Midway MCR power problem

[grueinthebox]

Have run into a bit of a snag trying to get audio on my Satan's Hollow project cabinet.  I had two shot power regulator boards (one wouldn't output above ~2.3v, one would output around 5, but the adjustment pot was shot and wouldn't adjust accurately), so picked up a switching power supply conversion from Arcadeshop.  This got the game up and running - it now plays, just with no audio.

The culprit is the fuse for the AC 12v line that goes to the J6 plug on the power board.  A 3 amp 250v slow blow fuse is called for, and that's what I'm using (although just for giggles I tried a 5 amp once).  Powering the machine up blows the fuse every time (went through about four) almost instantly.  I measured the AC voltage output with a working fuse at a little over 13v (at the wire), so it's a little high, but the AC amps only measured at about 1.86 across the terminals of the fuse holder.  I'm a little unclear why these fuses keep blowing here, as the amps seem to be below the rating of the fuses.

Essentially, my questions are these: 1) are there any common problems that would cause this fuse to blow, and 2) is there any other good way to get the voltage required to run the audio system, bypassing the big transformer altogether?  I'd really like to get some sound out of this thing.

[Brunnen-G]

Check out this page:

http://www.homearcade.org/BBBB/midway.html


He also has repair kits for shotgunning the MCR power supplies.

[grueinthebox]

Appreciate the assistance, but the problematic portion is actually the transformer/fuse portion - the large part in the bottom of the cabinet, encased in metal, with the two very large caps.  The power board shown there I've actually replaced with the Arcadeshop switching power supply kit.   

[Brunnen-G]

I haven't seen the replacement kit though.  Does the plug from J6 plug into it, or is the original MCR supply used for J6?  If the MCR power supply is still plugged into J6 at all, read on.


Unplug J6 and see if it still blows fuses.  If it does, it's a shorted wire somewhere, since the circuit basically goes from the transformer output to the input of the MCR power board.

If it doesn't blow the fuse, reconnect J6 and disconnect J5.  If it blows a fuse, the problem is on the MCR power board, possibly shorted or leaking D101-104.  If the fuse blows with a bright flash, look for a short.

If the fuse doesn't blow with J5 disconnected, then the problem is likely U1 or U2 on the audio amp board.  You might want to check the speakers themselves and make sure they aren't shorted if the amp or amps are blown.

You could also get a small stereo amp, even one in a kit form that MCM Electronics sells, and power it from the +12v from the switching power supply.  That way you'd bypass the circuit completely.

[grueinthebox]

You could also get a small stereo amp, even one in a kit form that MCM Electronics sells, and power it from the +12v from the switching power supply.  That way you'd bypass the circuit completely.

That's the kind of thing I was looking for...  It had never occured to me (missing the forest for the trees?) that the power in question was simply for the audio amp, and that I could just bypass that amp completely.  I'll head by Fry's and see if they have any kind of cheap 12v powered amp kits.

Thanks for the tip!

[Raven..]

Or just rip apart a cheap pair of powered computer speakers.  That's how I get my audio amps for MAME setups.

[grueinthebox]

Or just rip apart a cheap pair of powered computer speakers.  That's how I get my audio amps for MAME setups.

That's one of the things I'll be looking at when I hit Fry's...  I acutally tested it with a spare hacked up PC speaker amp I had laying around from my MAME(tm) compatible project, but they run off 9v AC - need to find a set that runs off 12v DC.

[Raven..]

they run off 9v AC - need to find a set that runs off 12v DC.

Oh yeah, duh!  I wasn't even thinking about it being 12v DC. 

[grueinthebox]

Actually I ended up using the amp board that I'd tested yesterday after all.  Picked up some molex headers and did some soldering so I didn't have to cut into the cabinet's wiring harness.  Scraficed an old extention cord and spliced it into the marquee light power lines with some vampire taps.  That gave me a switched power outlet I could plug the power adapter from the PC speakers into.  Works great!  Now all I need to do is pick up some of those nylon allen wrenches so I can adjust the width (height) of the screen, do some degaussing, and the machine should be functionally mahvelous and ready to start on the cosmetic stuffs.

Thanks for the advice, guys.

[Raven..]

Congratulations!  That's great man.