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

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By Macross_VF1

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09-08-2019, 21:27

I tried swapping around the Q9 and Q10 transistors to see if the problem shifts to the green signal, however the problem stays the same. I think we can at least rule out those transistors. I'm starting to believe that the problem may lie with the daughterboard.

By Grauw

Ascended (8406)

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09-08-2019, 22:15

Ah I see, the other one covers the three other revisions /00 /01 and /19 so I thought it covered all, but the /20 has its own manual Smile.

Seems that in the /20 there’s no capacitors on the RGB signal path to the SCART connector. Maybe resistors R63 or R68? It really does seem like just the blue component is affected and that doesn’t involve the daughterboard.

The daughterboard provides the composite video (CVBS) signal, which doubles as csync. You could try to remove the daughterboard and use a pure csync signal rather than CVBS just like the SCART-only French version does (see “French version only” block on p. 14). I’m not sure how easy it is to make that change, but if R76 is present on the board then it’s probably easy, look at the signal path from them to LF40 and change it so that it connects to R76 instead of R71, if you’re lucky there’s even a jumper. Otherwise you would need to populate that part.

However I dunno, the issues I’m seeing here don’t make me suspect the sync. But it would at least take the daughterboard out of the equation (also the RGB inputs it has, though that should not affect the SCART output, but maybe Q3 is broken or sth). And idk, electronics can be fickle, and I don’t know what your TV does with the CVBS signal so who knows.

By Macross_VF1

Rookie (18)

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10-08-2019, 15:05

I've done some experimenting just to rule out as much as possible. R63 and 68 give the proper values and as stated earlier the Q9 transistor doesn't seem to be broken. I even tried a bypass by rigging up a simple circuit, disconnecting Q9, and taking the blue signal directly from pin 24 on the video chip and routing it to the SCART socket. No improvement. Well technically, it was worse, since the blue was almost black, but that can be attributed to my homemade circuit. The warping and shadow was still present and that was all I needed to see.

The same can be said for the Csync signal. To rule out the daughterboard I removed it, disconnected Q12, rigged another simple circuit, and made a bypass from pin 6 on U14 to SCART pin 19. I also swapped the HSync and Csync transistors Q7 and Q8 just to eliminate the possibility of a bad transistor on the Csync line. And while I was at it I measured the R60 and R59 resistors on the Csync line, and they also give the proper readings. All of this and no improvement. If there is a problem on the Csync line, and I agree it's unlikely, it would be U14 but I don't have one of those lying around.

By Grauw

Ascended (8406)

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10-08-2019, 15:08

Have you tried yout flat screen TV with other SCART equipment and similarly blue backgrounds?

By Macross_VF1

Rookie (18)

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10-08-2019, 17:57

Grauw wrote:

Have you tried yout flat screen TV with other SCART equipment and similarly blue backgrounds?

Now that was a really good question. I had not tested that so I set about doing so. I dug out my MSX2+ and wired a scart connector for it and it works flawlessly during boot up. Also, I brought the VG-8235 over to my brother and tested it on his TV and the colour problem do NOT manifest.

One thing I noticed now that I have dug around in the guts of this computer for quite some time and have studied the schematics is that it has been modified some time during its' life. I already knew the FDD had been replaced, but there are a fair few wires added as well, and a ROM chip as well. Could this have anything to do with the boot up colour?

By Grauw

Ascended (8406)

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10-08-2019, 18:10

Don’t see how that would affect the boot up colour unless the modifications are touching the video circuitry. Or maybe something to do with power draw? Do you see video interference when the floppy drive activates?

I think what this may turn out to be is that your TV’s analog RGB digitising circuitry is just pretty crappy (not surprising nowadays) and is really sensitive about the signal without much tolerances. Maybe again something to do with impedances a well. If you use an OSSC or similar you’ll probably get a perfectly fine picture.

I wonder if you modify your cable to change the impedance of the signals, if it will fix this problem. Dunno how that works, but I’m hazarding a guess Smile.

Otherwise, if you’re not willing to shell out for an OSSC (tho I would recommend, picture quality is amazing), probably even a cheap Chinese SCART -> HDMI converter off eBay will fix this problem for you.

By Macross_VF1

Rookie (18)

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10-08-2019, 18:56

Yeah, didn't think it was the ROM chip but I had to ask.

The currently most likely explanation for the problem is of course the TV. I have considered buying an OSSC but haven't really wanted to shell out the necessary cash as I already own a Retrotink 2X. If anything, what I want is an RGB to YPbPr converter instead.

I actually just bought a SCART to HDMI converter that was supposed to be a good deal better than the bog standard converters you can find on Ebay. It was even reviewed favorably by several sources. Sadly, the one I got must have been defective, as it only converted the composite signals, not RGB. So I had to return it.

Anyway, thanks for the help even if it likely just was a waste of time since the TV seems to be the culprit. And even if it isn't, the problem isn't exactly huge. I can live with the wrong colour during start up as long as no games also show the same problem.

By Manuel

Ascended (15697)

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10-08-2019, 19:23

I'm pretty sure this won't be only happening at the start. Try a game with a blue background, like Pooyan or Fire Rescue.

By Macross_VF1

Rookie (18)

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10-08-2019, 23:42

Manuel wrote:

I'm pretty sure this won't be only happening at the start. Try a game with a blue background, like Pooyan or Fire Rescue.

The only game with a lot of blue that I have would be Twinbee and sure enough it does display some interesting problems. The title screen show the same problem with blue but when I start the game, the water in the background is not affected. Admittedly it's a paler blue compared to intense blue of the title screen.

By Manuel

Ascended (15697)

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10-08-2019, 23:52

You could download the games I mentioned and try that as well... If you're interested.

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