OKI B6200 Fault

Huh, I can't find that part number anywhere... it's almost certainly an MOV though. Used for transient suppression from the mains. Problem is when they explode like that, they've usually failed into a short briefly before exploding into an open. This short is what blows your fuse and unfortunately, a lot of times, other components.

I've seen this happen a lot with MOVs that were underrated. Some designers like to put the smallest possible value and call it good. Problem is, the thing gets hit with spikes over and over, and it's a cumulative degradation of the device since it's being overloaded. Eventually it gives in and this is the result. Now, the markings make me believe it's a 470V MOV... but, I don't know if that's an AC or a DC rating, if this is a 120/240Vac input, 277Vac input, etc.?

In theory, you can remove the bad MOV(s) (as you have done) replace the fuse, and see what happens. I have seen some very random components fail due to an MOV failing, though, so be wary. For instance, a forward converter SMPS had a unit w/ exploded MOVs come back. No Vcc, but the Vcc generation circuitry all looked peachy. Turns out a gate protection zener of the high side FET was shorted, and kept the unit from starting since only the low side was trying to switch, but never got the ball rolling to generate Vcc, and just shut the whole thing down. The symptoms were like you're reporting. I never would've found that fault if I hadn't decided to inject Vcc from an external supply and see what the deal was. Still can't explain how an MOV blowing caused that zener to short... but anyhow, you get my drift. You'd probably need to do further troubleshooting to get it running again.

You can run it without the MOV, with the risk of transients hurting something over time. But I would advise replacing it, if we can figure out what it was rated for. Perhaps go a little higher on the voltage/energy ratings. I'll keep looking around.

Edit: VZ05E471KBS
That's a candidate for that part... is it about 5mm thick? If so, it's rated for 300Vac, so I wouldn't expect too many problems replacing it (perhaps with something that has a higher energy rating, like the VZ07E471KBS) That would be a little thicker, but looks like you have room. If you're running off of a 277Vac mains, you may want to go up to the next higher voltage rating, to be safe. Others here may be able to step in and tell you for sure.

Again, if you've clipped it out and replaced the fuse but it still doesn't work... you have other problems.

 

See edit above, added info.

Ah, 240Vac mains... I would expect a 300Vac rating to be "ok" but honestly I would go up to something like the VZ10E511KBS, which gives you a 320Vac rating with about four times the energy capacity for transient absorbtion. This will be a bit bigger than what was used, though I don't see room being an issue.

Anyone else have any input?

If you can't find that part anywhere, any good MOV should work: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=P7211-ND

That's a 67 joule, 320Vac rated part... double the thickness. The varistor voltage (at which it clamps) goes up a bit, so it may be prudent to find the specs of the switching FET(s) / bulk caps, and make sure it's still got good headroom. (Original was 470V, the one I linked is 510V.) The FETs should be 600V rated, as a guess... Raising the clamping voltage should do away with these exploding MOVs due to swells and the like, and hopefully the designers didn't spec everything with "just enough" headroom to only survive under 'typical' conditions.

And once again, I highly doubt that just replacing that part will bring it back to life.

 

Good to hear it's running again, almost surprising!

I looked through their stock, but it seems all they have are MOVs with really high clamping voltages (in the right AC rating range anyway.) 775V clamping is the lowest I can find there, which may be a bit excessive, I'm not sure if it would do you any good in circuit if other components aren't capable of dealing with that kind of surge voltage.

It would be better than nothing though.