Sharp twin - Messed up graphics

[Ritch]

Hi everyone, I wonder if somebody might be able to help me. I've got a sharp famicom twin, which i repaired the belt on and loads/plays from disks fine now. However, loading from cartridge will load some cartridges but with messed up graphics (random characters/tiles in the wrong places). Other carts will just show a grey screen, and it's fairly consistent how each cart behaves.

Any ideas what i should be looking at checking, ppu/ram chips? Or are they ruled out because games load fine from disks? Or the cartridge connector?

One extra point, the psu i got with it is 7v, not 7.6v, and 1000mA. Could that be causing an issue?

Thanks!

[P]

If disks works you should definitely rule out bad chips and virtually everything else except the cartridge connector. Try cleaning it with 90%+ isopropyl alcohol and an old (but clean) toothbrush and let it dry before turning on the power. Another trick is to use a t-shirt or other thin cloth soaked in isopropyl alcohol and wrap it around an old credit card and swipe it in the slot many times. Clean your cartridge edge connectors the same way using cotton swabs and/or a toothbrush, both dipped in isopropyl alcohol. In all cases you may need to scrub quite a lot to get out all the grime and the layer of oxidized metal that prevents the electrical conduction.
In worst case you may need to reflow the solder joints in the cartridge connector or even replace it for a new one.

Not sure if the slight difference of voltage in the PSU is a problem or not. 7 V is right at the minimum end of the 7805 voltage regulator's tolerance. But it shouldn't be the cause of your graphical problem, since disks do work fine.

[Ritch]

That's great news, thanks for your suggestions. I'll give it a try cleaning out the connectors.