PD170 : incoming Audio cancels itself out !!!

[Ken Beals]

Was adjusting the audio levels up when all of a sudden NO audio... dead air... total silence in my headphones !!!

Then noticed that channel 1 and channel 2 were set at the exact same level.

As soon as I changed channel 1 or channel 2 the audio would return but as soon as I set them at the same level again - POOF ! .... no audio.

Also on playback whenever the levels same no audio in headphones.

That is SCARY. I'm thankful to a Mentor who kept pounding into my brain the importance of using headphones !

Anyway here's the camera settings :

The slide button in the Record Channel Select is on Ch1 Ch2.
The audio settings in the menu :

Audio Mode : FS48K
Mic NR : ON
Audio Set : AGC Ch1: OFF AGC Ch2OFF
Man Gain : SEPARATE

Other info that may more may be helpful :

Heads have been cleaned periodically.

Current Hours Meter Read :

OPERATION
151 x10H
DRUM RUN
80 x10H
TAPE RUN
51 x10H
THREADING
229 x10H

Any theories ?

Thanks,

Ken

[Mike Rehmus]

No theories. You didn't say what the source of the sound is and how it is fed in.

I'd guess you have two separate feeds from a single source?

Observations:

Turn the NR off. it is only of limited use when you are using the stock on-camera microphone. It does put artifacts into the audio. Usually exhibited by a slight ringing after the end of any sound.

I'd guess you have two signals that are exactly opposite. Either someone has inverted the feed on one channel or have the wires crossed on one of the cables.

You didn't say whether you are hearing a good stereo image when the sound comes back or if it comes back on one side or the other.

[Ken Beals]

The onboard mic is the single source.

The external slide switch on the other side of the XLR input was set on:

Ch1 Ch2

which should cause whatever is plugged into Audio input 1 to record onto channel 1 & 2.

Since the only way to hear audio in the headphone is to off set the levels so the stereo image is not even.

This phenomena has never happened before.

Am wondering if something is squirrely is happening with the circuitry ?

thanks for responding Mike.

K

[John Cline]

It may be that the ground wire has come loose, either in the headphone cable or the jack. If the ground is missing, then the headphone drivers are in series and are being driven by the difference between the two channels. If the channels are identical in level, then there is no sound.

I suspect that the audio recording itself is just fine.

John

[Eric Stemen]

When you captured the footage do you have audio?
The volume on the lcd isn't set at zero is it?

[Ken Beals]

It didn't record audio when the levels were equal but did when off-set.

Will be bringing cam in for repair to Armato's in Manhattan and post what they find out.

Appreciate everyone helping to CSI this one.

K

[Daniel Epstein]

Did the audio actually go away or was it just gone in the headphones? This can happen with Phase issues. If you were using a Lav Radio mic with a little digital delay and a boom mic set close to the same distance as the lav you can
end up with a phase issue which will show up only if you are listening to a mix of the two signals. By themselves they are both fine. If you lower the level of one then you should hear the other in the mix and vice Versa. If the audio wasn't recording then that is another issue indicating a problem with the inputs

[John Cline]

It is acoustically and electrically impossible for two dissimilar microphones to be exactly 180 degrees out of phase and cancel each other out.

I still maintain that the ground wire of either the headphones or the headphone jack has become disconnected and, therefore, the two headphone elements have become wired in series. Since the audio was a single mono source recorded to both channels, when you apply exactly the same level signal to the hot leads of the left and right channel with no ground, they will perfectly cancel out. If you raise or lower the level of one or the other channel, then the differential voltage between the two channels will then drive the headphones and you will hear something.

John

[J. Stephen McDonald]

Originally Posted by John Cline

It is acoustically and electrically impossible for two dissimilar microphones to be exactly 180 degrees out of phase and cancel each other out.

I still maintain that the ground wire of either the headphones or the headphone jack has become disconnected and, therefore, the two headphone elements have become wired in series. Since the audio was a single mono source recorded to both channels, when you apply exactly the same level signal to the hot leads of the left and right channel with no ground, they will perfectly cancel out. If you raise or lower the level of one or the other channel, then the differential voltage between the two channels will then drive the headphones and you will hear something.

John

I think that what John says here might be the answer, except that it may be in the switching circuits that route the monaural mike input into both recording channels. If the ground connection in the switching component is bad, then the circuit might be completed with a flow from one positive pole to the other, driven by a difference in the levels. Since Ken says that no sound was recorded during the times that the headphones were dead, then the broken ground connection wouldn't have been limited to just the headphone circuits. Have separate mikes been connected to the two inputs and tested?

[Allen Plowman]

my pd170 was having similar problems. I was also having issues with the headphones cutting in and out also. I know the headphone jack has a microswitch in it to disconnect the internal speaker. I connect the sennheiser G2, set to ch 1&2, set the volume levels identical, and they drop to zero. if the volume levels was set differently, they both worked (one mic set on 1&2) I replaced the headphone jack, and it seems to be working normal now. The headphone jack costs about 5 dollars plus shipping, however it was not a simple matter to replace it. I have an electronics background. I finished the repair about 5 minutes ago, or I would have mentioned it sooner :)