A friend brought one of these very common PA powered loudspeakers over to the workshop. An internal T4A mains fuse had blown and when I replaced it and re-powered there was a very loud 50hz hum for a few seconds and the fuse in the power cord blew. I assumed it was the audio path that was making the noise and I assumed that one one the probably faults was a short/open de-coupling capacitor that was letting 50hz through to the audio stage.
So - I made sure both fuses were good and with Dave Jones's words "thou shall always check the rails" in my ears I disconnected the output of the large toroid from the two rectifiers/smoothing caps and tried again. I figured I'd make sure the o/p of the transformer was good (there is some de-coupling on the primary). However I got the same loud 50hz for a couple of seconds and one of the fuses failed. The toroid was also noticeably warmer than it had been! So - checking the DC resistance of the primary side showed it to be less than six ohms (so it was pulling an inrush of at least 40A!).
A quick flail around the web showed that the transformer is a known weak point of this design.
The real bummer is that the transformer isn't stocked by Mackie (or indeed anyone else) and the folks at Save My Light only do a minimum order (ten pieces) from the Chinese factory that wind them when they have enough orders (and the chap there told me he sells an average of two a year).
So, do I just wait or pay him the thousand quid to have ten made?!
Time to keep my eye on eBay for a hopefully working second hand replacement?
So after lots of flailing around the web and finding a few reclaimed ones for more than £150 I came across AJAudio on eBay (Alex Mathew email@example.com was super-helpful) and he sold me one for £99.
For info - the failed toroid had around 5 ohms DC impedance across the primary whereas the replacement has 8.5 ohms.
It's been shaking the workshop with loud music all afternoon!