Phantom Power issues maybe?

[JMackenzie]

there is a loud 50 cycle hum as well as hiss.

 

Oh oh oh yeah ;) that really does sound like a mains earthing issue, even Beheringer shouldn't exhibit noticable 50hz hum :o , hiss? yes by the bucket load at full gain :P :o

 

Stripping it down / moving it will have highly likely changed something in the earth department, especially if the fault is in the desk rather than the 13a supply like I suggested.

 

Although your previous sparkie has suggested a possible problem.

 

"Earth potential difference" is basically where there is a voltage difference between two earth points, say two 13a sockets at different locations.

 

How can you have a voltage difference to earth? Surely Earth should be Zero volts.

Confused :unsure:

 

A concurrent post has been automatically merged from this point on.

 

Hi again guys.

Problem is now back. This becoming a pain in the neck

 

Now, one thing that is for certain, now that I think about it, is a possible temperature effect. My School theatre is normally very cool, except when full of people, like assemblies, presentations, or shows. It can get very hot, and all windows get opened .

When I was last testing it out in there, I was the only person in the room all day, and it was really quite cool( we do get some coolish weather in Scotland some days :huh: ).

 

Moving it today into my office/workshop meant that it was in a VERY warm environment this morning. I really do suspect it is a temperature thing, but I dont know what components would be most likely to be affected. Maybe , like power amps, it has some sort of protection built in against overheating. I would doubt it, because it is not a powered mixer. I am going to try and force cool it somehow, or even fit a large heatsink onto the Power Supply (there is plenty of room to do that inside the case.)

If anyone can suggest which components are most likely to be affected by heat, feel free to speak out. I know that resistors will change value with heat, but I cant see capacitors doing that. I presume the switching transistors might be affected, but I would expect them to just fail and not recover.

I will keep you all posted.

[MarkPAman]

For the PSU/phantom bit of the problem, one possibility that fits all the symptoms – I think :unsure:

 

There is a partial short circuit in the phantom side of things, somewhere between the PSU and the XLR inputs on the desk. This causes the PSU to warm up until a thermal protection circuit kicks in and turns it off (I have no idea if it has one of these – it’s possible though). It then cools & comes back on. In a cold room this takes longer to happen, or does not happen at all. Have a close look at all the cables & PCB tracks from PSU to phantom switch & from there to the XLRs. You may find a whisker of copper that shouldn’t be there or a blob of solder that’s too big.

 

Only an idea & probably wrong!

[jamesperrett]

I know that diagnosis at a distance can be a dangerous thing - but to me this sound like a combination of a dry joint with maybe an ageing component in the power supply. I've often seen faulty switch mode power supplies show the symptom of trying to switch on every few seconds - only to shut down almost immediately (though I'm not enough of an expert on switch mode PSU's to tell you where to look to fix it).

 

Cheers

 

James.

[sleah]

How can you have a voltage difference to earth? Surely Earth should be Zero volts.

Confused

 

Actually that's the point!

 

Yes, earth should be 0v, referenced to both true earth and neutral. If the earth bond is either missing or isn't making good contact it can result in a voltage between electrical earth and true earth, causing all manor of bizzare problems, not to mention the safety aspect.

 

One symptom is being able to feel a slight tingle from a metal case that is supposedly earthed. You often feel a similar tingle on class 2 devices such as video recorders that have a metal case but no earth. It is down to a slight leakage from devices such as transformers.

 

Even if it isn't the cause of your problems, as you've mentioned past issues relating to earths, I suggest you get it checked by a sparkie ASAP if only for your own safety and piece of mind.

Even if it all checks out OK, at least it will eliminate earth faults as the cause

[JMackenzie]

Hi again.

A little bit of an update on this mixer problem of mine. Hopefully this whole story will be of use to someone else someday.

I decided to take the power supply out of the mixer, just to have a visual check over it.

I was able to make a few measurements and found some rather strange things going on.

With the earth strap removed from the casing of the mixer, I was getting a voltage of 107V AC between the case and earth.

Switching on the Phantom power did not change this measured voltage.

However, when checking the voltage on the XLR sockets, which was reading 46.5 DC, when I then switched my meter to AC, I was getting a cycling AC voltage appearing at the same pins. This seemed to be going from mains voltage (236V here,) to zero, at a frequency of 1-2HZ. My meter probably could not keep up with this rate of change, so it may have been faster. NOT GOOD. No AC voltage is present at the XLR sockets when the phantom power is off.

Now going to check all the earthing in the Theatre that this thing is used in, before connecting anything else into the audio circuits.

Still cant find anyone to look at this mixer for me. If anyone near Edinburgh or the Borders knows of a good repair technician , could they let me know.

[Paul TC]

switched my meter to AC, I was getting a cycling AC voltage appearing at the same pins. This seemed to be going from mains voltage (236V here,) to zero, at a frequency of 1-2HZ. My meter probably could not keep up with this rate of change, so it may have been faster. NOT GOOD. No AC voltage is present at the XLR sockets when the phantom power is off.

 

Any possibility of putting a scope on these points to see what is actually there in terms of frequency ? I.e is it 50Hz or something else ?

 

Also what is happening on the PSU rails on the mixer ?

 

 

H&S Note : Only connect the scope if you fully understand how to take measurements from 240V ac with the instrument.