Repairing a Spirit Folio SX rack mixer

I’ve written before about my interest in sound equipment and engineering, which mostly relates to work at my church. As part of this I do general maintenance on the equipment and where possible do repairs to keep it all working. Given the cost of pro-audio gear this is generally cost-effective and also it’s satisfying to keep stuff working rather than buying new.

We have a mobile rig and part of this is a fairly old Spirit Folio SX rack mounted mixer. The one we have at the moment looks like this:

Soundcraft Folio SX labricox images

It’s not quite the same as ours has rack ears but the principle is the same. One of the main problems with these is the power connector, which because it is a rack mount is on the underside of the desk. It is a small proprietary plastic connector which looks like this:

It’s small, flimsy and easily broken. There are three pins because the power supply is a little unusual, being 2 17V AC lines with a 0V in the middle. I’m not entirely sure why it needs this, possibly something to do with the phantom power (which is 48V) used to power stage boxes and certain types of microphone.

When I was given the desk to look at the power was flickering on and off and it was immediately obvious that the power plug was loose in the socket, wobbling around all over the place. There was a small catch on the top of it and when I tried to move it it broke off in my hand. So the first step was to replace the power connection.

A lot of other people out there seem to have had very similar problems and there are a lot of people on various forums complaining about the same thing. There are a number of ideas, including removing the connector altogether and wiring the power supply straight in as a captive lead. Other people have used various types of XLR connector. We have loads of these lying around, but I thought it was better to avoid using these in case someone accidentally plugged a microphone into the power supply!

I finally settled on a locking 3 pin microphone connector, as seen on aircraft and various amateur radio equipment (another interest for another post).

To make it a neat job I decided to bolt a suitable socket to the bottom of the desk in place of the existing one. The connectors themselves are easily available on ebay:

connectors

The socket needs a 15mm hole which is too big for regular drill bits (the largest I have is 13mm) so I needed a suitable hole saw. Once again there was a wide choice on ebay and I found this for the bargain price of £2.79:

Image result for 15mm hole saw

Great service as usual from ebay traders and they were delivered quickly. The hole saw is from a Chinese manufacturer but I was impressed with the quality of it.

The bottom of the desk is a steel panel held on by a load of screws but fortunately there is quite a lot of space between the panel and the circuit board underneath. There is plenty of space to fit the new socket and connect the wires up without it fouling the board underneath.

First step was to remove the old socket which simply pulled out, and cutting the wires. Then I offered up the socket roughly to where I wanted and made sure that there was space to connect it up. Drilling the hole with the new cutter was pretty easy – I drilled a pilot hole first, and then used the cutter with a block of wood underneath. It did a great job and cut straight through leaving a very clean hole.

The new socket bolts through from the bottom with the supplied nut and washer. Then it is a matter of soldering the wires on to the connector. As above the power supply has two 17V AC rails and a ground connection. I traced these out using a meter and wired the plug and socket to match. The only problem was that the cable on the plug end was a bit thin for the cable clamp  so I had to bulk it up a bit with some insulating tape.

 

I also found that if I turned the panel around I could cover up the hole than the old power socket was in, which finished the job off nicely. After reassembling it, the power is now rock solid and various other odd problems have resolved.

You could use pretty much any connector to do this but this one was good because it was cheap and the locking mechanism made it very unlikely to fall out. I’d definitely recommend this as a modification as the original connector is a poor design and I’m not surprised that others have had problems with it.

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