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AMX240 Questions

Printed From: Avidyne
Category: Avidyne General
Forum Name: AMX240 Audio Panel
Forum Description: Topics on the Avidyne AMX240 Audio Panel
Printed Date: 19 May 2019 at 2:46am
Software Version: Web Wiz Forums 12.01 -

Topic: AMX240 Questions
Posted By: AviKent
Subject: AMX240 Questions
Date Posted: 14 Apr 2015 at 10:56pm
Question: I had an AMX240 installed. The display is not bright enough to see the "lighted" buttons in daytime? Even adjusting the panel dimming rheostat does not help. Any ideas?

Posted By: AviSimpson
Date Posted: 15 Apr 2015 at 5:10pm
Hi Kent,

Do you have a 14v or 28v aircraft?

Simpson Bennett
Avidyne Corporation
Product Manager

Posted By: scott
Date Posted: 24 Aug 2015 at 8:50pm
I am having the exact same problem. I have verified (twice) that we have pins 5 and 6 shorted together and connected to ground. Pin 7 varies between 15V and 24V (it's on battery at the moment). I also have another instrument connected to the same dimming circuit and it works fine. We talked to Avidyne tech support and they sent us a new unit - same exact problem. The display is totally unreadable in the daylight. I need some help here. 

Posted By: scott
Date Posted: 24 Aug 2015 at 9:05pm
One other piece of information. If I pull the breaker on the lighting circuit (pin 7 voltage goes to 0) then the lights go full bright which is indicated in the manual. This seems to indicate the unit is wired correctly.

Posted By: AviJake
Date Posted: 24 Aug 2015 at 9:15pm
Tech support will have to engage. There is no systemic design issue so it's virtually certain there is some installation gotcha we'll have to figure out.

Steve Jacobson

Posted By: scott
Date Posted: 24 Aug 2015 at 9:41pm
I even tried shorting the reostat so that pin 7 is getting the full 24 volts and bypassing the reostat completely. No change at all. Are you certain there is no configuration setting that enables the restate dimming?

Posted By: scott
Date Posted: 25 Aug 2015 at 4:40pm
I just connected the AMX 240 to a test bench and it behaves exactly the same as in the airplane. I cannot see how this is an install issue. Send me an e mail at and I will send you the test results. I am working with tech support and they are going to call PS Eng to see what is going on. 

Posted By: scott
Date Posted: 26 Aug 2015 at 12:43pm
Update: Avidyne tech support has reached out to PS Eng and they say that when they put a unit on their bench it works fine. I don't know if it is an AMX 240 or one of their units. I also don't know if it is wired exactly like how I have it on my bench or what. Or there is the possibility that I just have two defective units. 

Is there anyone out there who has installed an AMX 240 and the dimming DOES work correctly? 14 or 28V installation? Wired exactly per the manual?

Posted By: Gring
Date Posted: 26 Aug 2015 at 1:31pm
The dimming on my AMX240 works the same as my AXP340 with the dimming rheostat. 

Posted By: scott
Date Posted: 26 Aug 2015 at 5:26pm
Gring, 14 or 28V? Solid state or old style rheostat?

Posted By: Gring
Date Posted: 27 Aug 2015 at 8:13am
I have a 28v Bonanza with the original old style rheostat.  

Posted By: scott
Date Posted: 27 Aug 2015 at 4:55pm
I just got off the phone with Gary at PS Engineering. Here’s who this unit works. Basically there are two brightness modes: day and night. When the signal to pin 7 is below about 1.5V the LEDs are at full bright. This is “day” mode. When the signal is above 1.5V the unit transitions to “night mode”. Night mode only has a control span of 1.5V to about 14V even with a 28V buss. In other words there is no increase in brightness between 14V and 28V. Also, maximum brightness in night mode is much less than the day mode brightness. You can see this illustrated in my bench test results. The audio panel draws 410mA in day mode. In night mode minimum (about 1.6V)  it draws 240mA and in night mode maximum (15V) it draws maybe 250 mA. The bottom line is there is a very narrow brightness adjustment for night mode, and max night brightness is nowhere near day brightness. In my case my rheostat has a control range of 15 - 28V so I am always in max night mode, but that brightness level is very dim and unreadable in the daytime. I need to look at my rheostat to see how I can get 0V when it is turned all the way up. Also I need to see how that is going to affect my CDI backlighting. All this brightness logic was determined by Avidyne and they make the front panel. PS only makes the guts. If I were Avidyne I would have increased the adjustability span in night mode so that 28V is at or close to full bright. This could be changed by firmware but that is not something a user could do since there is no way to update it in the field. 

Bottom line: If you want to see the audio panel buttons and LEDs in the daytime, make sure your rheostat has the capability to go to 0V when turned all the way up.

Posted By: Gring
Date Posted: 27 Aug 2015 at 6:48pm
My rheostat work as follows:

Down is 0v and up is 28v (or something very close to those values)

When the rheostat is at 0v, the AMX240 and AXP340 buttons are full bright.  When the rheostat is turned slightly up and a little bit of voltage is provided, the buttons go full dim and my instrument lights connected to the same rheostat are full dim.  As I turn the rheostat up, the buttons get brighter and the instrument lights get brighter.

I set my IFD540s to use only the photocell for the time being since I just didn't have time to get it working properly with the rheostat.

Here is a poor quality iPhone video I took tonight where you can kind of see what happens.  The units are full bright, the rheostat is off.  I then turn up the rheostat slowly and you can see it dims completely then gets brighter as the rheostat goes up.

I think it may be time to upgrade from an iPhone 4..." rel="nofollow -

EDIT:  I just saw Scott's post above and my installation works exactly as he describes above.

Posted By: scott
Date Posted: 27 Aug 2015 at 9:06pm
Thanks Gring. The only remaining issue I have is my current rheostat starts off at 15V and goes to 28. Because the control span on the 240 is only 2-14V I don't really have any control My rheostat was designed for incandescent lamps so the 240 probably doesn't have enough load to bring to voltage down but I can always install a resistor. My rheostat also does't have the detent where it is basically off so I don't have that full bright setting. I need to replace it and hopefully that will fix the problem. My only beef is I wish Avidyne would have published the fine details on how the lighting works. That would have saved everyone (including their tech support) a lot of time. 

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