SC500 Master Hum

Founded in 1973, Soundcraft first made its mark with the launch of the Series 1, the first mixing console in a flightcase.

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SC500 Master Hum

Postby a.david.mackinnon » Fri Nov 04, 2016 8:11 pm

Hi Folks
I've just bought an amazingly clean 24 channel 500 series board. With the exception of the print being worn off of the channel on buttons it looks new and there's not a scratchy pot or fader to be found anywhere. Amazing!
There is however a hum in the master section that I can't chase down. It's in the CR, studio and headphone feeds. When i switch the master input to any of the 2 track inputs it goes away. Switch back to mix and there it is. It's present with all of the input channels switched off and out of the mix bus. Same goes for groups and returns. They don't add or subtract from the hum.
I just did a full recap on the power supply hoping that would clear it up but the recap has no effect.

Any ideas where I should look next? I've heard about people improving on the console grounding scheme but I don't think this is just normal ground hum. It's much louder than I'd expect from that.

Any advice would be greatly appreciated.
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Re: SC500 Master Hum

Postby Jim Williams » Sat Nov 05, 2016 10:21 am

Check the 1000 uf caps used to decouple the headphone/CR section. If they are dried up you could get hum. A scope will help find the place where it comes up. 2200 uf/35V Panasonic FR caps would be a good replacement.
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Re: SC500 Master Hum

Postby a.david.mackinnon » Sat Nov 05, 2016 12:46 pm

Thanks Jim. Will do.
Would those caps be upstream of the mix, 2tr a, 2 trc b, & 2trc c buttons? The 2 track inputs are fine, no hum. The hum is only when the mix input is assigned in the master section.
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Re: SC500 Master Hum

Postby Jim Williams » Sun Nov 06, 2016 9:50 am

Yep, they decouple the monitor path.
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Re: SC500 Master Hum

Postby a.david.mackinnon » Sun Nov 06, 2016 11:10 am

Thanks a million Jim. I'm picking up a parts console tomorrow and will be sending the spare master off to you for upgrades.
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Re: SC500 Master Hum

Postby a.david.mackinnon » Tue Nov 08, 2016 9:22 am

Feel like I'm going blind (or crazy) but I can't fine the 1000uf decoupling caps. that said I'm a bit rusty with schematics. Are you talking about C33 & 133? They're 2u2/50.
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Re: SC500 Master Hum

Postby a.david.mackinnon » Tue Nov 08, 2016 9:27 am

500-600 master.jpg
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Re: SC500 Master Hum

Postby Jim Williams » Tue Nov 08, 2016 11:59 am

Sorry, got my masters mixed up. C56 and C57 are the caps, 47 uf. I would replace those with 470 uf/25V Panasonic FR series, they fit.
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Re: SC500 Master Hum

Postby a.david.mackinnon » Thu Nov 10, 2016 8:48 am

Thanks Jim. What about C54-55 in that same section?
Also, what's the best way to get in touch with you?
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