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Old 28th March 2011, 12:42 PM   #1
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Default +/- 0.3V bumps on my ground

I hope this is the right forum - apologies if not.

I have a Behringer MX1602 Mixer which has +/- 0.3V "bumps" on the earth line, which make a rather nasty sound (oscilloscope trace top pic in the attached)

I have photoed traces of the voltages of the input supply and of each pin of the supply regulators 317 7815 & 7915 (rows 2,3,4,& 5 of attached)

Any ideas why this is and how I might fix it? One thing that is a bit odd is that the 7815 & 7915 get hot very quickly with no signal input. I suspect that something has failed and is drawing a lot of current which the 7815 & 7915 are not coping with.

Any thoughts gratefully received.

Simon.
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Old 28th March 2011, 12:56 PM   #2
Mooly is online now Mooly  United Kingdom
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A circuit would be a huge help.

If the 'scope ground lead is on PSU "ground" in the mixer then I can't quite see what your ground scope picture (2nd row middle) is actually measuring.... it can't be the same point.

It's great to see scope pictures... in this case more info is needed
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Old 28th March 2011, 01:16 PM   #3
AndrewT is online now AndrewT  Scotland
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Yes !!!
Real pictures. Very nice. And they show the V/div on screen.
Try turning down the trace brightness slightly. Can trace focus be improved?
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Old 28th March 2011, 01:49 PM   #4
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Originally Posted by Mooly View Post
A circuit would be a huge help.

If the 'scope ground lead is on PSU "ground" in the mixer then I can't quite see what your ground scope picture (2nd row middle) is actually measuring.... it can't be the same point.

It's great to see scope pictures... in this case more info is needed
ummm...quite. I don't understand that either.

Below is a picture of the 3 power regulators + the three pads for the input which is ~18VAC from a wall wart.

Unfortunately I did these photos a while ago with a borrowed scope so I cant do anything new, at least not in the short term. The voltages are all wrt the scopes earth which is tied to the mains earth. This I thought should be tied to the mixers power input earth (through the wall wart) and so as you say "be the same point" but there are these bumps and a nasty noise and all that heat.

Simon.
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Old 28th March 2011, 02:01 PM   #5
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I doubt the wall wart is earthed, but as we don't know you must prove it to yourself by measuring on ohms from the PSU ground to the plug top earth for continuity.

Without a scope to redo the measurements it's hard to say but I suspect those in your picture are down to the way you measured... they look a classic case of a missing ground in the measurement procedure.

Grounding of test gear is just as important as grounding in audio and it's easy to pick up noise etc... again without it in front me it's hard to advise.

The regs look like they should all be on heatsinks ? and the 78/79 type regs can and do fail so if there is PSU related noise don't discount them, but it would be best to work methodically with a scope measuring carefully.

Is the noise problem really bad ? could it be unrelated to the mixer and more a ground loop issue with other equipment ?
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Old 29th March 2011, 02:05 AM   #6
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they look a classic case of a missing ground in the measurement procedure
Yup! A second call for that here
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Old 29th March 2011, 09:09 AM   #7
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Originally Posted by Mooly View Post
I doubt the wall wart is earthed, but as we don't know you must prove it to yourself by measuring on ohms from the PSU ground to the plug top earth for continuity.

Without a scope to redo the measurements it's hard to say but I suspect those in your picture are down to the way you measured... they look a classic case of a missing ground in the measurement procedure.

Grounding of test gear is just as important as grounding in audio and it's easy to pick up noise etc... again without it in front me it's hard to advise.

The regs look like they should all be on heatsinks ? and the 78/79 type regs can and do fail so if there is PSU related noise don't discount them, but it would be best to work methodically with a scope measuring carefully.

Is the noise problem really bad ? could it be unrelated to the mixer and more a ground loop issue with other equipment ?
Mooly,

Thank you for your thoughts.

I was sloppy in my terminology. This is not a power mixer (ie there is no power amp) so when I said speaker out I should have said line out! So the noise is really bad.

The mixer came without a power supply (another ebay success - not) so I have modded a bought AC-AC wall wart. The earth is direct to the mains earth and I measure zero resistance between the mixer and the wall plug earth pin. The scope was plugged into the same mains plug board. I did not check the scope to mixer earth resistance through the plug board but I do not think this is the problem. My three reasons for this are a) the shape of the waveforms on the earth line - the truncated sine wave suggests something odd rather than a floating earth b) the heat generated in the 78/79 chips on what should be idle. Yes they are attached to the metal chassis as a heatsink and c) the low likelihood of there being a break in the earth from the scope through the plug board.

The mixer has been fiddled with before I acquired it. The 78/79 regulators have been replaced (or botched) and a 4580 op amp removed from one channel.

Simon.
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Old 30th March 2011, 10:11 AM   #8
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Not much I can suggest really, particularly as you say it has been worked on. Those top middle traces still look like a ground/wrong ground measurement point issue.

Are you sure the PSU you are using is OK ? as I see three wires. A small increase in voltage on the regs can increase power dissipation considerably if the reg is dropping to much voltage.
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Old 30th March 2011, 12:19 PM   #9
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Mooly - thanks for getting involved.

I think my next step is to bypass the power supply completely and provide +- 15vdc independently somehow.

Simon.
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Old 30th March 2011, 01:02 PM   #10
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Sounds a good idea... should prove if it's all working.
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