Avionics dimmer question

[Andy Alson]

So our 1981 Mooney has some older radios in it. Recently we've noticed that at night 3 items (#2 Com, ADF-yep, still have one of those in there, and the autopilot annunciator) all go dark with the glowing displays not visible unless you shine a flashlight into the light sensor on the unit. The other radios (Garmin 430W, Nav#2) and everything else seem to work normally, as do the questionable radios which work just fine except for the dimming of the display.

Question is, does the dimming circuit somehow use a ground connection as a reference of some sort which might be common to the units that are having the problem, and not used by the radios that work correctly, rather than all three developing internal problems at the same time?

During the day everything works normally.

Just wondering what everyone thinks before we discuss it with our avionics guy whose response will predictably be time to replace all of our radios with a new stack which is not gonna happen <G>.

Andy

[Ray Tackett]

In an airplane, ground and DC return are the same thing. There is nothing separate about thedisplay dimmer. If you were somehow missing a ground connection (just about impossible with a metal case radio and the coaxial antenna cable), the radio would not work at all. Disconnecting ground, if you could do it (unlikely) would behave identically with disconnecting power.

Back in the incandescent radio light days, there might have been a connection from the radio to the panel light dimmer, but no more.

I suspect the "dim" setting circuit has died or gotten out of calibration such that "dim" is "too dim". The electronic details of the dimming cicruit depend upon the type of display. A good radio tech should be able to fix it, but given the costs, it may well be upgrade time.

The radio removal and replacement labor is the same either way. A couple hours of bench time plus parts would be some percentage of a new radio. FAA paperwork costs would likely be similar either way. Your friendly radio tech can run a cost comparison for you.

[Russell Holton]

I suspect the "dim" setting circuit has died or gotten out of calibration such that "dim" is "too dim".

If it hasn't completely opened up.

The electronic details of the dimming cicruit depend upon the type of display.

Some units might have a function that sets a "minimum dim" no matter what the dimming circuit is set to.

Either way, I suspect the problem is with the airplane's wiring/dimmer, not the radios.

[Andy Alson]

I agree that replacement is a better option than trying to repair these old units. Just really curious that three of them developed the same symptom at the same time and it wouldn't be something other than the radios themselves? I know we have rotary dimmer controls for the instrument lighting and for the eyebrow lights at the top of the panel, but I don't know that we have any "airplane's wiring/dimmer" that controls the radio brightness.

Andy

[Rod Madsen]

Those radios apparently thinks it's very dark in the cockpit, so they need to go dim. Is there some object blocking the ambient light sensing device?

[Andy Alson]

Those radios apparently thinks it's very dark in the cockpit, so they need to go dim. Is there some object blocking the ambient light sensing device?

Nope. Each of the three items that dim have their own sensor that makes the radio brighten when we shine a flashlight on the sensor. Nothing blocking any of the three.

[Bruce Gorrell]

Nope. Each of the three items that dim have their own sensor that makes the radio brighten when we shine a flashlight on the sensor. Nothing blocking any of the three.

Dumb question. Did you try cleaning the sensor?

[Andy Alson]

Dumb question. Did you try cleaning the sensor?

Wouldn’t seem likely that those three would get dirty at the same time while the remaining two radios would have no problem, but i’ll Try that anyway next time.

[Terry Carraway]

If three separate devices start misbehaving the same way at the same time, my money is on some that is common to all three, not a similar failure in all 3 devices at the same time.

[Andy Alson]

If three separate devices start misbehaving the same way at the same time, my money is on some that is common to all three, not a similar failure in all 3 devices at the same time.

That's what I thought and why I asked about whether those three avionics items might have one common connection to something that the other radios didn't. Haven't found anything that fits that as yet.

[Russell Holton]

That's what I thought and why I asked about whether those three avionics items might have one common connection to something that the other radios didn't. Haven't found anything that fits that as yet.

It's possible that all the radios are supposed to have something in common, but because of a failed splice, those three are now separated.

[Andy Alson]

It's possible that all the radios are supposed to have something in common, but because of a failed splice, those three are now separated.

That's a possibility. Not sure I'd be able to trace the wires behind the panel to see it though. But maybe my avionics guy might have an idea of where they might be joined.

[John Gaitskill]

We taped a small flat LED light from Radio Shack over the sensor when ours quit. Had to tape a small battery and wires out of the way, but it worked.

[Andy Alson]

We taped a small flat LED light from Radio Shack over the sensor when ours quit. Had to tape a small battery and wires out of the way, but it worked.

Interesting idea and one to consider. Thanks.