Bose X Failures

[mosquito]

Hi everyone,

I've suffered two Bose X failures in the last 5 flight hours of my -6. These are the conventional dual plug models. Both exactly the same symptoms - after startup, I don the headset, and there is no audio, no functioning microphone, no sidetone. The ANR function is still working fine, however.

I plan to spend some time this weekend looking for any obvious causes. I expect to find perhaps stray voltage is getting to them, or a spike at some point. The avionics master, and thus the intercom, is never on at startup, and it's generally off before shutdown.

Intercom is a PS3000 wired for stereo only.

The headsets were each acquired used from VAF classifieds from different sellers, so they aren't sequential serial numbers or anything.

Has anyone experienced this failure mode? I suppose it's possible I've just won the cosmic lottery and there's nothing wrong with the wiring, but I am skeptical.

Thanks,

-jon

[Doug]

A simple test of the headsets with an alternate audio source would be a good first step.
Perhaps just plug them into a home audio system.

[mosquito]

Originally Posted by Doug

A simple test of the headsets with an alternate audio source would be a good first step.
Perhaps just plug them into a home audio system.

Thanks for the reply, Doug! I've tried them with a couple of appropriate audio sources. No audio, and no signal from the microphones either. The ANR function does still work fine, though. Maybe the sidetone circuitry has been fried? Total guess.

Heading to the hangar this evening, will put a multimeter across the headset ouputs and microphone inputs and hopefully find something amiss.

I haven't heard of anyone else with this particular failure mode for the Bose, so having two pairs do the same thing within 5 hrs of each other is rather odd.

-jon

[Bavafa]

Originally Posted by mosquito

Hi everyone,

I've suffered two Bose X failures in the last 5 flight hours of my -6. These are the conventional dual plug models. Both exactly the same symptoms - after startup, I don the headset, and there is no audio, no functioning microphone, no sidetone. The ANR function is still working fine, however.

I plan to spend some time this weekend looking for any obvious causes. I expect to find perhaps stray voltage is getting to them, or a spike at some point. The avionics master, and thus the intercom, is never on at startup, and it's generally off before shutdown.

Intercom is a PS3000 wired for stereo only.

The headsets were each acquired used from VAF classifieds from different sellers, so they aren't sequential serial numbers or anything.

Has anyone experienced this failure mode? I suppose it's possible I've just won the cosmic lottery and there's nothing wrong with the wiring, but I am skeptical.

Thanks,

-jon

Straight cable or coiled cable?

When I finished my RV and started flying, my Bose headset with the straight cable kept failing and Bose kept replacing the cord portion. After 4-5 cable replacement, I asked for a coiled cable which they sent me and that has worked for the last 6 years without any failure. I don?t know what is the difference or was causing it to fail but have not had any issues since I switched to the coiled cable

[mosquito]

Originally Posted by Bavafa

Straight cable or coiled cable?

One of each, actually. I'm doubting it's a mechanical connection failure because I would expect the microphone to still function, as it's on it's own cable.

I did some more testing with my backup, non-ANR headsets and discovered everything sounded terrible. Music input had no gain, radio distorted and tinny. Peculiar.

Poked around with a multimeter and discovered the resistance across headset output and ground to be around 130ohms, which seemed very low. I would expect something in the megaohm range? I checked co-pilot side, also ~130ohms.

I made a jumper 25pin DIN to remove the PS3000 and wire the pilots headset directly to the radio. Still 130 ohms. So the PS3000 is off the hook at least.

Next suspect, radio. I pulled the radio and reconnected the PS3000. Music sounds good again, and plenty of resistance across signal and ground, in the 3 megaohm range.

Thinking I'd clearly located the problem, I reinstalled the radio to confirm. And... everything is now fine with the radio installed. Still 3 megaohms resistance, stuff sounds good. Removed and reinstalled a few times, hoping for the problem to come back. No joy.

My least favorite outcome, I would really like to find the actual issue, not just push it off. Not sure if I should suspect the radio itself or wiring. Also not sure this semi-shorted state of 130ohms resistance between signal and ground would kill Boses... but it seems very possible.

Any and all input extremely welcome!

-jon

[Doug]

It is possible the intercom output is transformer coupled in which case 130ohms would be a reasonable measurement across the output. If I designed an intercom like this I would use a transformer to match to impedance.

I am a little confused with your last, does 'Music sounds good again' mean the Bose headsets are working but not when connected to the intercom?

[BobTurner]

Originally Posted by mosquito

Poked around with a multimeter and discovered the resistance across headset output and ground to be around 130ohms, which seemed very low. I would expect something in the megaohm range? I checked co-pilot side, also ~130ohms.

I made a jumper 25pin DIN to remove the PS3000 and wire the pilots headset directly to the radio. Still 130 ohms. So the PS3000 is off the hook at least.

-jon

Do you mean the PS engineering PM3000 intercom? It has a fail-safe connection to the #1 com, so when you measured the resistance of the intercom's output (presumably power off) you actually were going straight thru to the com. With the com pulled you get a very high number.

I would look very carefully at the wiring going into the intercom pins. You're looking for a single strand of stray wire that might have been shorting the output to power - now slightly moved after the removal and reinsertion.

[mosquito]

Sorry, I think my description of my troubleshooting was confusing, will try to clarify. I really appreciate the views and input!

Originally Posted by Doug

It is possible the intercom output is transformer coupled in which case 130ohms would be a reasonable measurement across the output. If I designed an intercom like this I would use a transformer to match to impedance.

I measured the 130ohms on the output of the PS3000. I then constructed a dummy/jumper 25pin DIN to plug into the aircraft harness. The dummy cable allowed complete removal of the PS3000 from the aircraft. It hardwires the Radio in/out directly to the pilot headset, and the PTT to the radio's keyline. With the PS3000 removed, I still measured 130ohms across the headset output. I then removed the radio from it's tray, and had much higher impedance. Upon reinstalling the radio, the high impedance remained, and I have not been able to duplicate the 130ohms. I removed the jumper and reinstalled the PS3000, and the impedance remained ~3megaohms.

Originally Posted by Doug

I am a little confused with your last, does 'Music sounds good again' mean the Bose headsets are working but not when connected to the intercom?

No, the Boses are dead as doornails. I've tried them in a couple of airplanes and a couple of stereos. Zero output. The microphones are also confirmed dead. They do still power on, and the ANR engages and seems to work fine. All of my tests yesterday were conducted with a pair of passive Dave Clarks.

Music had been sounding very bad -- no headroom, i.e. no gain before distortion. Same for the radio. Once the resistance measured high again, the headroom returned and things were back to normal.

Originally Posted by BobTurner

Do you mean the PS engineering PM3000 intercom? It has a fail-safe connection to the #1 com, so when you measured the resistance of the intercom's output (presumably power off) you actually were going straight thru to the com. With the com pulled you get a very high number.

I think (?) the PS3000's failsafe mode only works when the unit is switched 'on' but has no power? It doesn't seem to pass through when actually switched off, and with the power off, the radio is also powered down.

Originally Posted by BobTurner

I would look very carefully at the wiring going into the intercom pins. You're looking for a single strand of stray wire that might have been shorting the output to power - now slightly moved after the removal and reinsertion.

That seems like the likely culprit, except I suspect the single strand may have been in the radio harness or tray. Is 130ohms consistent with a single strand of wire? Test time!

The 'stray wire' theory seems like the probable cause for the resistance issue and the general bad sound.

But to the original issue that started all of this chasing -- would 130ohms of resistance between signal and ground cause Boses to die? For that matter, would a full short?

Thanks again for the input, I truly appreciate it.

-jon

[wirejock]

How about a loose ball of solder bouncing around inside? Or a cold solder joint?