Thursday, December 22, 2016

Power supply toroid in Mackie SRM450 PA loudspeakers

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.

I found the schematic at Mackie-SRM450 actspk.zip (although this is for the rev.C of the board, but the PSU appears not to have changed).

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?

UPDATE 16/01/2017: 

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 sales@ajaudio.co.uk was super-helpful) and he sold me one for £99.
Job done.

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!

3 comments:

nealk said...

These guys made the transformers for Meridian when I worked for them (I believe they were known as St. Ives Windings back then) http://www.carnhill.co.uk/. They might be able to help with something similar if you can describe the requirements!?!

With warm regards,

Neal

Michael Gossen said...

It sounds like I have the same issue with a pair of MR5mkII that have been sitting on my shelf for almost a year...I wonder if it could also be the same transformer? I haven't really had time to get into them, but sounds exactly like what I was seeing.

Phil Crawley said...

Looks like I've found an alternative;

https://www.livingtechnology.co.uk/avstore/index.php?route=product/product&product_id=129&search=srm450

I'll be ordering in the next couple of days and will update the post accordingly.