Megadrive with sound but no image (no RGB, no COMP)

[Jean Marc]

Hello,
a friend asked me if I could do anything for his Megadrive
which has sound but no picture. I'm an electromechanical technician in tramways
myself, far from an electronic technician...
yes, I enjoy doing hacks or mods of all kinds, but I've never done this before.

I tested the voltage regulators, they're giving a good 5V.

I also tested the RGB encoder, here are the results:
we have both 5V (one above the RGB outputs, the other in the upper right),

and a stable 2V at all six RGB inputs and outputs.
















Also, there are five small yellow capacitors on the PCB that are puzzling me: one on top of the PCB, and the other four underneath.

Even though my Ardutester doesn't tell me they're faulty, what it's showing is quite strange.

The next photo shows the five odd results, and the following one shows the result for a normal capacitor.

Note that I haven't detached even a single leg.





Does anyone have any idea? Thanks in advance for your help.
Thank you for your attention, Jean-Marc

 

[Tiviat]

Hunt around the MD's gpu the VDP chip. look for faults and any imperfections on the motherboard, rust around any of the components or contacts. Maybe it just need a little cleaning...hopefully not a bad VDP chip....or a dead capacitor.

 

[Jean Marc]

hi thank you for answering. the pcb is so clean, i searched and seached and the only thing
i found is a shard, between the riding C63 and FB13, but certainly made intentionnally, see pictures: it seems a factory modification,
'cause there is a wire underneath doing the job.

 

[Tiviat]

Great. But odd capacitor readings can cause all kinds of power related problems...including not giving components enough wattage to function properly or at all. Is the odd capacitor reading anywhere within the vicinity of the gpu chip area , or along the video display route on the motherboard ?

 

[Jean Marc]

as we can see, there is one close to the 68000, one close to the RGB encoder - C-SYNC OUT signal - , and an another close to the 315 bus arbiter.