Re: TECH: 6.5VAC on ground? (Bally)

TheKorn wrote:
Chris Moates <cmoates@xxxxxxxxx> wrote in
news:h096g2$q8d$1@xxxxxxxxxxxxxxxxxx:

TheKorn wrote:

Where are you measuring from? (i.e. where are you putting your
probes?) Normally you have one probe on ground, for reference, then
measure to a secondary point. If you have both probes on a rail and
you're getting an AC reading, then your meter is busted.
I get the AC reading going from GND inside the pin (off the MPU) to
ground on the outlet. This _should_ be the same, and tests for
continuity just fine. I would have said that it's a break in the
ground, but I shouldn't get continuity in that case.

I've tested with two digital meters and one analog meter. And I have
an undeniable symptom; when I have a ground wire connected between the
pin and something else, it gets warm, trying to carry current. That's
how I noticed it in the first place.

(The "this and something else" is an RS232 cable. The shield of the
cable connects to a 9 pin RS232 port on a replacement MPU and the
RS232 port of a computer. I've tried multiple computers, multiple
RS232 cables and multiple serial ports, both on board and USB.)

Saying all of this, it almost seems like I _have_ to have a break in
the ground; as you mentioned, you can't measure AC from a single rail.
I'll poke around for that first, I suppose.

I don't think you have a broken ground, but rather, a cruddy one. Put
your meter on the lowest ohm resistance it'll read, and read the
resistance between those two points. I'm guessing you'll have something
like 5-10 ohms of resistance, which is way too much.

I'd check to see where the ground wire for the plug attaches in the game.
(It should connect to the transformer IIRC, but double-check that.) Then
I'd check the gronud connection between the head and the body, make sure
that's all hunky dory. And just start working the chain from there;
someplace there's something amiss.


Ok, I have 3 points I'm working with:
1) Ground from the wall (chassis ground)
2) Ground test point on AS-2518-22 (Solenoid Driver Board)
3) Ground at MPU H4 (bottom of header)

#2 and #3 should be in the same ground plane, as best as I can determine.

Resistance checks between them seem fine. I get 0.2 - 0.1 If I run a
jumper wire from chassis ground to the GND test point, it is
consistently 0.1 Ohms of resistance.

However, if I power up the machine with that jumper wire from chassis
ground to test point ground, all solenoids immediately fire and hold.

I don't yet have the schematics for these boards, but I'm thinking
something must be seriously wrong if grounding board ground to chassis
ground causes it to fire.

Cheers,
Chris
.

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