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Old April 20th, 2011, 05:52 PM   #1
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Phantom power broken on ex1r?

Hello,

I'm using an AKG SE300B mic with phantom power on a very short XLR cable (20 cm) as an onboard mic for my Ex1r.

Problem:
Sometimes, but not always I get audio noise. It sounds like a bad plug/contact with a bit of crackling, but its continuesly and sometimes tends to increase in volume. This was getting annoying so I decided to take a closer look...

What I tried:
-Plugging the mic into the other channel (channel 1) gives crystal clear audio. Always.
-On channel 2 the mic sounds overal (regardless of noise) lower in volume!
-Double checked, changed all audio settings in camera. No difference
-Using a sennheiser wireless mic (whithout phantom power) give no problems at all.

-I measured the phantom power with a multimeter: Channel 1 gives around 16 volts over pin 2+1 and 3+1 Channel 2 gives only 1.6 volts (!!!)

I'm affraid my phantom power is broken??? And this surely explains the lower volume on channel 2 and perhaps too the noise...

I really hope I am missing something, because it just sucks when your expensive equipment brakes down... Ojh and while im on the subject, any ideas how this happened and how to make sure it doesnt happen again?

Ok, small update: When I disable the phantom power on channel 2, the phantom power on channel 1 becomes 48 volts. When I enable phantom power on channel 2 too, the phantom power on channel 1 drops to 16 volts...(!) No mics attached btw...


Thnx
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Old April 21st, 2011, 08:25 AM   #2
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re: Phantom power broken on ex1r?

There are a number of places in your chain of equipment that could be causing the problem, but the measurements of the phantom power do seem to point to a problem with the camera's phantom power supply. Hopefully you can get this tested and fixed.
If you can't afford to be without the camera or if it's out of warranty, you may need to use the clean channel for phantom powered mics and leave phantom off on the problem jack (connected with non-phantom devices like your wireless receiver).
However, you should also make sure the XLR input jack on the problem input has clean pin sockets.
Also make sure the pins on your XLR cable are clean and test using a different cable as well.
Remember the SE300b is just the power supply and the connection between it and the detachable mic capsule can also become oxidized. Simply connect and reconnect the capsule to the SE300b body a couple of times.
Your mic will operate with phantom voltage from 9 to 52 volts. Worst case if there is a problem with the camera that's expensive to fix, you can use a battery-powered phantom supply with this mic. I know that's not as convenient, but it's a handy box to have on hand when clean phantom power isn't available.
For example you could also use a phantom box if you ever needed to feed your SE300b and mic capsule into a wireless transmitter that doesn't supply phantom power.
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Old April 21st, 2011, 10:29 AM   #3
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re: Phantom power broken on ex1r?

Thnx for your reply. I'n pretty postive its just a problem with the camcorder. I already checked the connection between actual mic and mic power and the XLR cable seems to be ok too.

Getting warranty will be difficult I guess. I bought it at DigiBroadcast UK (i'm from Holland) and if i recall correctly it had a one year warranty. I'll probably just live with it, since I didn't even notice this particular problem for a long time. 99.5% of what I do is with the sennheiser wireless sets (no phantom power needed) and the AKG mic in the ok working channel.
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Old April 21st, 2011, 10:47 AM   #4
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re: Phantom power broken on ex1r?

if switching on the faulty channel phantom impacts on the good channel, then almost certainly there is 'proper' fault. it will be worth checking the resistance between pins two and three - obviously not an impedance check, but a good indication and something you can compare from ch to ch. I'd hazard a guess that the pin 2 to 3 resistance on the dodgy channel is much less, and worth checking if there's a similar change between 2 and 3, down to earth on pin 1. Battery operated cameras can't supply much in the way of current, so if the impedance is lower than it should be, it will have a severe impact on the voltage. If you can open it up - you may be able to find a simple solution but testing each component on the two channels in sequence from the two identical resistors that apply the phantom across 2 and 3, back to the socket. Not really beginners stuff. It may be something very silly - perhaps just two legs close together that may have flexed when a cable was in the socket and given a bit of a pull - if there is a near short across 1 and 2, or 1 and 3, then the audio will still be present as it's just virtually an unbalanced input then, but phantom voltage dropping down a long way will mess that up.

Try a dynamic mic and see what happens - if you get good results, but just a few dBs lower on the meters, suspect a low res near short on 2 or 3 down to 1.
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Old April 22nd, 2011, 05:37 AM   #5
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Re: Phantom power broken on ex1r?

Thank you for your extensive reply. I'll look into it when I have some spare time!
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Old April 22nd, 2011, 10:34 AM   #6
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Re: Phantom power broken on ex1r?

Ok, I did some testing. Camcorder off and no mic's attached. Multimeter set to 2000 khm scale

[Channel 1 - the OK channel]
-Resistance between pin 1 and 3 = 203
-Resistance between pin 1 and 2 = 203

-Resistance between pin 2 and 3 = 013


[Channel 2 - the 'bad' channel]
-Resistance between pin 1 and 3 = 007
-Resistance between pin 1 and 2 = 007

-Resistance between pin 2 and 3 = 013


Changing inputs to line, mic of mic + phantom makes no difference at all
When I open up the camcorder and disconnect the flat cable connecting the 'XLR box' to the logic board all resistance drops to infinity (no shorts detected in the xlr box)

So I think it's a problem with the logic board? The camcorder always was very good taken care off, so the scenario where a sudden pull on a xlr cable somehow broke the xlr box on the camera is quite unlikely...
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Old April 22nd, 2011, 02:35 PM   #7
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Re: Phantom power broken on ex1r?

Quote:
Originally Posted by Bram Corstjens
So I think it's a problem with the logic board?
Good troubleshooting. Unfortunately, I think your conclusion is probably correct.

There are a few slim possibilities you might want to consider. If one of these is actually the issue, it might be less serious than a logic board problem.

1.) I don't know where you disconnected the ribbon cable. Is there a possibility that there's a fault in the ribbon cable or its connector? Perhaps a wire that was pinched to ground when everything was assembled?

2.) A very long shot... if the ribbon cable uses one conductor for PIN 1 and a different conductor for "ground," then perhaps there might be a fault on the XLR board (between P2 or P3, and the actual "ground") which is not the same as a fault to P1, and, therefore, does not show up when metering to P1 with the ribbon cable disconnected.

It would be nice to have a schematic, to try to figure out what's happening on the logic board. OTOH, a schematic might just serve to prolong the agony.

Good luck!
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