Help needed to pinpoint and hopefully repair a 1CHIP SNES

Hiya!

several days ago I installed in my 1CHIP SNES the superCIC + IGR + 50/60hz switch mods... They all seem to work fine (except the snes powerpak in ntsc mode, in pal it worked OK) until I had to brilliant idea to flick the switch on the scart cable I had and thus make an on-the-fly switch from RGB to composite. (the switch is at the pin16 of the scart plug, the "rgb blanking" pin , it either provides current to the pin so it is rgb mode or cuts it off completely so it's composite) This worked for some seconds and then the screen got distorted and finally went black.
Now when powered I see no image/sound... plus you can't power it on, turn it off and then on again, it does not work... you need to unplug it from the mains and plug it after a while to work. If you leave it enough time it will show an image for 1sec then go dead again...

Today after ~3 days I tried it again and it boots and plays fine... it's been running for 10mins OK but at some point the image was lost again and all I could hear was the music from the intro loop of the simpsons game I left it with. When powered off it doesn't power on again, only for 0.5sec the led comes up and it goes off again. As before you need to completely remove the psu, put it in power-on position and plug the psu back again... then it boots for a while, you get distortion on the screen and then it dies...

None of the chips on it seem hot, could it be the S-RGB chip that died because I switched on the fly the scart mode from rgb to composite?

I have done a couple of things on the snes except the mods , I use now a glass tube fuse on stands and I have replaced the stock 7805 with a MC 78T05CT one that I could find locally (it provides 3A instead of 1A and has a higher temperature tolerance to my understanding).

Does anyone here has any idea of what the fault is? I already got a 1chip snes replacement but I would really like to repair this one too

Thanks in advance for any help!

 

Following Bad_ad84's advice I found out that there is indeed something short-circuiting on the motherboard...

I did this:
I have completely removed the 7805, put my multimeter in continuity test and touch the 2 sides of the fuse and it beeps normally. Then I remove the probe from the right side of the fuse (towards the cartridge slot) and touch the GND plane... it still beeps for about 5-7 seconds and then it stops.

I assume this behavior is not normal... anyone has any idea what part to check?
TIA for any info/ideas

edit:

the same thing happens when testing with the VA1 capacitor that is connected between 9v+GND... I think this is normal right? You probe for a while the "I" point from 7805 and the left capacitor leg and then move from "I" point to "G"ND... you get a connection for a while and then it disconnects...

 

Off topic, but how did you get the fuse holder to attach to the board? I have the run a short set of wires since the solder seems to refuse to stick to it.

 

flux+good heating, I put some solder on the mobo pads and flux on the underside of the holders.
Then I put them in place and using some pliers to hold it in place I heat on the top side of the holder. Eventually the solder below melts and makes a nice connection.

Back on topic, I think this is not where my problem lies... it's normal for the cap to make a connection for a little time... so back to square 1 for me

 

Yes this is normal with caps... though 5-7 seconds is quite a long time I presume thats just because the cap(s) is/are fairly large.

However, the problem does sound like it could be something to do with a cap. Normally when there is an intermittent fault like this check for shorts, bad connections, faulty caps, and then faulty other components, in that order. Unfortunately its a bit hit and miss to figure out exactly what's going wrong, especially if nothing is visibly damaged. I normally start in the power supply section, then I guess you should look at the video section. If you arent getting power then it sounds like something with a capacitor... I'd suggest checking power output voltages while it isnt working and when it transitions from good->bad. Get docs for anything power regulation related (there's got to be more than just a 7805? Well, I guess possibly not, but seems a little basic) and check that the outputs are correct. Check power supplies to chips to make sure the correct voltages are present. Hopefully you can find the source of the issue. Oscilloscope might be handy.

Having said this I know nothing about the SNES, so cant give any specific help. But emu-docs.org and mmmonkey are your friends!