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Soundcraft Spirit M12 PSU issue.
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author Soundcraft Spirit M12 PSU issue.
MAXTHEDOG
Hi All, I’ve got one of the above mixers. It needs attention to the PSU. With reference to the photo 3 of the 4 resistors seem to be 3 ohms, the fourth well who knows. My question is why would there be 3 x3 ohm + X fourth in parallel (according to the PCB tracks). The right hand side comes from the -ve terminal of the electrolytic capacitor, the left side goes to the MOSFET leg as marked. The MOSFET is a P6NK60ZF.
I realise that it’s difficult but how would you go about trying to repair this. A replacement is around £110 from SC.
Regards
MtD.
JohnLRice
I think it would be worth the money to just buy the replacement? Unless you know for sure what caused it to fry in the first place and have a service manual with schematic so you can determine which components may or may not be working, you may end up spending a lot of time and money trying to fix it.

And if you do buy a replacement, you can use that to determine values as you try to repair the fried one if you really want to, then you'll either have a spare or could sell one of them?

Best of luck!
MAXTHEDOG
Hi John,
Thanks for your reply. I’m tending to agree due to not having a service manual (or enough knowledge!) to tackle this. I’m wanting to sell but the few M12’s on eBay are on at about £175 and are not exactly getting snapped up. At the mo’ I can’t bring myself to just chuck in the recycling.
Regards,
MtD.
Mungo
You can see a clear view of that board to pick out the values here:
http://studiorepair.com/gallery/Soundcraft/Spirit_M12/slides/Soundcraf t_Spirit_M12_PSU__STUDIOREPAIR_16120501_1612125962.html

Its a relatively simple power supply and won't be far off the reference implementation in the data sheet:
http://www.ti.com/product/UC3842

If you don't know whats broken and want to play around just replace the burnt resistors along with the chip and mostfet. Check all the diodes as they are easy to test. An hours work and $10 might just get you back and going without needing to understand the problem.
MAXTHEDOG
Hi Mungo,
Many, many thanks for that. I’ll do some more track tracing and draw it out.
Could I ask if you could see what the resistor values are?
I get 3x 1 ohm @ 5% ( Br, Bl, Gd, Gd) and 1x 4k6@ 0.1% (Y, B, R, Vio).
From looking at the traces underneath, it doesn’t seem to make any sense to have 3 one ohm and a 4k6 in parallel, or would it be 4R6 + 0.33R to get close to 5 ohms?
Your eyes on this would be greatly appreciated.
Regards,
MtD.
Mungo
MAXTHEDOG wrote:
Hi Mungo,
Many, many thanks for that. I’ll do some more track tracing and draw it out.
I think you need to do a little more tracing and drawing, the burnt out mystery resistor is not in parallel. More pictures of other boards:
https://www.fullcompass.com/prod/124517-soundcraft-rf4105a-power-suppl y-pcb-for-spirit-m8-and-m12
https://brashleraudio.wordpress.com/2012/08/12/power-supply-woes/

MAXTHEDOG wrote:
Could I ask if you could see what the resistor values are?
I get 3x 1 ohm @ 5% ( Br, Bl, Gd, Gd) and 1x 4k6@ 0.1% (Y, B, R, Vio).
From looking at the traces underneath, it doesn’t seem to make any sense to have 3 one ohm and a 4k6 in parallel, or would it be 4R6 + 0.33R to get close to 5 ohms?
Your eyes on this would be greatly appreciated.
Regards,
MtD.
Looks to be a 470R, from the data sheet thats Rcsf. They parallel the 1R resistors to increase the power handling, it wouldn't make sense to have 3x 1R in parallel then a 4R7 in series with that.
MAXTHEDOG
Thanks again, Mungo for all that. Mine did exactly the same as his - fuse blows, replace fuse, magic smoke.
Seems to be a weak point in that PSU there. Glad that you confirmed them resistors as the Rcsf from the data sheet, I did wonder.
More tracing and drawing for me.’
Regards,
MtD.
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