Need help to diagnose noise in intercom

[Geoff]

Ok here are the symptoms on my 82 M20k

1. Alternating medium and high pulsating noise in intercom headsets started after avionics shop installed JPI 900 and removed GEM 601

2. Radios in the panel: GNS 430w, GMA340 audio panel, kx165, strike finder, 696 in doc

3. Noise occurs at all four seats in different brands of headsets

4. Noise does not occur on speaker

5. Noise occurs with engine off

6. We sequentially pulled each breaker and noise still occurs regardless of any individual or combination of pulled breakers

7. Swapped the GMA340 for another one, noise still occurs 

8. Noise is not the ELT

9. Powered off each radio by the radio power switch and noise still occurs

10. When pilot isolation is presses, noise stops in pilot position only, noise still occurs at all other stations  

11.  Noise stops when speaking in intercom or when radios are receiving audio but returns when all becomes quiet again

12.  Unplugged all headsets and tested each station noise still occurs  

Both me and the avionics shop are stumped so I turn to the wisdom of the board....

 

Thabks for you ideas. 

[LANCECASPER]

I'm sure you checked this, but just in case . .  is there a usb outlet plugged into the cigarette lighter socket?

[takair]

The jacks should be isolated from airframe ground by plastic washers, one that drops in the oversized hole and one is flat.  Maybe one of the PAX jacks got bumped and you are now picking up noisy airframe ground.  Happens often when a plane gets worked on...

[Geoff]

No USB charger plugged in but a good thought. 

[Yetti]

what circuit is the JPI 900 on? what else is on that circuit?

Edited December 16, 2017 by Yetti

[Geoff]

1 hour ago, Yetti said:

what circuit is the JPI 900 on? what else is on that circuit?

I’ll ha e to ask installer. 

[Piloto]

Check the intercom squelch setting.

José

[Geoff]

Checked the squelch. When mike is open noise ceases but returns as soon as squelch is reduced to eliminate hot intercom. 

[Marauder]

Ok here are the symptoms on my 82 M20k
1. Alternating medium and high pulsating noise in intercom headsets started after avionics shop installed JPI 900 and removed GEM 601
2. Radios in the panel: GNS 430w, GMA340 audio panel, kx165, strike finder, 696 in doc
3. Noise occurs at all four seats in different brands of headsets
4. Noise does not occur on speaker
5. Noise occurs with engine off
6. We sequentially pulled each breaker and noise still occurs regardless of any individual or combination of pulled breakers
7. Swapped the GMA340 for another one, noise still occurs 
8. Noise is not the ELT
9. Powered off each radio by the radio power switch and noise still occurs
10. When pilot isolation is presses, noise stops in pilot position only, noise still occurs at all other stations  
11.  Noise stops when speaking in intercom or when radios are receiving audio but returns when all becomes quiet again
12.  Unplugged all headsets and tested each station noise still occurs  
Both me and the avionics shop are stumped so I turn to the wisdom of the board....
 
Thabks for you ideas. 

I had a similar experience with an open squelch after a JPI 900 install. Does it go away when you pull the JPI breaker? Is the remote annunicator installed?




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[Geoff]

The noise remains after the JPI breaker is pulled and the remote annunciator is not installed. Not needed due to location of JPI. 

What was your solution?

[Yetti]

I am guessing you grounded the ring on your copilot station.

The rear stations are combined.   The pilot station is clear when isolated. That points to the copilot station.

check with VOM

Then check the rear stations if that does not find anything.

[Guitarmaster]

Alternating high and low sounds... Like a French siren?

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[Yetti]

Does it go away when you transmit?

[Marauder]

The noise remains after the JPI breaker is pulled and the remote annunciator is not installed. Not needed due to location of JPI. 
What was your solution?

If you pulled the breaker on the JPI, then it can’t be it. Mine when away when I pulled the breaker and later discovered that when I disconnected the remote annunciator on the JPI it went away. JPI sent me a ferrite filter to attach to the lead coming out of the unit for the remote annunciator.


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[Geoff]

3 hours ago, Guitarmaster said:

Alternating high and low sounds... Like a French siren?

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Yes alternating high and low but cracking or rough not clear. 

[Geoff]

3 hours ago, Yetti said:

Does it go away when you transmit?

Yes noise goes away when transmitting or receiving. 

[Geoff]

3 hours ago, Yetti said:

I am guessing you grounded the ring on your copilot station.

The rear stations are combined.   The pilot station is clear when isolated. That points to the copilot station.

check with VOM

Then check the rear stations if that does not find anything.

Headed to the airport now to check the copilot ground. Plastic washers in hand just in case. 

[Geoff]

22 hours ago, Yetti said:

what circuit is the JPI 900 on? what else is on that circuit?

I’ll have to ask installer. 

[Geoff]

Found the culprit but not the solution.  The noise stops when I unplug the panel docked 696 from ships power. 

Any ideas on why it only makes noise when plugged into ships power?

[Guitarmaster]

I amazes me that a $120 car stereo can be completely devoid of noise whatsoever, but all this high-dollar, TSO  gear is so affected.  Does the experimental world have these problems or just the hamstrung TSO manufacturers?

Rant out.

[LANCECASPER]

15 minutes ago, Geoff said:

Found the culprit but not the solution.  The noise stops when I unplug the panel docked 696 from ships power. 

Any ideas on why it only makes noise when plugged into ships power?

 

[Yetti]

what circuit is the 696 on? What else is on it? Is the Data cable used, what is it connected to?

[Yetti]

Use the "What changed methodology?'  When did it last work properly?"

[bradp]

It’s probably because the ground for the 696 goes to a different place than the audio panel ground and is creating a ground loop.  It may be as simple as your A&P moving a single ground wire to the common airframe return.  Wondering if it got moved during the JPI Install. 

[Yetti]

That's what I am thinking too.  or something did not get a good ground.  Or someone grounded to the frame instead of the skin. Or the 696 is on the same circuit as the JPI and they have  different ground as the JPI.  Or a ground screw is not tight.