Allen & Heath RPS11 random fuse blow issue

[pecorporation]

Hi guys,

I have an RPS11 supply on the bench here and the customer says that it blows the mains fuse once out of 4 to 5 power on cycles. On the bench, I've not been able to get the fuse to blow at all. Currently, the customer is using another backup power supply for the same mixing console and hasn't had a problem since.

The unit specs a 5 amp time lag fuse but the customer used a fast-blo 5 amp. I've been using a 5 amp slo-blo for about 20 start-ups with no failures. Voltages are on-spec +/-16vdc and +48vdc.

The blown fuse the customer used was blackened (blew hard) on the inside of it. So first, I brought the unit up on the variac, successfully, and then had several successful cold starts. I inspected the casings of the power switch, AC inlet/fuse box and power transformer for compromised insulation which may cause a temporary arch shorting to ground but did not find any evidence of such.

Anyone come across anything like this before?

[sidneystreet]

...The unit specs a 5 amp time lag fuse but the customer used a fast-blo 5 amp. I've been using a 5 amp slo-blo for about 20 start-ups with no failures. Voltages are on-spec +/-16vdc and +48vdc...

The customer was using the wrong fuse. Problem solved, right?

[Enzo]

Put a fast blow in it, and see how many times it takes flipping the power switch to blow it.

That is the whole point of slow blow fuses - to survive the power surge when you turn the unit on. Bringing it up on a variac will allow a fast blow fuse to survive. The unit works fine with the CORRECT fuse, right?

As Sydney said, customer used wrong fuse.

And yes, I have encountered customers using wrong fuses before.

[pecorporation]

Working so far, 10 start-ups with a 5 amp fast-blo, no mixer load. It was just the way his 5 amp fuse blew. It was a black, splatted mess inside the 5 amp fast-blo fuse glass. I would think if it were the start up surge popping the fuse, it would've looked more like a burned out light bulb filament with a break in the middle of it.

[Enzo]

No, it will blacken because it burst open, it didn;t melt over 10 seconds.

Think about the AC waveform in your mains power. Starts at zero, rises to +170v, then back through zero to peak at -170v and back to zero. Repeat as long as needed.

When you close the power switch, at that instant, the waveform is at SOME point on that curve. If you flip the switch right at a zero crossing, there is no instant current, after all the voltage is at zero. But if you happen to flip the switch on a peak, you just introduced a 170v step waveform into your transformer. There will therefore be a LARGE instant current surge. That will vaporize the fuse in an instant. Black. And that is in addition to the initial charge up of empty filter caps.

That is why it works sometimes and blows fuses other times - the current surge is not consistent, it depends upon where in the cycle of mains power you connected.

[pecorporation]

Thanks again, Enzo!

[bigbassman]

RPS11 Fuse blowing.

I had the same problem; replaced the fuse and it was OK.........

A month after the fuse blew, a hum gradually became apparent in the main output. After 2-3 weeks of that, one day I smelled that smell; cooking components, then within an hour, the board lit up randomly, then all over, and no audio. I shut down, and am about to look at the PS to repair it. I just hope that's where the problem lies.

Bottom line, it seems that, as with some other console manufacturers, the power supplies for consoles drawing 200 to 300 watts or more, there are long term reliability issues. You'd think that for real pro audio situations, gear and their (expensive) power supplies should have a sufficiently large margin of over-design so we would not have to be worrying about the central part of our setup failing; a catastrophic event on several levels. To be fair, I'll have a better idea of that margin when I get into the PS. However, my unit's fuse blew during normal operation well after power-up; I suspect this power supply may have some unresolved design-related reliability issues that may go beyond random fuse blowing.

BTW, I could use a copy of the schematic or manual; The A & H site has the service manual for the console, but not the RPS11.

[bigbassman]

I had the same problem; replaced the fuse and it was OK.........

A month after the fuse blew, a hum gradually became apparent in the main output. After 2-3 weeks of that, one day I smelled that smell; cooking components, then within an hour, the board lit up randomly, then all over, and no audio. I shut down, and am about to look at the PS to repair it. I just hope that's where the problem lies.

Bottom line, it seems that, as with some other console manufacturers, the power supplies for consoles drawing 200 to 300 watts or more, there are long term reliability issues. You'd think that for real pro audio situations, gear and their (expensive) power supplies should have a sufficiently large margin of over-design so we would not have to be worrying about the central part of our setup failing; a catastrophic event on several levels. To be fair, I'll have a better idea of that margin when I get into the PS. However, my unit's fuse blew during normal operation well after power-up; I suspect this power supply may have some unresolved design-related reliability issues that may go beyond random fuse blowing.

BTW, I could use a copy of the schematic or manual; The A & H site has the service manual for the console, but not the RPS11.

OK. I took the power supply apart and found that the Base lead to the negative supply output transistor had come off the pin on the circuit board, due to a cold solder joint. Both output transistors (MJ11016) were gone, and both uA723 regulator IC's as well.