My new mixer *almost* works perfectly!

[chriskorff]

Hello people!

I've just picked up a rather fantastic Soundcraft 600 series 24-channel desk. I've whipped out a couple of the modules, and they all look in fantastic shape - it even looks like the whole lot might have been re-capped recently, which is nice! Also, the VU meters work perfectly, right down to the bulbs!

Only problem is... it's buzzing. I've tried the control room outputs, the headphone outs and the studio outputs, and they all suffer from the same low-frequency hum. It's not a grounding thing (too high a frequency - I'm guessing around 800Hz), and it also happened to the person I bought it off in his house, so that rules that out. I'm thinking it might be something awry with the master module - the noise actually cancels when the master fader is down at around -50dB, but gets louder either above or below that. Also, using the auxes seems to modulate the noise, and the aux returns are also in the master module...

Now, I've used big desks many times before, but I've never actually owned one - what do you reckon should be the first thing I try? I haven't done anything to it at all (I just set it up this afternoon!) but I'm hoping a bit of compressed air and maybe some contact cleaner might jog the thing into working properly!

Any ideas?

Cheers!

Chris

[garyb]

bad fader? bad caps? you'll either need a tech, or you'll have to track it down bit by bit. i'd look for bad solder on the master module first. congratulations! you just got a boatload of great mic pres....

[netguyjoel]

Not to mention the wonderful EQ Soundcraft is known for... congrats....good luck tracking down the hum. I would exercise ALL faders & pots a lot...you may find one that is tempermental enough to cause it to hum more or less...for what that's worth..

[chriskorff]

Cheers guys!

I'm dead chuffed with it actually - one of my workmates bought it off ebay in perfect condition (ie. working cleanly), with PSU, for £250!!! He got a demo of the guy that sold it to him, showing it working fine, then when he set it up at home it was chucking out this buzzing sound... Seeing as my mate can't be arsed to fix it though, he decided to sell it to me for what he bought it for - not bad, considering it also included an EDAC loom!!!

I'm just having a play now, trying various different inputs/outputs - my nuclear option is basically to take it apart and put it back together again - i figure it must be something fairly simple to resolve, and there's even a tech nearby who can service these things properly. All the caps look clean and fresh though, nothing leaking there, so I reckon it's something pretty simple like a dry solder or something.

Cheers!

Chris

[chriskorff]

That's odd... I just noticed that the Solo light on the master module stays on permanently, even though nothing is solo'd... Also, the LED ladder meters are all hopping around fairly crazily!

If anyone has the faintest idea how to sort this out, please let me know.

Cheers!

Chris

[garyb]

i'd look into the cold solder thing. there could be a bad switch or op amp, but the first thing is to retouch all the solder on the master module. disconnect all channels but one, so that if there is a channel card that is damaged, you can isolate it. you also might get some edge connectors and make an extender card, so that you can trace the signal with the card connected to the bus and ps....

someone who KNOWS what he is doing could probably track down the issue quickly. even if you paid a couple hundred dollars, it would be money well spent.

[chriskorff]

Hi Gary,

I agree 100% - I'm good at general fault-finding, but not very confident in getting my mits dirty with electronics. My preliminary tests so far point to the master module; I've checked the individual channels quite thoroughly and they all seem to be working fine (hum/buzz notwithstanding). Apparently there's some dude in the Cambridgeshire (UK) area that knows how to sort these things out, and he even does call-outs, so he'll be my first port of call.

I've still got plenty of testing to do, mind, but then in a slightly perverse way, I kind of enjoy it! There's definitely a knack to checking every last link in the chain to see and rule out what could possibly be causing problems, but since I'm not au fait with the guts of these old desks, I think I'll leave it to someone who is.

Cheers all for the help!

Chris

[garyb]

you are CERTAINLY doing the right thing! it's good for you to be an engineer and have some idea have the thing works. isn't that a beuatifully made desk? it really shows how crappy things are constructed today. they meant for that console to be first quality for the rest of the owner's life or longer....

i have a model 1600 modified for 24track with a tt patchbay. i don't use it for a console anymore, but i surely use the mic pres!

[chriskorff]

Oh, while I remember, I've got a quick question for anyone who knows these desks well:

There are 16 ladder LED meters, and two VU meters. The VU meters are doing the mix outs and PFL, the first eight LED meters are doing the busses... but does anyone know what the LED meters from 9 - 16 are metering!?! As I said earlier, they're going a bit haywire at the moment (although they seem to have chilled out a bit now - I gave the desk a clonk on one of the end-cheeks and they're just sitting at 2 LEDs now, instead of spazzing around half-way!)

Cheers!

Chris

[garyb]

maybe the desk was connected to a 16 track machine....
the fact that you can hit it and things work better says there's definitely some cold or cracked solder issues, probably from moving and storage....

btw-don't forget to check the solder and caps in the power supply!

[chriskorff]

Hi again,

Cross-posting, I think!

Yep, I can't help but admire the thing. Christ, I've seen *furniture* that's cost several times that, and it doesn't look half as nice! I think the end-cheeks are made of ash! It's just got a fantastic presence to it, and an amazingly reassuring sturdiness to it. It's a blatant keeper, and you're right - they really don't make 'em like that any more, sadly.

Did I mention it's got the built-in Bantam patchbay? That should come in handy for fault-checking, since you can patch in/out of the desk almost anywhere in the path - unfortunately I don't have any bantam/jack leads handy, otherwise I really could test my master-module theory out by monitoring the direct outs or something.

What's the 1600, by the way - a larger version from around the same time?
Cheers!

Chris

* The PSU is a definite culprit there, I think! I'll pop the lid off now...

[astroman]

That's odd... I just noticed that the Solo light on the master module stays on permanently, even though nothing is solo'd... Also, the LED ladder meters are all hopping around fairly crazily! ...

together with the low freq hum you mention this may point to some short circuit in the control path, not the signal path. A failed cap or diode might be the source.
I've no idea about the internals of your desk, tho.
Just to give you an anchor where to start (in case you decide for some action yourself) and where a multimeter can do the job...

cheers and good luck, Tom

ps: as you mention the PSU an exxagerated voltage (failed regulator) is also suspect

[chriskorff]

Hello Tom!

Thanks for the input mate - I'll pass it on to the competent person who I'm going to pay to fix the bleedin' thing! I'm still very much on the learning curve as far as the 'inside stuff' is concerned, but make no mistake - I'll be picking the brains of whoever fixes it for me! I've got a 'rough/vague/general' idea of how diodes and caps work (diodes have two 'chambers' of semiconductors in 'em and can only conduct AC electricity in one direction or something, right??) but while I'm still learning, I think I'll leave it to those that know.

Thanks once again for all your help guys - and have a good one!

Chris

[garyb]

yes, the 1600 was the 16bus pro recording console. mine has been fitted for 24 track recording(some auxilliary channels added and was mated to a 24 track 2 inch Scully machine that i sold a few years ago(didn't need it with Scope!). yes, it's about the same age as yours...

[Neutron]

I would look at the capacitors in the power supply if everything is humming, if one has worn out (and big caps wear out faster than small ones) if you can get hold of a scope, have a look at the power supply rails, and if you cant you can set a digital multimeter to "ac" and look at what AC is escaping the filtering.

[the19thbear]

sorry for stealing the thread for a sec.
neutron... could you explain about the ac escaping?? i know how to use a scope.. but i dont know much about electronic i'm not reparing a console... but i want to learn about the trick you mentioned

thanks!

[garyb]

the AC isn't escaping. what he said was that the AC may not be getting filtered because of the caps being old and not working.

[Neutron]

a non switching power supply is usually just a transformer, a rectifier, sometimes a voltage regulator and capacitors to smooth out the AC pulses, because the capacitors have to be large values they are always electrolytic ones. those type of capacitors eventually become weaker and weaker, usually they dont last more than 10 or 15 years before they start to lose their value,
they start to lose the amount of filtering and sometimes the internal resistance breaks down. they are just foil wrapped around with treated material, sometimes even paper. chemical treating breaks down and sometimes micro "threads" of the chemical used for treating causes low current short circuits.

around year 1998-2000 some Taiwanese and Chinese capacitor manufacturers used a formula for the capacitors which was deliberately leaked to an industrial spy by a Japanese manufacturer, there was a plague of bad capacitors on motherboards and other electronics.
http://en.wikipedia.org/wiki/Capacitor_plague

some of our beloved scope boards will soon be needing new capacitors for sure.

some power amplifiers have really big value caps that will still work for ages, but you will start to get less and less headroom, since we usually only use the first watt or 2 of power its not really a big concern, but re-capping an older amplifiers (or mixing board1) power supply with modern caps almost always makes it sound better when its turned up a bit.

[chriskorff]

FUCKING POW!!!

Whipped out the master module this morning, looked at it a bit, decided I didn't know what most of the little bits inside it were, put it back in, and powered the mixer up, just for a larf like...

Yep, you've guessed it! It works, there's no buzzing, the Solo light isn't permanently on (and works when something *is* solo'd)!!!

There are a couple of scratchy faders on the busses (though these get less scratchy after a bit of a wobble), and the meter for Bus 1 is much lower than the rest (although the bus itself passes audio at the same level as the others)... so all in all, I'm fucking chuffed!

It obviously needs a bit of TLC, as there's a bit of audible scratchiness when the master fader gets close to unity, but as I suspected, I think all it needs is a bit of compressed air and some contact cleaner. Speaking of which, can anyone recommend something to clean the pots and faders? I've heard there are some types of cleaner that actually corrode stuff, which is clearly the last thing I want.

Cheers!

Chris

[garyb]

Deoxit always works well...sounds like the card just needed to be reseated.