Fixed: sudden deafening hum - help!

[Mfresh]

My Earthshaker was just sitting in attract mode in the other room when it suddenly started emitting a very very loud hum which was audible from the other side of the house.

I came in and switched the game off, and of course the humming stopped. Switched it on and the very loud hum came back. Turning up the machine volume control does seem to make the hum get louder, but even on zero volume it is loud. I was fiddling with the machine's volume when there was a loud pop like a knocker, and two fuses on the power supply board, both 7A, went.

I have replaced the fuses and the noise is back, but as far as I can tell the machine seems to be working apart from the loud noise. I have the volume on very low and I think I can hear some normal game sound, but maybe not - hard to say with the loud noise.

Anyone know what the problem could be?

 

[Spandangler]

My WW did this after I moved it once.
Horrid noise. I just reseated everything and twiddled the volume pot a bit and it seems to be fine now!!!

 

[Mfresh]

Well it took a while but this one is now fixed. In case anyone has the same problem:

Spoke to Andy Netherwood who said that hum is caused by AC on the DC power supplies, and to test the power supply board DC test points for AC voltage.

Sure 'nuff, the -12V DC was showing 10V AC on it - enough to make the amp on the sound board hum. Probable suspect was the 1000MFD capacitor which is meant to smooth the -12V supply, so I replaced that. Made no difference though, but one of the two 7A fuses blew. And the capacitor exploded.

Mmmm. Upsteam of the capacitor there is nothing much but the bridge rectifier itself, which displayed some pretty strange behaviour during diode tests between the legs. Replaced that, replaced the exploded capacitor, and replaced the fuse, and... it's all working great.

Haven't quite got my head round why a bust bridge would blow a fuse and blow the capacitor. Can anyone enlighten me?

 

[Sven Normansson]

The bridge essentially converts AC to DC, the cap works on DC so putting AC thru it would blow it. The fuse blows because it's a short.

 

[Mfresh]

Makes sense. Thanks @Sven Normansson! I only have a rudimentary but expanding grasp of all this, so all help gratefully received.

Is there any point in replacing all the other caps (for the +12V and 5V supply) given that they are working but aged? Would it make the sound quality better for example? I've seen that you can get rebuild kits containing all the caps needed, but is that just because the caps will fail eventually so if you have the board out you might as well replace the lot?

 

[AlanJ]

I'd replace the caps, fairly cheap and also easy to do. (In fact a lot easier than say replacing a MOSFET transistor or a 20 pin IC).

 

[Sgt GrizZ]

Makes sense. Thanks @Sven Normansson! I only have a rudimentary but expanding grasp of all this, so all help gratefully received.

Is there any point in replacing all the other caps (for the +12V and 5V supply) given that they are working but aged? Would it make the sound quality better for example? I've seen that you can get rebuild kits containing all the caps needed, but is that just because the caps will fail eventually so if you have the board out you might as well replace the lot?

I had a similar horrible sound on a Flintstones. Ended up replacing a couple of caps on the soundboard to sort it.

 

[Mfresh]

I'd replace the caps, fairly cheap and also easy to do. (In fact a lot easier than say replacing a MOSFET transistor or a 20 pin IC).

Thanks for the advice. But my question is: why replace them? Just to avoid them failing in the near future, or would it actually make a difference to the machine now? (Like less hum from the speakers)

 

[AlanJ]

yes, if they are not working 100% then the DC supply will be "noisy"-er than it could be, that in turn causes hum