Soundcraft Folio SX mixer problems - Replaced PSU as suggested... still there.

[TheGuitarist]

So i just finished making a new PSU for the mixer, which works great. Still a grounding hum but there is no ground going back to the houses earth. So i guess every time i track i need to keep a hand on the mixer.
I'm getting a hiss from the preamps, but i think that might just be noisy preamps, because it only happens when there is no microphone plugged in. Makes it like tape hiss  ;D. But there is also a... well farty sound i guess is the best way to describe it, its really low and only obvious when amplified  or when 8 channels are all open at once. I'm starting to think it could just be cheap pres but oddly enough it only happens when there are mics plugged in. So i either get hiss, or a farty sound.




Original Post:

Just wondering if you guys could help me troubleshoot some problems i'm having with my mixer. First problem is a low hum on all channels, i think its probably a grounding issue but i can't work out where from, its on the same power circuit as all my other gear and it happens on other circuits too. I am however running it through a stepdown transformer. A replacement 240v one for me costs around $180 so i decided to go the cheap route. Could this be the problem?

Also if i leave my mixer on for a few hours, the LED VU meters start metering when there is nothing even plugged in, it hovers around -10 db but sort of moves a bit. I'm guessing this would be a psu problem too. Would i be best off ordering a replacement? or even just making my own. Either way how should i go about troubleshooting this.


On a side note, i also have a sub thats also an amp... the amp works fine, none of the connections are burnt or anything but the fans don't run at all. Or the blue LED indicating the unit is on. Any tips on this also?

 

[helterbelter]

Well, about the sub, it could be that the fans don't run until it needs to cool the amp. Many amps have that feature. The Led... well, maybe the manual will say something about that. What type sub do you have ?

The folio.
Well, you mention a stepdown transformer. That can be the problem. Is it powerfull enough ? On the other hand, faulty PSU's can cause hum as well, but let's take one step at the time. Check the power requirements.

180 bucks for a replacement PSU.. well, how much power is there needed ? Which voltages ?

 

[TheGuitarist]

Thats australian dollars, but still its 150 US. Thats what the supplier told me. I'll try contacting them again, but this desk is well discontinued now.

Its 18v, thats why i refused to pay that amount, its ridiculous! 180 dollars for an 18v psu! Its a pretty chunky thing too, but its got a different connector on it. I could splice that easily enough but i'm not 100% on what pin is which.

Its a B-52 Matrix 2000 sub, and nah they used to run all the time, then they just stopped one time, don't know what happened. Must be a burnt out component somewhere.

 

[helterbelter]

18 volts... How many amperes ? (or VA ?).

The sub. Okay, I suggest to open it up and check if there's juice on the pins of the fan. Maybe the fan itself is broken, maybe the chip that regulates the fan or the temperature sensor. But, one step at the time. Check if there's a voltage running on the pins of the connector of the fan. (be careful, don't short the pins).

 

[TheGuitarist]

I'm not at home at the moment, didn't expect to get help so fast! (thanks!) but if memory serves its no more than 1.5v. Its not a power hog by any means... actually the mixer has been working fine on a 500va stepdown transformer but it could be 50. Actually looking at some of the pricings it looks like it might be 50 because the 500va ones go for about 130. and i payed 30.

 

[TheGuitarist]

Ok here are the details on the PSU and stepdown tranny.

stepdown, 50w

psu input 115vac~60hz 300mA

output 18.3v-0-18.3v~600mA

 

[helterbelter]

Hmmm, that should do it. But the hum, is it a 50 Hz hum ? (assuming you have 50 Hz mains). Then it could be a faulty cap or just a simple groundloop.

(I have searched for a manual, but couldn't find a schematic or info about the PSU (not that I expected to find that anyway).)

Before I mention other possibilities, you simply have to do a few things.
- Disconnect all equipment that's connected to the Folio.
- Connect 1 soundsource (Mic, cd player, whatever).
- Plug in a headphone.
Check if there's a hum /noise.
In case there's a hum, then there could be something wrong with the PSU or the Folio itself.

In case there's no hum, check every input with the soundsource. Still nu hum ? Then the PSU and Folio should be okay.
-Now, connect all the equipment again, one at the time, and check for hum. Begin with the stuff connected to the inputs. No hum ? connect the auxiliaries. No Hum ? and as last, connect the outputs. 

If the hum doesn't appear until you have connected the outputs, then it's very likely that you have a groundloop.

When it's a groundloop, you should isolate the earth from the outputs (assuming it has XLR outputs). By that I mean that you have to cut the screen in the XLR on one side of both output cables (preferrably at the recepting side of the cable). In case the folio has unbalanced out only, then you can't just simply cut the screens, but there are other ways to lift the ground as well.

But I'm also just guessing, I'm no electronics expert.

Good luck,
Paul

 

[Harpo]

Ok here are the details on the PSU and stepdown tranny.

stepdown, 50w

psu input 115vac~60hz 300mA

output 18.3v-0-18.3v~600mA

Just curious, your 240VAC wall outlet feeds the stepdown transformer, giving 36.6VAC across its secondaries.
This 36.6VAC now feeds your 115VAC folio psu ? If so, this will hum.

 

[TheGuitarist]

Whats happening is the mains goes into the step down transformer, which hits 240 down to 110, then i plug the psu (which is american) into the stepdown transformer. Its odd though, the hum is coming in on every channel, not the outputs, when i bring up all the faders its there and as i bring them down one by one it reduces.

 

[pucho812]

hey lets take this one at a time. are we worried more about the sub or the mixer/console.


You mentioned that when you pull the faders up, you hum. It's possible you have a ground loop but I am suspecting it is related to the PSU.... I would first make sure your stepdown transformer is correct... then take it from there.  

Is it on every channel regardless of signal comming into a channel or not? Since hum appears on every channel, I would make an educated guess that there is something wrong with the PSU it might even be in how you have it set up. take it one piece at a time. and work your way forward. 

 

[TheGuitarist]

The mixer, the sub is annoying but still works fine.


So is it possible to have a ground loop on just the inputs?

If it helps the step down transformer is only 2 pins, not 3 so i'm guessing its missing a ground. So is the PSU

 

[helterbelter]

Well, you did mention the PSU had 18.3/0/18.3 AC.... Is there a pin broken ?

Anyhow, the PSU supplies an AC output. This is good, because then you could use a simple transformer with dual 18v secondaries instead of the expensive Soundcraft 240v PSU.

 

[TheGuitarist]

Well thats the thing, the proprietary connector has 3 pins. That cable goes into the PSU box, and then comes out in an american plug with only 2 pins, not the third grounding one. Its not that one is broken, the plug just simply does not have one.

Its looking like the best course of action is just to make my own one and then just use the existing cable. If this is the way to go, any tips on working out what pin does what? I'd prefer not to pull the entire mixer apart if i can avoid it. Could i test with just a multimeter and seeing what ratings i get?

And sorry but what do you mean by dual 18v secondaries, i'm getting there electronic knowledge wise, but PSU's arent where i'm up to yet.

 

[pucho812]

the duel 18V would be for +/-18V if I am understanding your description correctly

oh and if you can take a photo or two or more... that would help  

 

[TheGuitarist]

What in particular do you need photos of and i'll get them asap.

 

[pucho812]

What in particular do you need photos of and i'll get them asap.

everything. I suspect there is a fault in your PSU which is causing your hum as well as your console meter issues. was it ever recapped?

 

[TheGuitarist]

No it wasn't, I'll get the pictures tonight though.

Is recapping something i should look at doing? If i do that is there anything else i should look at doing, its looking like i'll need to build or buy a new PSU so i might as well upgrade the whole thing while i'm at it. I can't find a schematic for the mixer at all, so i'm not sure how plausable recapping will be. The mixer isn't worth sending to get recapped, i love it, but its not worth spending a couple hundred on.

 

[TheGuitarist]

Ok hopefully these pictures are whats needed.

http://img695.imageshack.us/img695/6007/dsc00629.jpg
http://img35.imageshack.us/img35/3262/dsc00630bk.jpg
http://img94.imageshack.us/img94/204/dsc00634j.jpg
http://img11.imageshack.us/img11/6937/dsc00635cm.jpg
http://img691.imageshack.us/img691/9634/dsc00638a.jpg

 

[moamps]

Hi,
did you check the power supply connector?
I replaced these on two Soundcraft mixers (Folio and Fx)
with metal DIN connectors after experienced similar problems (the middle pin was lousy).
Regards,
Milan

 

[TheGuitarist]

Nah i didn't, i didn't think you could get replacement parts. Unless you mean change both ends. In either case, do you have any suggestions?