Bellanca Super Viking Communications Issue

[JimJ]

Last week I acquired a 76 17-30A with KX135 nav/coms, a KMA20Tso audio panel and an old GNC300 Gps/with the transceiver disabled. A little more than 3/4 the way from Dallas area to Phoenix area, it appeared that she lost the ability to transmit. Landed in Goodyear squawking 7600. After sliding in a different KMA20, it appears that what was lost is the ability to receive on headsets as now I can transmit on headsets but can only receive on the overhead speaker. Headsets are brand new and have since tested good in another airplane.
Does anyone have any ideas? Thank you.

 

[Dan Thomas]

Last week I acquired a 76 17-30A with KX135 nav/coms, a KMA20Tso audio panel and an old GNC300 Gps/with the transceiver disabled. A little more than 3/4 the way from Dallas area to Phoenix area, it appeared that she lost the ability to transmit. Landed in Goodyear squawking 7600. After sliding in a different KMA20, it appears that what was lost is the ability to receive on headsets as now I can transmit on headsets but can only receive on the overhead speaker. Headsets are brand new and have since tested good in another airplane.
Does anyone have any ideas? Thank you.

Well, after 48 years....

Sounds like an audio panel problem. Try pulling it out and pushing it back in. Old connector pins get oxidized/dirty/whatever. People that smoked in their airplanes really gummed things up, including the gyros.

 

[JimJ]

I was wondering if it isn't a wire that got shaken off of the connector attached to the back of the mounting box or maybe worse, old brittle wire broke. Just before the issue appeared, we had experienced some afternoon turbulence.

 

[SCCutler]

Not addressed, and I expect you are familiar with the use of the audio panel - but the KMA20 has a “Phone/Speaker” toggle for each COM radio, and an “Auto” switch on the left. The toggle for each COM should be in the center position, and the “Auto” switch set to Speaker or Phone, and thus set, the audio for the COM selected with the rotary “Mic” switch will be directed to whichever output the Auto switch is set for (Speaker or Phone). It’s easy to accidentally get the switches set wrong - been there, done that - and the symptoms you’ve described match the expected behavior with misdirected switches.

Here’s a sample manual with operating instructions: https://www.csobeech.com/files/KMA20-Manual.pdf

If all that is set properly, you might want to check the installed Intercom unit (it would be a separate unit in nearly all installations). Can’t troubleshoot that without knowing what unit it is.

 

[JimJ]

Not addressed, and I expect you are familiar with the use of the audio panel - but the KMA20 has a “Phone/Speaker” toggle for each COM radio, and an “Auto” switch on the left. The toggle for each COM should be in the center position, and the “Auto” switch set to Speaker or Phone, and thus set, the audio for the COM selected with the rotary “Mic” switch will be directed to whichever output the Auto switch is set for (Speaker or Phone). It’s easy to accidentally get the switches set wrong - been there, done that - and the symptoms you’ve described match the expected behavior with misdirected switches.

Here’s a sample manual with operating instructions: https://www.csobeech.com/files/KMA20-Manual.pdf

If all that is set properly, you might want to check the installed Intercom unit (it would be a separate unit in nearly all installations). Can’t troubleshoot that without knowing what unit it is.

Thanx Spike.

I believe all switch position combinations were tried but I'll have to give it another go to verify. Your explanation of how it all works helps to make sense of it!

 

[JimJ]

Problem Solved:
After the second replacement KMA 20 Audio panel was not the answer, I removed it and while sitting in the right seat, Stared into the mounting box, thinking, "Now what?" There is a connector in the back of the mounting box that connects the wires to the audio control panel when the connectors meet as the panel is seated in the box. The left side screw which fastens that connector to the back of the mounting box was missing!!
After replacement fasteners were applied and an audio panel reinstalled I was talking to controllers. The only question that remains is; Where did that screw go? I found the nut rattling around in the wire box that surrounds the connector but if the screw fell into the mounting box, it had no way to get out.