nice

By liveinabin

Rookie (20)

07-06-2020, 19:50

Wow. Thanks for all the suggestions everyone. I wondered whether it was true RGB or not. I don’t have a CRT but I do have several other LCDs I should try. Recapping is also a possibility I guess. Do MSXs suffer much in this regard?
I have the bits on order to build a composite cable (the HB-75B uses that 6 pin Din connector) so that might be a good thing to check first.
About the pots, am I OK jamming a screwdriver in these without an oscilloscope to hand? I’ve had both good and bad experiences with this on other old machines (Atari VCS and Vic-20 were fine, ZX Spectrum was very not.)
I have a deep suspicion that a previous owner did indeed muck about with the pots.

While I’m waiting for the cable bits to arrive, I guess I could be checking that pin 16 on the SCART too.
A lot to consider. Thanks so much. I’m truly loving this computer

By Grauw

Ascended (9578)

07-06-2020, 20:05

There are specific models which are known to have capacitor issues, but this is not one of them. I think for this machine there are other easier things to try out before recapping, unless you see any obviously bad electrolytic capacitors (bulging / leaking) of course.

I would check the composite signal and then try to fix the sync first, adjusting the VR4 pot. I would not expect any issues, if a system can be damaged by turning pots then it seems a bit irresponsible to put them on the board in the first place. But just tweak ’em carefully with a live video feed and if the signal gets worse then turn the other way. Use a plastic screwdriver if you have one. Also definitely try other TVs!

If I would judge the RGB quality of this machine, it’s not as good as the output of machines with Yamaha V9938 VDPs, but still better than composite and I think the best you will get on an MSX1 with TMS9918.

By liveinabin

Rookie (20)

07-06-2020, 22:23

Just ran a multimeter over pin 16 of the SCART. I’m getting 4.37V
That’s rather too much isn’t it?

By Pentarou

Champion (315)

07-06-2020, 22:40

Should be 1-3V, but most TVs will work fine with 5v.
Did you measure with the TV connected or disconnected?

By liveinabin

Rookie (20)

07-06-2020, 22:43

Disconnected. I took the reading from the pins of the SCART lead with the computer on.

By Pentarou

Champion (315)

07-06-2020, 23:16

The TV input circuit will affect the voltage level, so you should measure with the TV connected.
You should also measure if you are getting 5v on Scart pin 8 (for this you don't need the TV connected).

Edit: Grauw is correct, according to the schematic you should be getting ~2v on pin 16. [5/(330+220)*220]

By Nprod

Expert (88)

08-06-2020, 21:34

A little something about the capacitors - there are three X-type Rifa-branded filter capacitors on the board in the bottom left corner. They are located under a black plastic sheet and need to be replaced if possible. These old Rifa metallized paper capacitors are notorious for having their potting material start to crack and degrade after 30 years, and sometimes they can explode in a spectacular fashion. AFAIK the machine can power on without them, but you will lose the AC line filtering across live and neutral.