Dead Power Circuit?

[lolitsdaelan]

So I received a Wizard of Wor coffee table styled cabinet from a guy at work. I had an old computer laying around and decided to get a MAME cabinet going.

I got the front end working, gutted everything inside the cabinet except the power supply, hooked it up external (computer sitting outside the cabinet), and had it all running fine.

Then all of a sudden, I plug power in, and NOTHING comes on. Here are the two things that happened prior to this.
1) I tipped the arcade cabinet over (taking precaution to be gentle) so see if there was some way of removing the wood panels used to enclose the coin case.
2) While attempting to fit the computer inside, the case of the computer got pulled in by the magnetic field of the speaker and they made contact, however, neither the cabinet or the computer were actually plugged in.

So 2 seemingly innocent actions somehow stopped my cabinet from working.

I've probed around, and I can see that the 120VAC is present on both transformers. However, there is no voltage on any of the secondary windings.

So I feel confident that either somehow both transformers are toast or the AC filter toasted.

Anyone have any experience with and issue like this or have any ideas as to what may of happened???

Thanks in advance!!

[Ed_McCarron]

If you've got 120 on the primary and nothing on the secondary, something odd has happened -- a short would cause the transformer to smoke, but an open would fit your symptom.

With the power off, does the secondary winding show a resistance reading?

Are you talking about a power supply transformer or isolation transformers?

How are you using an old cabinet supply to power a PC?

Pics?

I wouldn't expect the speaker and case touching to cause you any grief, even if it WAS powered at the time.  Flipping the cab over may have caused a mechanical problem; something broke, dislodged, etc.

[lolitsdaelan]

I havent checked the secondarys for a resistance reading, I'll be sure to try that when I get home.

This problem applies to both transformers. So the isolation transformer that simply takes the 120VAC in and puts that on the 120VAC line of the monitor, and the actual supply transformer that steps it down to 14.5 and 12 for the rest of the cabinet.

One thing I recently thought of, is the little silver box, the EMI filter, may of gone bad. Since voltage shows up on both set of primary windings, and it would be pretty rare for both transformers to fail at the same time, I'm wondering if the GND connection on the filter is bad. This would explain why the primary voltage is present, but none on the secondary, since there is no path for current to flow.

So I guess my question now is, is this filter necessary? I'm assuming it protects against transients, which I believe could also be accomplished by a surge protector? To answer your other question, thats how I planned on powering the computer. Tuck a surge protector inside the cabinet, plug that into the wall, and use it to power the computer/cabinet/speakers.

I'll poke around once I'm off work and see if I can post some pictures.

Thanks!

[Ed_McCarron]

When you're checking for 120v, are you checking ACROSS the primary coil, or are you checking from one side to ground?

Might be an open neutral in there somewhere.

[lolitsdaelan]

Ok so correction, I don't measure 120VAC across the transformer. Not sure what I did to confuse that reading. Also, I measured shorts across the windings, so I know that the transformer didnt fail open.

One thing I did nail down for sure, is that pre EMI filter, I can measure 120 no problem, but immediately after, I get nothing across the 2 leads.

Is the EMI filter necessary? I assume it blocks transients and whatnot, which a surge protector would do. Is it some outdated part that I can simply leave out, and go straight from the wall to the transformer?

[Ed_McCarron]

OK, the transformer 'short' is hopefully just the winding resistance.

Technically, no, the EMI filter is not necessary to make it work.  Pass some FCC requirements, sure.  Protect the equipment, sure.  Required?  Probably not.

Try bypassing it with a fuse or two -- just in case -- and see what happens.

[lolitsdaelan]

Fixed it! Although I still have no idea what caused the initial failure.

I disconnected the other transformer and all other wiring that I was no longer using. Hooked the AC line straight through the EMI filter and then straight to the fuses that went to the primary and tada! Powers up just fine.

Again, no clue what could of gone wrong, but whatever it was, it's been worked around now 

Thanks for all your help!