Best / least painful (?) way to wire in 2nd COM?

[blaplante]

Situation - older plane (tip up), has single COM radio and a SPA-400 intercom.
To Do: install 2nd COM, mixer (AP-60), and some switches allowing PTT to go to 1 or 2, and to monitor COM 1 , 2, or both.
Eventually other inputs to the AP-60...

Looking at wiring diagrams for the SPA-400, all the audio outputs are wired in parallel, that is the output of the SPA-400, and the output of COM1 are wired together, and wired to the headset jacks.

The AP-60 mixer wiring has you run the output of the SPA-400, into an input of the mixer, then of course run COM1 audio in as another input, ditto with COM2.

Here's the question. Clearly it is going to be 'fun' (NOT) under the panel tracing wires, trying to figure out what to cut into. Ignoring the PTT, I need to find the COM1 audio out, the SPA-400 audio out, and the audio wire to the headsets. Then splice in the new equipment. Any recommendations on how to approach the problem? Suck it up and spend 3 hours cramping up? Or? For what it is worth the COM1 radio is a recent upgrade, but I haven't looked at the wiring to it yet.

Boy I wish avionics used separate power, audio in, audio out connectors!

 

[Carl Froehlich]

There is the “stick your head under the panel and make more of a rats nest of wires” approach, or bite the bullet, take the panel out and rewire. I suggest the latter.

The prime rule, “never on your back with your head under the panel”. Make the panel removable and work it on the bench.

Carl

 

[blaplante]

There is the “stick your head under the panel and make more of a rats nest of wires” approach, or bite the bullet, take the panel out and rewire. I suggest the latter.

The prime rule, “never on your back with your head under the panel”. Make the panel removable and work it on the bench.

Carl

Ouch. Estimated time to drill out a riveted in panel and since most wires have to be cut to get the panel out... a full re-wire... ?

Also on my plane, I did make the panel removable. Never actually have pulled it as I look at the effort to pull v. the couple hours under the panel... and bite the bullet and crawl down there.

 

[Carl Froehlich]

Ouch. Estimated time to drill out a riveted in panel and since most wires have to be cut to get the panel out... a full re-wire... ?

Also on my plane, I did make the panel removable. Never actually have pulled it as I look at the effort to pull v. the couple hours under the panel... and bite the bullet and crawl down there.

It sounds like your panel is far from removable. Drill out rivets? This should be a 15 min process.

So, “pay me now or pay me later”. There will be future mods and maintenance so perhaps you can consider this near term pain as an investment.

Carl

 

[blaplante]

The time to drill out the rivets isn't relavant. I meant the time to do the full pull, re-wire, and reinstall (making removable with platenuts).

Also perhaps not clear... this isn't MY plane. So 'investment in the future' is in someone else's future work....

The alternate thought I have is to make up a full new harness for existing COM and the SPA-400, rather than try to patch in.

 

[BobTurner]

The alternate thought I have is to make up a full new harness for existing COM and the SPA-400, rather than try to patch in.

This.^^^^
Wire and pins are cheap. There will be some ‘on your back time’, but much of the cutting, crimping, inserting pins can be done sitting at a table.

 

[YellerDaisy]

To Do: install 2nd COM, mixer (AP-60), and some switches allowing PTT to go to 1 or 2, and to monitor COM 1 , 2, or both.

Not an answer (sorry) and don't know your situation/goals but perhaps considering a transceiver that has active/standby frequencies and allows you to monitor the standby freq - would be less work and satisfy the need?? Some also have a built-in intercom as well.

Garmin SL-40, for example - not sure what a 'modern' equivalent might be (maybe a GTR-200??).

 

[blaplante]

Not an answer (sorry) and don't know your situation/goals but perhaps considering a transceiver that has active/standby frequencies and allows you to monitor the standby freq - would be less work and satisfy the need?? Some also have a built-in intercom as well.

Garmin SL-40, for example - not sure what a 'modern' equivalent might be (maybe a GTR-200??).

NO. The intent is to make the plane IFR capable.

In fact yesterday was an example of why not 1 radio. The single COM radio failed. No handheld on board. Oh, and BTW... the failed radio is a modern Garmin! No, the failure wasn't the Garmin's fault. It was the fault of whoever installed that radio. I'll add that the failure mode would not have taken out a 2nd radio.

 

[Canadian_JOY]

WARNING: It's soapbox time...

I'll provide the same guidance here, to you as a stranger, as I have provided to friends and family members. This advice has not changed in almost four decades as an avionics professional.

Think about this for a moment. You get in your airplane, fasten your seatbelts, then what do you do? You don your headset. At the end of the flight you shut down the engine. Only then do you remove your headset. Your ears are the primary man-machine interface in aviation. For this reason I tell everybody the same thing. Put in the best audio you can possibly afford. Cut no corners. Wire the audio exactly in accordance with the manufacturer's instructions.

This is normally where I tell folks to buy a PS Engineering audio panel and start with that as the basic building block of their avionics equipment. Buy the best and never regret that decision.

As for the "rip and replace" decision, that's up to you. Please know that your currently-planned path will result in more limitations in the future. The next time you want to add an audio source you'll be back in the same position. Better to bite the bullet now, do it right, and build in provisions for future audio equipment. It's easy to do. One doesn't have to re-wire the entire panel but with your current radio equipment there really aren't that many wires.

I'll second the "never on your back under the panel" motto. That's a recipe for a poorly executed installation.

 

[blaplante]

This is normally where I tell folks to buy a PS Engineering audio panel and start with that as the basic building block of their avionics equipment. Buy the best and never regret that decision.

I agree on the advice w/installing an audio panel.
But - No room in the stack without major panel re-work.


To everyone reading...
I was looking for helpful tips on how to spend less time under the panel doing some (emphasis SOME) rework.
I was about to vent about the suggestions ... but I've decided to delete that portion of this post.

 

[blaplante]

Thread Closed

 

[PaulvS]

Adapter harness

Back to the original post/question; it may be feasible to make an adapter harness that will work along with the existing panel wiring to integrate all of the desired components. The harness would be made on the bench and reduce the amount of time working under the panel. Assuming you have pinouts for the current equipment (and it has been wired correctly), then create an intermediate converter or adapter harness that will plug between the existing harness and the old and new equipment.

 

[rzbill]

Bryan,
The least painful way to add a second com is to go back a number of years to the time before audio panels and do it like that.


There are two audio related connections needed.


One is the microphone push button. You need a miniature toggle switch to flip the PTT line from one radio to the other. SPDT Single Pole Double Throw meaning it controls one circuit and swaps between two connections. Microphone PTT (from your intercom) to the center tab and two radio PTT lines on the outer two tabs.


Second is radio audio output. The audio outputs cannot be simply twisted together or the amplifiers will smoke. They need to be isolated to prevent backfeeding. This can be simply done with resistors on the order of 100 ohm to 150 ohm size. Put a resistor in the audio output line of each radio output (Including the Nav audio if you have that). Connect the free ends of the resistors together and send that to your intercom radio input.


In use, the toggle determines the transmit radio and use the radio volume controls to "turn off" the volume from the idle radio ( or leave it low for listening to AWOS and ATC at the same time)


Good Luck