Bad video signal on MSX2 Philips VG-8235

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By tipostupendo

Supporter (11)

tipostupendo さんの画像

11-06-2009, 20:09

First of all; hello all and many thanks for maintain live the msx spirit

After about 12 years without use; I recovered my old MSX2 from the vault.

But it seen to have a problem somewhere at the power supply or the video signal regulator. All i get on screen is a darkgreen picture, with a moving shadow that appears to be the MSX moving logo at the start. The floppy disk drive had loose the transmission gums but everything else looks to work ok, sounds, music from cartridge games...ect.

I have opened the case, and all looks ok. I dont have a multimeter to verify voltages. So i wonder if you have seen this problem before or if you have any idea what could be wrong.

Thanks in advance!
This is the image displayed:
farm4.static.flickr.com/3598/3617498060_809629cba7_o.jpg

The machine:
farm4.static.flickr.com/3341/3617497996_9050f2df4f_o.jpg

PD: the image is perfectly sintonized, thats not the problem.
PD2: Please, could you post the voltages of the cable coming from psu to motherboard to measure them?

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By Jipe

Paragon (1594)

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11-06-2009, 20:59

you use what video signal : RVB with Péritel / UHF / Din8 with video composite ?

By tipostupendo

Supporter (11)

tipostupendo さんの画像

11-06-2009, 21:03

Hi! Im using the UHF/TV cable.

By tipostupendo

Supporter (11)

tipostupendo さんの画像

11-06-2009, 21:19

Just tried the SCART connector with the same result.

The MSX shadow moving at the start makes me think of a weak video signal for some reason.

This is a capture with a filter applied, you can guess the MSX logo Crying

farm4.static.flickr.com/3400/3617701574_baa9526931_o.jpg

By Jipe

Paragon (1594)

Jipe さんの画像

11-06-2009, 21:50

which model of 8235 is it ? /19 /39 /00 /20 or other number ?

is important for schematic !!

turn and see on label under MSX

By tipostupendo

Supporter (11)

tipostupendo さんの画像

11-06-2009, 21:52

TYPE VG8235/20

By tipostupendo

Supporter (11)

tipostupendo さんの画像

11-06-2009, 22:55

Ok, just found the service manual for the msx, and discover that there is only 5+,12+ & 12- connections from the PSU to both the floppy drive and the MB; so I will try to get a multimeter and measure the PSU.
In case of a PSU fault ill try to replace it with an ATX PC power supply. If the problem is at the encoder unit sadly ill have to say RIP to my precious MSX. Crying

Here is the service manual:
msx.retro8bits.com/msxdocs/vg8235sm20.pdf

By Sonic_aka_T

Enlighted (4130)

Sonic_aka_T さんの画像

11-06-2009, 23:15

Don't RIP it just yet. Perhaps someone like repair-bas has a few ideas you could try. Also, try looking if any of the caps look 'odd', replacing those is usually inexpensive and easy, and it could help...

By flyguille

Prophet (3031)

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12-06-2009, 00:48

IT has RTC? if it has, maybe the battery deaded a lot time ago and oxidate everythings just like happened in my msx2.

By RetroTechie

Paragon (1563)

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12-06-2009, 01:17

(..) so I will try to get a multimeter and measure the PSU
Yes, definately start with that... Murdoch the power supply is like the 'low hanging fruit' in computer repair - easy to diagnose, easy to fix. Tip: unplug the floppy drive power cable - it also uses +12V (if original drive). It's very unlikely the floppy drive does something to nuke the +12V line, but you don't need the floppy drive for diagnosing this problem anyway (=make the system simpler, narrow your search area).

Btw. you can ignore the -12V: most MSX computers don't even use it for *anything* (only for supply to cartridge connectors)

If power (+5 and +12) is okay and system running normal otherwise, then you might be looking at a broken transistor somewhere in the R/G/B ouput stages, or even dead video output(s) on the VDP. Question

One other quick thing to try: if I'm not mistaken, the 8235 has a small circuit board with the video encoding stuff (+modulator) on it, separate from the mainboard, with a short connector to the mainboard. Try unplugging and re-plugging that connector one or two times (while powered off), and see if that makes any difference. Probably not, but if so it might be a simple connector/wiring issue.

By tipostupendo

Supporter (11)

tipostupendo さんの画像

12-06-2009, 01:26

IT has RTC? if it has, maybe the battery deaded a lot time ago and oxidate everythings just like happened in my msx2.


Neither not exploded or visible leaking caps. But Ill swear the problem is on one of the caps afecting the video encoder, making the signal so weak.

Many thanks for your help and ideas!!!

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