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.

 

[rorirub]

That looks like a french model Megadrive which was factory modified to output RGB only, so it's not strange to see it have some bodge wires or cut traces around the video area. From what I remember it had composite output cut from the video connector, and replaced it with C-SYNC output from the video encoder. It did so without any filtering, so it's possible that the pin got burnt out, same goes for the RGB pins, the filtering was inside the RGB SCART cables for the Megadrive...

Try probing both the input and output of the video encoder (CXA1145?) with a scope, as well as the c-sync output, to see if there's anything going in or out. If you don't have a scope, you should at least see the voltage fluctuate on a fast multimeter. If the video encoder RGB input pins show no activity but just a very steady signal, that means there's no output from the VDP, so probably game over.

On the long term you may also want to replace the voltage regulators and the capacitors but that may not be the fault.

 

[Jean Marc]

very thanks for these informations. so the stable 2V seen at RGB IN on the encoder by multimeter is worrying you mean ? i got a scope, i 'm going to try to measure. if speaking about 100nF ceramic capacitor, i only got four here at my place and they are five on the board.. wich one of them worth replacing first ?

 

[rorirub]

so the stable 2V seen at RGB IN on the encoder by multimeter is worrying you mean ?

I recall RGB pins varying in voltage depending on the image, but I could very well be wrong. Nevertheless checking it with a scope should tell you if there's any video output from the VDP or not (preferably test it while listening to the music of some game, so you know you are not testing a blank black screen...).