Extremely Strange Behaviour - RF noise, capacitance?? - Page 1
Forum section: electronics / beginners

author: GadgetUK
author_rank: Contributor
timestamp: December 02, 2013, 02:30:27 pm
content: Hi,I really hope someone can help me with this problem. I've worked in electronics previously (20 years ago) for a good 10 years+ and recently got back into electronics (thanks to Dave and others - Alison Challis). Anyway, long story short:-I have a problem with a game system where a game cart is causing strange things to occur. What happens is I get slight artifacting on the screen - like there's some noise somewhere. If I hold my finger over one spot of the main board the problem vanishes, and I mean vanishes - completely. Move finger 1cm away and problem is back. I can repeat this over and over on demand. Now the game cart works, and the console works. And infact i've ruled out everything here by using 2 other consoles that work flawlessly and both behave the same way with the cart.What I know about the cart is it has a couple of CPLD's on there and operates at 3.3v, whereas the systems operate at 5v. I am wondering if anyone has ever encountered behaviour in other electronic devices / systems like his before, and perhaps some ideas on where to look? I am thinking that some decoupling / bypass caps could help somewhere?

author: GadgetUK
author_rank: Contributor
timestamp: December 02, 2013, 08:31:58 pm
content: I am wondering if its capacitive reactance? Does anyone know anything about it - practically?I am wondering if its due to 3.3v chip logic levels outputting to the 5v based MVS board. Any help or advice welcome.

author: AndyC_772
author_rank: Super Contributor
timestamp: December 02, 2013, 08:50:10 pm
content: This reminds me very much of some odd behaviour I saw on my electronics A-level project many years ago.It was built on breadboards, and consisted of (amongst other things) a 6502 CPU, a two digit LED display and a keypad. At one stage, if I brought my hand near the keypad, the display would fade to a dim '88', and as soon as I withdrew it, the correct display would return.To this day I can't say with any certainty what the root cause was, but I know what fixed it: improving the power connections between the various breadboards, and adding a load of decoupling capacitors. It was a very memorable lesson in physical layout, and one which has served me well ever since.If I were you I'd take a critical look at how the affected cartridge is laid out. Does it have a good solid, unbroken power and ground plane? Are there decoupling capacitors near all the power pins of the ICs? If those two basic rules haven't been followed, there could be scope for some simple improvements. It may be that all you need is 10nF or less, connected in exactly the right place.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 02, 2013, 08:50:15 pm
content: Quote from: GadgetUK on December 02, 2013, 02:30:27 pmI don't think it is decoupling since the cartridge is the problem, not the console and all you can decouple is power lines and they are not coming from the cartridge... to me it looks like there is a problem with a data or address line, and is 'smells' like a missing/broken pull-up resistor.If you don't have a scope you could use a multimeter and measure the resistance between the data lines and ground/vcc, and the same for the address lines. You should be able to see if one of these is different from the others: work from there.And in the end there could be a faillure in one of the chips in the cartridge of course.

author: marshallh
author_rank: Unknown
timestamp: December 02, 2013, 08:53:38 pm
content: Is it a multiple-slot MVS board? the 6 slot boards are extremely notorious for not working except in perfect circumstances.Are you running a pirate cartridge, or a 100-in-1 cartridge? Many of those are so poorly designed I laugh just thinking about it.Can you post some pictures of the hardware you're working with