Philips VG-8235 - Wrong colour on boot and basic prompt

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By Macross_VF1

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07-08-2019, 16:53

As the title says I have a problem with my VG-8235. When I boot the computer I first get a short flash of the standard blue, black and white boot screen, before it changes blue to black and white to yellow. The colour change persists to the basic prompt and only goes back to normal once a game is fully loaded. For instance I have the Penguin Adventure cart, and the Konami logo that shows up before the game starts is also black and yellow but after that the game works as normal.

This only happens on my flat screen TV, while my old CRT does not show this problem. Yeah I know, a modern TV isn't really suitable for an old timer like the MSX, but sometimes I just need to do a quick check, and digging up my old CRT (normally only used for testing purposes) is so damn inconvenient.

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By Grauw

Ascended (8307)

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07-08-2019, 18:54

The blue component of the RGB seems to be missing. An impedance problem?

By sdsnatcher73

Champion (487)

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07-08-2019, 21:07

@Grauw: then again, if it is working fine on the CRT...

@Macross_VF1: How did you hook it up to your LCD screen? I see it is on the SCART input, is the SCART cable fully connected (all 21 pins)? Could you try RF or RCA (you’ll need a DIN to RCA cable)?

By Grauw

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07-08-2019, 21:17

Idk, the analog circuitry of the CRT may have more tolerance for that kind of thing…

By Macross_VF1

Rookie (18)

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07-08-2019, 22:37

sdsnatcher73 wrote:

@Grauw: then again, if it is working fine on the CRT...

@Macross_VF1: How did you hook it up to your LCD screen? I see it is on the SCART input, is the SCART cable fully connected (all 21 pins)? Could you try RF or RCA (you’ll need a DIN to RCA cable)?

It's SCART from the back of the computer to the back of the TV. I've tried wiggling each side of the SCART connector and I get some rather peculiar results.

If I manage to get the connector at the back of the computer just right, enough pins on the SCART remain unconnected that the picture instantly reverts to composite. While in composite mode, the colours are the standard white and blue. Also, if I leave the computer turned on and remove and connect the plug completely, the picture reverts to the proper colours, if only for a short while, just like on start up. So clearly my TV recognize something in the RGB signals but do not seem to properly read them. Could it be possible that they are not properly transmitted?

Just to be extra sure I've tried with a different SCART cable and no difference. I also opened up the computer and looked at the solderpoints of the SCART connector on the PCB. While I can't be completely certain I can at the very least not see any cracks or broken connections.

Last minute addition before I submit this. I went and tried my CRT again and noticed one thing that might be important. During start up, the MSX logo is slightly warped and have a "shadow" on both sides. I've included a picture though it is difficult to see on a photo. The warping and shadow is more pronounced on the right side of the MSX logo. You might have to zoom in somewhat to make it more noticeable.

By Grauw

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07-08-2019, 23:25

I would check the capacitors and the resistors on the blue component output path of the MSX main board. I’m not an electrical engineer but it sounds like some kind of DC offset emerges, which I think it could vary based on characteristics of the receiver, and is normally regulated by capacitors and resistors.

First I would try to replace the electrolytic capacitors C28 (47µF) and C27 (22µF) since electrolytic capacitors are a common failure point. If a visual inspection shows they are bulging or leaking they’re bad for sure. If you don’t have those capacitor values on hand then as a test you could try to swap them with the caps of the green line (C35, C34) and see if the output changes (MSX letters turning magenta).

And if that doesn’t do the trick I would check the resistors on the blue component output path (R66, R67, R68, R9, etc.), and lastly other components. If you have some measuring and soldering skills it sounds like it’ll be relatively easy to pinpoint this problem.

See the VG8235 service manual, the relevant path is shown in the middle crossing pages 13-14 of the schematic, annotated with “B”.

By Macross_VF1

Rookie (18)

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07-08-2019, 23:31

Oh I most certainly have the necessary capacitors, no worries on that front. Also, thanks for the manual.

By Grauw

Ascended (8307)

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08-08-2019, 00:33

I hope that was helpful Smile. No harm to replace ’em at least, and hopefully it fixes the problem.

Keep us updated!

By Macross_VF1

Rookie (18)

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08-08-2019, 11:06

I've done some testing and I'm pretty sure the service manual isn't completely correct. For instance, I've been able to locate some of the components that should be connected to the Blue RGB line but they don't appear to have the correct value. As an example resistor R66 gives a 100 ohm reading and is clearly marked as 100 ohm on the resistor, but is supposed to be 1.3k ohm if the manual is anything to go by.

Despite that, I did manage to get some interesting readings. First I started by measuring the voltage output on the RGB pins of the SCART plug. The Red and Green line outputs a fairly steady 2-2.1V. The Blue line starts by outputting the same, but jumps to 2.6V when the boot screen switches colour. It continues at 2.6 until a game is fully loaded and the colours revert to the correct set.

By this time I was really curious and with the help of the manual I traced the lines back to the video chip. Pins 22,23 and 24 are responsible for RGB. And when I measure voltage here I get pretty much the same result. All pins start at about 2.6V but the blue quickly jumps to 3.2V and stays there until a game is completely loaded.

By Grauw

Ascended (8307)

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08-08-2019, 11:48

Macross_VF1 wrote:

Despite that, I did manage to get some interesting readings. First I started by measuring the voltage output on the RGB pins of the SCART plug. The Red and Green line outputs a fairly steady 2-2.1V. The Blue line starts by outputting the same, but jumps to 2.6V when the boot screen switches colour. It continues at 2.6 until a game is fully loaded and the colours revert to the correct set.

Doesn’t that just coincide with the screen turning from black (off) to predominantly blue, and then back to predominantly black/red/green (depending on the game)?

With a blue background colour (7/7) the blue component it will be at maximum output while the green and red components are at near-minimum output (1/7). Looking at the datasheet of the V9938 (p. 127), the voltage difference between min and max output is 0.8V (though the averages you’re reading will be a bit less because it outputs black during erase time).

In Basic, assuming you left the background colour at the default COLOR 15,4,4, try COLOR=(4,r,g,b), e.g. COLOR=(4,0,0,0) and COLOR=(4,7,7,7) to change the RGB values of the background colour, and check your readings.

By Macross_VF1

Rookie (18)

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08-08-2019, 13:34

Turns out you were quite right about the voltage levels. That's a relief, I really wouldn't have liked it if the video chip was faulty.

Anyway, it turns out I have the VG-8235/20 version so I needed a different service manual. Pages 14-15 show the relevant bits.

Compared to the other service manual, there are a lot less parts between the video chip and the blue RGB pin out. Really, the only part that's relevant is the Q9 transistor. However, there's also a daughter board that interacts with the RGB signal as well. It's called encoder unit in the manual (grey part of page 15-16) and while it mostly seem to deal with RF and composite it's not all it does. If I remove it and power on the computer my CRT can't find the sync signal.

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