Ms Pac Trouble

ManiN

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Picked up a board and harness. I bench tested them and things seemed fine until the monitor went out and a few fuses blew. The fuses are for 12V on both the SB and FA blocks. The fuse on the monitor chassis blew and it's still getting power. The monitor thing could be a separate issue.

I tried to test out why the fuses blew and can play the game blind with one fuse replaced. The remaining 12V fuse on the FA block keeps blowing and for some reason I test 15V coming back at the transformer! I try to test at the transformer for 12V and have nothing. Fact is, I haven't gotten a decent reading out of any part of the transformers when I put ground on 0V and hot on 12V or even 7V and so on. How the hell can there be 15V coming from the board to the transformer? Is the power supply on the board putting power back through the harness to the fuse then to the transformer to then short it out and blow the fuse? F7 in the pic below is the one that keeps blowing and has 15V coming from the board side of the harness and not the transformer. All help is greatly appreciated!

pacfuse.jpg
 
Ok, maybe I can simplify this. What is the best way to test these transformers to start? Is it as simple as put one lead on 0V and one lead on say 12V and it should read respectively? For example, the 7V is reading 0.22V. Wtf is that? I am worried the transformer or (s) are dead as I just got them w/ harness and was told it was pulled from a working unit.

A recap, the game plays even though this 'problem' 12V AFB is disconnected. Not the xformer for the monitor and lights, but the one that has 7V, 12V and so on.
 
You should be reading AC voltage, and you should be measuring the 7vac between the 7v and Center Tap lugs. Same thing with the 12v....

MsPactransformer.jpg
 
Ok, check. Now how about the fact that when I remove the 12VAC fuse and the board is hooked to the harness I am getting voltage on the board side of the fuse block? Isn't that some kind of back feed from the board? I'm concerned there's a short on the board of some kind feeding voltage backwards through that 12V line.
 
well, there are two 12vac lines coming into the PCB at 19/X and 20/W, each with it's own fuse. Once on the board, they each go through a diode to rectify it to DC, and then are tied to the same line on the PCB as they go to the Audio section.

So, if you are measuring the 12v after the diodes, then you'd expect to see 12v as long as one of the fuses was good. If you are seeing 12vac at the pads BEFORE the diodes, and the fuse to that pad has been removed, then I'd check to make sure the diode on that line isn't shorted.....
 
Yeah, I thought the same on the 16VDC coming back and looked at the schematic to see those two diodes. Checked D4 and she's shorted! Thanks for helping! Hopefully that's all it needs... The caps and resistor seem fine online w/ D4.
 
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