PM3000 problem

[ChiefPilot]

I found today that plugging in the mic cable on the copilot side to my PS Engineering PM3000 intercom results in odd behavior. Specifically, the pilot mic is blocked, and the xmit led which is normally either red or green turns orange. Without the copilot mic plugged in, things works normally.

I've checked the wiring but it tests ok with an ohmmeter. I see good continuity between pin 3 on the db25 connector and the plug center conductor, as well as ground between pin 1 on the db25 and the plug barrel/ring. PTT isn't used, and there is not a short between things.

Again, this only happens when a headset is plugged in (I've tried three different ones). I'm starting to think it might be the PM3000 - I'm wondering if the orange LED indicates a problem.

 

[BobTurner]

I would also check that pin 3 does not go to ground when you plug in the plug. Sometimes the jack can rotate to a position where the little arms contact the side panel when they move outwards with plug insertion.
Is the radio transmitting? Does the copilot mike work in the intercom?

Wait, I just re-read your post. The mike hi goes to the middle, sometimes called ring, terminal. Ground to the long barrel leading towards the handle. PTT uses the tip of the mike plug. Is that what you have?

 

[ChiefPilot]

I would also check that pin 3 does not go to ground when you plug in the plug. Sometimes the jack can rotate to a position where the little arms contact the side panel when they move outwards with plug insertion.
Is the radio transmitting? Does the copilot mike work in the intercom?

Wait, I just re-read your post. The mike hi goes to the middle, sometimes called ring, terminal. Ground to the long barrel leading towards the handle. PTT uses the tip of the mike plug. Is that what you have?

Yes, the mic input going to the ring, PTT to the tip, and the barrel is ground. The copilot mic does not work at all for the intercom; instead, at best it produces a buzzing/clicking noise.

I was curious if something being ground would produce the condition, but it does not - the ring and barrel together don't produce the same effect: the LED remains green, and the pilot mic functions normally.

 

[BobTurner]

Another idea

Check to see if pin 16 (ptt) and pin 3 are shorted. Check both the d-sub connector going to the jacks, and the corresponding pins on the intercom.
Or, with everything hooked up, slide a small screwdriver into the jack and ground the mike hi (middle) pin. If the transmit lights come on, those pins are shorted.

 

[ChiefPilot]

Hmmm - doesn't appear to be a PTT problem. When I short 16 to ground I see the LED turn red, indicating transmit. When I short 3 to ground I see no visible change. When I insert the mic plug, the LED turns orange.

 

[BobTurner]

And what happens if you put an ohmeter on those pins, or short the mike hi to ground at the jack, with everything connected?
The logic here is that the mike is fed 12 volts thru a bias resistor (which would also be true of the ptt line). When the mike is plugged in it pulls the voltage down to around 6 volts. PTT line is green with 12, red with zero volts. I wonder if 6 volts makes orange?

 

[ChiefPilot]

And what happens if you put an ohmeter on those pins, or short the mike hi to ground at the jack, with everything connected?
The logic here is that the mike is fed 12 volts thru a bias resistor (which would also be true of the ptt line). When the mike is plugged in it pulls the voltage down to around 6 volts. PTT line is green with 12, red with zero volts. I wonder if 6 volts makes orange?

Good question - I will check it out when I'm back out at the hangar tomorrow. Thanks for your help!

 

[ChiefPilot]

Resolved

Found it! With a clear head this morning and the schematic I put together I retraced your suggestions and found that the tip and barrel connections were reversed. This explains why PTT seemed to work. Further, shorting the "real" mic and PTT produces an orange led. Swapping the wires for ground and PTT to their correct terminals resolved the problem.

Bob, thanks again for your help - I really appreciate it!