Philips VG-8020/00 - No Video Signal

Door NC92

Supporter (3)

afbeelding van NC92

01-04-2020, 00:22

Hi.
I bought my first MSX computer very cheaply but unfortunately it doesn't work.
All power supply voltages are present, Red LED is on, CapsLock is working.

I measured the voltage at the CVBS output.
I was expecting below 1V but it is 2.2V.
Isn't that way too much?

Looking at schematics, the first suspect is LVA510.
Do you have any other ideas?
Thanks!

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Van Wlcracks

Champion (421)

afbeelding van Wlcracks

01-04-2020, 16:13

The monochrome video output is broken too? on pin 4 of the din plug? That would bypass the LVA510.

Maybe C13, but that should give no voltage at all. There are a lot of Electrolytic on the U-100 video board. Maybe check them.
Pretty boring story, but using a scope at the VDP video signal outputs Y B-Y R-Y, follow the path where it stops.
I guess you don't have any. Checking pin 37 on the VDP clock output should be something like 3.47 Mhz I think.
The video voltage can be that high because video has to be terminated with 75 ohms. Measure it with the monitor connected this could be lower it to the required voltages.

Van NC92

Supporter (3)

afbeelding van NC92

04-04-2020, 01:31

Thanks for the tips.
VDP is getting quite hot.
Y, B-Y, R-Y signals are not present.
There is communication between the CPU and VDP as well as between VDP and VRAM.
The resonators work correctly, but unfortunately nothing on the output.

I think you meant pin 38 of TMS9918A.
TMS9929A has a different pinout.

I wonder if faulty VRAM can cause this problem.
Probably I will replace VDP first.

Van Wlcracks

Champion (421)

afbeelding van Wlcracks

04-04-2020, 09:47

The vdp can get "hot". I think not finger burning hot.
The vdp generates the 50hz interrupt for the Z80. If caps lock works that give a bit away the VDP still generating interrupts.
Difficult case. I repaired some hard case died MSX1 computers but RAM is always more the problem then the custom chips. Maybe i got the wrong pin sorry.
Checking RAM is possible with a logic analyzer but still a b@#$ch.

Van NC92

Supporter (3)

afbeelding van NC92

11-07-2020, 20:59

I looked more closely at the communication between TMS9929A and VRAM.
It didn't look right. More like random noise.
I decided to replace TMS9929A.
After that the computer works like new.
Thanks for the help.

Beware of fake chips from ebay.
The first TMS9929A I bought was fake.