Soundcraft DeltaDLX issue.

[parcell]

Hi all,
Long time reader, first time poster.

As the title suggests, I have an issue with a Delta that is a little perplexing..
Was recording the other day when the mixer suddenly shut off. I noticed the +/- 17 LEDs on the supply were very dim/close to off (48v was fine) so immediately shut off the supply.
Got the supply on the bench and was able to have the same behavior without it plugged into the mixer - surely must be the supply. Also noticed a respectable scorch mark on the transformer PCB.
Managed to find another CPS150. Got it plugged into the mixer, still the same behavior.. Narrowed it down to the master channel bringing down the power supply.
Now my question is, what could be bringing down the supply? My initial thought was the sudden shutdown wreaked some havoc but most of the components here aren't too sensitive..

Any ideas would be greatly appreciated. I do like this mixer and would like to get it back up and running.

 

[NoisyIndividual]

I'm pretty sure these will have the usual 10 ohm resistors on the 17v rails at the point near the main connector.
These should normally blow if you have a short on the main outs. If there are 2 separate L R boards there will be a pair per board.
A partial short might not blow the resistors but may well colour them, and they will certainly feel and probably smell hot.
Have a look for those resistors and see what you can see.

They may be standing off the PCB which would make them easier to find.

 

[parcell]

I'm pretty sure these will have the usual 10 ohm resistors on the 17v rails at the point near the main connector.
These should normally blow if you have a short on the main outs. If there are 2 separate L R boards there will be a pair per board.
A partial short might not blow the resistors but may well colour them, and they will certainly feel and probably smell hot.
Have a look for those resistors and see what you can see.

They may be standing off the PCB which would make them easier to find.

Had problems with the 10Ω resistors on individual channels so that was one of the first things I checked out on the master. Nothing visual unfortunately but will get them under a multimeter tomorrow.

 

[parcell]

Had problems with the 10Ω resistors on individual channels so that was one of the first things I checked out on the master. Nothing visual unfortunately but will get them under a multimeter tomorrow.

Can confirm that the handful of 10ohm resistors are all reading 10ohm.

 

[NoisyIndividual]

Well this is a bit confusing.
Let me get this right:
Original PSU connected to desk, leds go dim.
Original PSU disconnected from desk, leds go dim.

So there is a problem with the PSU.


Different PSU connected to desk, leds go dim, but not when Master channels disconnected.

Might be that its maximum capacity is compromised and is also faulty - do the leds go dim when ONLY the master section is connected?


If the master section is indeed faulty, then this excessive fault current should be going through at least a pair of these 10 ohm resistors, which should be getting warm at least.
Measuring the voltage across them will indicate how much current is actually going through them.
If this is feasible to do.


Having two or more faults at the same time is not unheard of, but can be mightily confusing.



Ah, hang on, also I think the CPS supplies have some sort of crowbar protection on the audio rails at least, which, when operated, may dim the LEDs.
So maybe if there is some short the PSU's just shut themselves down and won't heat up the 10 ohm resistors at all.
Though that doesn't rule out the first PSU being faulty just sitting there with dim leds.


If that is the case you could try and measure the resistance between the two 10R resistors, and also between each one and ground to see if you get a low ohmage reading downstream of them. (Compare this with a channel). A faulty op-amp can short between power rails but not between power rails and ground, so measure all 3 permutations.