Damn Generators!!

[lowell]

Played an outdoor gig with a generator, supplied by the buyer, and within 5 minutes our PA power amp fuse blew. It's a Stewart PA-1200. Replaced the fuse and boom blew again. SOB. Anyone had this happen before? I will be demanding power conditioners for all future outdoor gigs.

Anyone have a schematic or know where to find one? Also, being a SMPS amp where would one look for faults from my failure description? I DO smell that "fried" smell internally.

[Steve Conner]

I would tell you where to look, but it might not make sense if you don't know what the parts of a SMPS are called. Basically check the switching transistors, these can usually be found on a heat sink nearer the AC input end of the power supply. There may be several on different heatsinks. If you're lucky, you'll find a shorted one.

Same goes for components in the AC line filter. Look near the AC input terminals, maybe there is a MOV or capacitor that has shorted.

SMPS repair is not for the faint-hearted so maybe call Stewart and see if they would exchange the power supply board.

I can think of plenty of cases where the power from a generator might be "bad" in ways that a power conditioner would do nothing to help. For example I once saw one with a malfunctioning governor causing it to put out the right voltage, but the wrong frequency, nearer 25Hz than 50, and this burnt out some motors it was driving.

If you get to choose the generator, the Honda Inverter ones are awesome. The generator output is converted to DC and then back to AC with regulated voltage and frequency.

[Enzo]

Son't just assume it is the SMPS, a blown output channel in the amp can also blow fuses, as well as smell burnt.

[Mick Bailey]

This year I've had 6 pieces of equipment that have been connected to generators - all with either burnt out mains transformers or failed PSU components. Whilst it's easy to blame the generator, stuff fails anyhow and some amps don't have much tolerance for surges or spikes - the damage is done before the fuse has time to blow. If a generator has a shared load with the band (such as a catering van) and isn't correctly sized, the governor in the generator sometimes can't maintain a constant voltage if it suddenly comes off load, causing an over-voltage condition affecting the remaining equipment. I doubt whether the output of hired-in generators for functions ever gets checked - if it lights a bulb it's probably deemed ok.

With some of the cheap inverters I've seen, output can be at best roughly sinusoidal - sometimes a rounded-off triangle where filtering has removed the crests of the waveform. With others there can be a high level of harmonic content when scoped. Honda has always been ahead of the pack with generators and when you look inside at the quality of engineering and thoughtful design that's why.

Good luck with the SMPS troubleshooting - they're a closed-loop system and you have to fix everything in one go, otherwise it will most likely blow again. There are a few good references on the web. Understanding the principle is the first stage - random parts replacement sometimes can strike lucky, but most often not. Most SMPS won't start with a shorted load, and many won't start with the load disconnected.

After failed mains input components, then rectifier shorts are a common failure. Check the bridge made up of quite hefty diodes that rectifies the incoming mains. Then check the main cap (or caps) for shorts. When either of these components fail there's a good chance that everything else is OK. Also, if you check the switching transistors and they're shorted the associated PWM controller IC can also be blown. If it is, some need to be soldered back onto the board even though it's tempting to install a socket. Failed rectifiers or caps on the secondary side can also cause problems.

Whilst a block diagram of a SMPS looks simple, in practice there may be custom semiconductors, obscure SMD devices and a whole load of other stuff that complicates finding a fault. Don't forget that the primary side is at mains potential. To set up for SMPS repair the first thing to think about is an isolation transformer. Then a variac, test loads, scope etc.