PTT Problem

Scott Sherer

I am member of Cessna Owner Organization and i am A&p responsible for the maintenence of our airplanes. We have a 1978 172n 17269979 and a issue with the ptt.
The plane has the Collins 251 vhf with the arc 300 audio panel.
The speaker is directly wired to com 1. No matter what you select on the audio panel. The speaker is in the position.
Intermittently the ptt stops working. I replaced the ptt and checked the wires on the back side of the instrument panel but no luck. I replaced the audio panel and the speaker is still directly wired com 1.
When the ptt stops working the mic also does not work. However the velcro ptt on the passanger side will transmit (we have a 2 place external intercom box with a spare ptt for the co-pilot and hardwired ptt on the pilot side).
Do you have the cessna electronics manual with the Collins arc.300 wire schematic. I cannot find a good copy and cessna wants $300 for a copy of the manual.
I have taken the airplane to Rochelle Avionics and Rockford avionics and they will not repair the arc audio panel. They want to install a new audio panel and intercom for 3k
We are going to install a 4 place intercom this winter but need to figure out what is causing out ptt issue.
One forum said the audio amplifier could cause this issue. Where is the amp. I know where the radio 24-12v transformer are but cannot find a speaker amp.
Please advise

Scott Sherer

A lot there, so I'll boil it down; if you have confirmed aircraft wiring at the audio panel and between the com and audio panel (I'm not going to detail how to do all of that due to space) the only thing you can do is try a known good audio panel and com to see if anything changes. If it fixes your issues the box is the problem. If good boxes don't solve the issues you missed something in the aircraft wiring. You can prove your findings by putting the boxes from the problem airplane into a known good airplane and checking the results.



You mentioned you are an A&P if that's correct it is legal for you to install a used audio panel off eBay or wherever at a price point less than $3,000.




You asked about a schematic, do you mean a pin out for the connector? If so a copy of "quick shots" available from Edmo is probably a great addition to your toolbox, it has pin outs for hundreds of popular legacy boxes in it.




There is probably not a remote speaker amp anywhere, its likely in com 1 and not wired correctly.




Calling a shop and asking for prints is not a sin, all they can do is say no, and you call another one...




Just the view from my hangar~




~Erich Rempert, A&P Consultant

Scott Sherer

Hi Guys, Problem one is that ARC audio panels were produced individually for the specific aircraft as it came down the assembly line. Unlike most other avionics, the arc 300 audio panel in most cases is not interchangeable with another. You say you replaced the audio panel? I guess you got very lucky if it works. Perhaps this is why the avionics shops are telling you that you must replace it. I am also pretty sure that they did not have an audio amp and used the audio amp in one of the navcoms. In your case, that's why it is wired to your #1. A PTT is pretty simple. one side is wired to the correct pin on the back of the audio panel or in the case of a panel mounted intercom, it is wired into the intercom and the other wire on the PTT goes to ground. Pushing the PTT grounds the circuit and opens the mic and activates transmit. Bad grounds can drive you crazy! Make sure your ground side of the PTT is solid. You should have a plug in your portable intercom for a Pilot PTT. Move the copilot PTT to that plug and see if it transmits? Old Cessna install manuals are very hard to find. Portable intercoms usually plug into the pilots mic and phone jack and then the pilot and the copilot plug their headsets into the portable intercom. Your portable intercom is triggering transmit from the copilot side. If it also triggers it from the pilot side than I would suggest that the audio panel is bad. There are plenty of used audio panels on the used market. I would recommend one with a built in intercom and I always recommend PS Engineering. Frankly, their PMA-6000B is $995 new and I see them used for $800. Buy the new one. Labor is 16-24 hours most places so that's another $1300-2000 depending on your panel so the $3000 quotes are real.
​​​​​​​Bob Hart www.AvionixHelp.com

Scott Sherer

Hi and thanks for all of the messages.

We found our Ptt problem. The coiled wire under the yoke and a short from the yoke map light to the ptt circuit.

Scott Sherer

Question 2

Same 172n 1977 with the Collins 251 radios. Why does the audio panel not shut off the overhead speaker. Do you have a electrical schematic of the ARC 300 audio panel(0570115-6-28)? We tried two different panel and the overhead speaker is always on. I have found schematics for the maker beacon but not the audio panel portion
Tony Zurek

Bob Hart

Hi Guys, I'm sure you know how to work your audio panel. Switches in the up position sends the audio to the speaker, switches down to the headphones. If your switches are down and the speaker is till working, you likely have a bad audio panel. Sorry! Bob Hart www.AvionixHelp.com