Philips VG8235 - Blurry screen

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Door philgood1351fr

Rookie (23)

afbeelding van philgood1351fr

20-07-2021, 19:45

Hi,
My VG8235 has some screen problems. When I turn it on, the MSX Logo appears but is very dark. Then after few seconds, on MSX Basic, the luminosity is shiny.
And if I start Multimente, for exemple, or a game, the colors are blurry, bad.
Made a little vidéo here https://youtu.be/MgNNqvrxM5w

Connected by scart cable (Tried other cable, other TV - no change). Changed the fuse too.

Anyone have an advice ?

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Van Grauw

Ascended (10010)

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21-07-2021, 01:30

The smears in MultiMente make me think it is a capacitor problem on the R, G and B output lines… Check if the electrolytic ones have leaked or are bulging. It could also be an issue with the DAC of the VDP itself, but I’ve never seen this failure mode before. I would inspect the video circuitry first.

Which VG8235 model type do you have exactly? E.g. VG8235/20? Look at the bottom of the machine.

Do you have any electronics measuring equipment and soldering experience?

Van Meits

Scribe (6432)

afbeelding van Meits

21-07-2021, 01:45

To philgood1351fr, sorry for not being totally on topic, but I wanted to react only on one line by Grauw:

Grauw wrote:

Which VG8235 model type do you have exactly? E.g. VG8235/20? Look at the bottom of the machine.

There is a difference between the /00 and /20. /00 does not have a white MSX logo during boot and the overall color output is darker.
From left to right /20, /00, /20. It's not a screen or cable thing. I did check that.

edit:
To philgood1351fr. Can you check if the voltages coming from the PSU are what they should be?

Van philgood1351fr

Rookie (23)

afbeelding van philgood1351fr

21-07-2021, 11:21

Hi, thanks for your help.
Mine is a 8235/39 - French
I checked the voltage on the Fuse, it shows only 8.8V.
It seems low no ? shouldn't be 12v or near ?

Van Meits

Scribe (6432)

afbeelding van Meits

21-07-2021, 14:32

That would be the french version of the /20. In the front right of the PSU board the indivicual voltages can be meassured.

Also check if the big STK Voltage Reg chip has a solid connection. As well on the board side as on the chip side. They're not always placed the way they supposed to be placed.
I had a -9.86V on the -12V read just recently and it was allright.

Van philgood1351fr

Rookie (23)

afbeelding van philgood1351fr

21-07-2021, 18:26

Thanks for the TIP. So my values : Blue (-12v) = -11,9 / Yellow (+12v) = 1,9 / Red (5v) = 5v.

I checked the STK voltage is well connected. But values on Pins 2,3,10 and 11 are 1.9v too (rest good).

So do we have enough clues ? What is the next thing to look at ?

Van Meits

Scribe (6432)

afbeelding van Meits

21-07-2021, 19:46

philgood1351fr wrote:

Yellow (+12v) = 1,9

Is that 1,9 or 11,9?

Van philgood1351fr

Rookie (23)

afbeelding van philgood1351fr

21-07-2021, 21:55

Yes 1.9..... very low

On the other way, on the blue cable of the PSU (Big bloc name JA 10W SMD), only 8.8v is measured. That should be 12v no ?

Van AxelF

Champion (386)

afbeelding van AxelF

21-07-2021, 22:15

Meits wrote:

There is a difference between the /00 and /20. /00 does not have a white MSX logo during boot......

Wow did not know this, do you have a photo of the not white MSX logo. ?

Van Briqunullus

Champion (392)

afbeelding van Briqunullus

21-07-2021, 22:30

What? You mean the MSX logo scrolling up? My 8235/00 has it.
The only difference with later models afaik is that only VRAM is reported, not main RAM.

Van Pentarou

Champion (389)

afbeelding van Pentarou

21-07-2021, 22:55

philgood1351fr wrote:

Yes 1.9..... very low

Is the measurement taken with the 2 cables (disk drive - main PCB power) connected or disconnected?
You should measure with both disconnected.

philgood1351fr wrote:

On the other way, on the blue cable of the PSU (Big bloc name JA 10W SMD), only 8.8v is measured. That should be 12v no ?

The transformer? Are you measuring in DC or AC mode?

@Meits: Why is your ZD1 diode cut?

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