Testing pads on a circuit board with a multimeter

[wongojack]

I'm right in the middle of my Flashback 2 (Atari) cart port mod. Going good so far, but I want to test if two different pairs of 'pads' on the board share the same ground. These are coupled pads that when jumped or open determine if the FB is in Cart or Onboard game mode.

How do I apply the leads from the multimeter to determine which side is ground and how can I determine if they share a connected ground?

To find out if they share the same ground, I think I should be able to test for continuity between the two pairs
1) Find ground for both pairs
2) Touch black to ground of one pair (dunno if color matters)
3) Touch red to NOT ground of other pair
4) Listen for beep

That should tell me if they have the same ground, right? How do I figure out which side is ground?

[wongojack]

As a follow up to this question, I was able to test for continuity between 2 pads that were a shared ground. "Tinning" the pad allowed me to test them quickly. Basically just touch both ends of the meter to the pads and if they are connected then beep. Worked great and was just like the diagram indicated on AAge.

[miner2049er]

I'm not sure how you work out which side is ground. Did you do it?

[wongojack]

Well one pad on each jumper set were connected (loud beep on the meter) which means ground. I'll poke around and see if I can test polarity somehow, but I'm not sure really how to be 100% without other clues.

[SubaruBrat]

I has been a while since I did my FB2, but what I did was put a SPST (on/off) switch on J3 to select the cart port or the built in cart. It does not matter which side is ground, they are either connected or not.

On a separate note, I have seen the FB3s in stores for Xmass, I am going to snag a bunch in the after Xmass sales.

[wongojack]

There are several different revisions. J3 needs to be closed for cart, but J9 needs to be opened (on a Rev C). I wanted to connect the common ground for both to the common lead of an SPST as opposed to using a DPST . . . .

Anyway, I have other problems now besides the switch. Garbled screen . . . .

[SubaruBrat]

Yeah, mine is a VERY early release and had just the one pad. Let me check into that for you.

[retroshaun]

Scott: which stores did you see them in? I didn't see any yet but I really wanna pick one up.

[wongojack]

Yeah, mine is a VERY early release and had just the one pad. Let me check into that for you.

Ok Scott, I know you have better things to do than answer my bungling beginner electronics questions, but I do have a "Garbled" screen after installing my cart mod. It looks like this:

http://www.atariage.com/forums/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=224321


I had some trouble wiring the switch but I've repaired that and I can test with a multimeter that my repair has created less than 1 Ohm resistance (.02) whatevs - its connected. I also tested the repair and the flashback would display its own menu. I still think that my repair is a troublespot because it controls what displays on the screen, but I've tested it over and over and it reads fine. What do you think could cause a display like the one in the picture?

I'll say that while doing the mod, the AV wires became disconnected from their "sub board." This was annoying because while they were partially connected I was testing the display thinking that my work was bad. Anyway, I stripped and resoldered the AV wires to this "sub board" but when I test the connections from the tips of the AV cable that plugs into the TV to the end connection on the bard I get something like 2 ohm resistance. That is still really low, but I'm wondering if it could be causing my problem. When I test a regular set of RCA cables not associated with this mod at all there is less than 1 ohm resistance from end to end.

I don't think that I've damaged the onboard ROM, that seems unlikely, but I guess its possible. I would think that if I Damaged the "Michelle" chip, I would get nothing at all. Also, both of these chips are sealed under high grade epoxy (er something).

I think there must be an explanation to whats happened here that I should be able to figure out. I just was wondering if you could point me in any directions to begin testing resistors or power or something else specific that you think could jumble things up like that.

If I can't figure anything else out, my next step is to completely remove all my connections for the cart mod and test again. Basic t'shooting to just undo what I did and go back to the last state I tested successfully.

[SubaruBrat]

Scott: which stores did you see them in? I didn't see any yet but I really wanna pick one up.

We have a pharmacy chain here called Rite Aid, they have them $39 as I recall. I would check Amazon as well, if worse comes to worse I can grab one and mail it to you.

Unfortunately for your display problem your just going to have to dig, it is one of two things, something that you messed up that you can undo, or you damaged the AOAC and it is just plain unrepairable. I wish I could offer more advice but it is pretty much one of the two. You are going down the right road by undoing what you have done and sorting it out. It happens, part of the learning curve.

[wongojack]

or you damaged the AOAC and it is just plain unrepairable.

I tried to look up AOAC, but I don't think I'm finding what you are referring to

[SubaruBrat]

Atari On A Chip, the IC under the epoxy (aka glop top)

[wongojack]

Ah yes, thats what I was referring to as the "Michelle"

[wongojack]

I disconnected my cart mod wires from pads 13-24 and the screen was still jumbled up like in the picture. I then disconnected 1-12 and the flashback works again. I guess I had some problem with my soldering technique or maybe there is a bad pin on the edge connector or something.

[Nurmix]

Ok Scott, I know you have better things to do than answer my bungling beginner electronics questions, but I do have a "Garbled" screen after installing my cart mod. It looks like this:

http://www.atariage.com/forums/index.php?app=core&module=attach&section=attach&attach_rel_module=post&attach_id=224321


No advice for you on your trouble, but I just had to say that I have that exact same TV hooked up to my retro consoles.
:AT

[wongojack]

Not sure why this was moved to the Pinball Wizard forum when its mostly about the Flashback . . .

I got my Flashback cart mod to work. I had to de-solder all 24 wires and then put them back. I'm not sure what I did wrong the first time, but all is well now.

Finding the ground connection on the pads came down mainly to identifying other ground on the board and then testing for continuity between known ground and the unknown pads. In this case the board diagrams on Atariage showed me known ground.

Anyway, I've put pics with captions for my mod here:
https://skydrive.live.com/redir.aspx?cid=ec7bd1a81d3e9f22&resid=EC7BD1A81D3E9F22!127&parid=root