I have a radio installation with two KX-155 Nav-Comms. These radios are equipped with a photocell to automatically dim the displays to match cabin lighting. In dark lighting, the digits fade to the point of unreadability, with some segments being visible and some not. I can brighten and read the displays by aiming a flashlight into the photocells.

It seems like there is a bias set incorrectly or an excitation voltage which is wrong.

Why might this be happening?


1 Answer 1


Yes, there is a trim pot on the display circuit board that adjusts the dimmer level.

The display intensity pot is R438. The red arrow points to it: enter image description here

The dimmer pre-set should be marked on the outside cover, and is adjustable without removing the covers, but it only adjusts the max brightness.

It could also be the photo cell that is bad or dirty. When the display starts to loose it's brightness, that's usually a sign that it's about to go south. It can be readjusted with the pot a couple of times, then is only visible when the radio has warmed up, then finito. Try a new photocell, very easy to replace.

Be sure to observe all regulations concerning avionics maintenance!!

Source of info: http://www.337skymaster.com/messages/showthread.php?t=478

  • 2
    $\begingroup$ The fact that the segments aren't dimming equally indicates that the gas is starting to leak out of some of the segments. When enough leaks out the display will fail and you will need a new display. But since a KX-155 is at least 25 years old, it may be time for new radios. $\endgroup$
    – Gerry
    Oct 8, 2020 at 2:40
  • 1
    $\begingroup$ The content in this answer seems to be curated from thread, which I found after posting my question: 337skymaster.com/messages/showthread.php?t=478 I suggest editing for attribution. $\endgroup$ Oct 9, 2020 at 20:46
  • 1
    $\begingroup$ done as requested $\endgroup$ Oct 11, 2020 at 13:36

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .