Radio and Transponder problems

[km74]

My radio and transponder work fine when I first leave on a flight and they don't work very well when I get back. My signal was a 2 today at the field after I got back with the engine running or stopped. There is power going to the radio and transponder so maybe my battery is on the way out? It starts fine after refuelling when I return. I would have thought if the battery can crank an engine then it should be able to run the radios, maybe I just don't notice because it starts very well. If it sits overnight it is fine the next day at the start and after awhile it is bad again. It has done this since I have owned it. In all temps from 20 above to -25 today. Thanks for any suggestions.

Ken

[lowNslow]

I am not much of a avionics guy, but since both the transponder and radio are affected it must be something common to both. I would check for a bad or corroded connection or possibly a bad radio master switch.

[N1277D]

It sounds like the voltage regulator needs to be reset. As it warms up the voltage cut off set point changes and goes lower. It might be 14.5V cold and 12.5V warm, or it could have drifted way low or you might have a cell dead in the battery. First check the battery voltage if good I would fly it and check the buse voltage as the flight progressess. You can use a voltmeter and measure the voltage at one of the fuses. Just unscrew the fuse cap till you can get a good contact with the voltage probe. Better done by a copilot.

If it measures low after heating up, you should pull the regulator cover off off after flying and adjust the set screw for the generator cut off voltage up to about 14.5 or so volts with the regulator hot. You can do the adjustment by trial and error with various engine restarts or with a voltage source. The generator regulator usually needs attention and adjusted/reset in summer and winter.

This is my guess, it could be something else but sounds like low voltage due to voltage regulator drift.

[Watkinsnv]

My radio did the same thing after it was installed. I found that after I chased down the ground wire that was used before that it went no where and was not hooked to anything. Check your ground wire and for a Good ground. It would work fine but after it built up a static charge I could hear but not send for beans. Lance

[wa4jr]

If you do find that it is a regulator problem, you might want to go with a newer solid state type such as the Zeftronics. I find they are much better than the finnicky coil and relay mechanisms. Yes, I have several wires that appear to go nowhere behind my panel. I just completed the install of a KLN-88 loran, and I was amazed at the number of old cloth covered wires that just termintate without connecting to anything. Guess I'll start pulling them out this summer and find out what the other end is connected to!

[km74]

Thanks for all the replys. I am not an electrical guy at all but I do like to try, so I have a few dumb questions. What does the voltage regulator look like and where is it located? Does the voltage regulator work just with the generator? Because mine also did this with the engine shut off as well. How do I check If it was the battery wouldn't you think that if it had enough power to turn over the engine that it should have enough power to run the radios? How would a person go about checking if it was the battery? I am a little reluctant to take it to the mechanics here because avionics and electrical aren't their specialty. I won't be able to get to my plane for a week due to work but at least I will have somewhere to start with now. Thanks for the replys.

Ken

[Dave Clark]

Ken
"Ockams Razor"
I would really suggest you start with the simplest thing first and that is the ground and other connections at the bus, switch etc. Check for tightness and at the grounds pull them apart and make sure the parts that connect are free from dirt, corrosion and paint. Check all the grounds you can find on the plane not just for the radios.

[wa4jr]

Another thing that might be coming into play here is avionics cooling. My '54 does not have a cooling air intake, and my KLN-88 needs cooling air over the unit, so I am installing one. Don't know what type radio or xpndr you have, but they may be getting hot during the flight and failing slowly as they heat up. Some avionics are REALLY sensitive to proper 13.8 volts DC. When my battery was failing and the gen was not turning fast enough to provide power, my transmitters went downhill VERY fast. I have a KX-170 and had a KX-145 at the time. Interesting to note that the battery did indeed have enough power to crank the engine...if slowly. My voltage regulator is on the right side of the firewall...easily accessible via the right cowl inspection hatch. Would be an interesting trick to come back from a flight with your radio failing, get a voltage reading on your battery with a good test meter, and then hook a battery charger up providing proper voltage and then see if your radio starts working again. Like Dave said, check all those grounds and basic connections first...the simple stuff that does not cost you anything!

[km74]

I went out to the airport before work the other day and did some checking around. All of the ground wires are connected. the radio worked fine when I tried it and the battery was fully charged. I hooked up a charger to it to check. I am going to check the connections and wiring to the fuse on my days off. The wiring is old so it might be kinked and wire strands may be broken inside the sheathing? If thats not the case then they may be overheating? The radios are both Kings. KX170 and ??76? Anyway, Thanks for the replys.

Ken
51 170A

[km74]

As for the ??76? I think that was the ADF. I never use that.

Ken

[wa4jr]

Good idea to check the fuses. I have found a few over the years that looked really cruddy inside...like the one that eventually blew in my generator regualator circuit. Check the resistance of each fuse in your radio stack with a good ohmeter to make sure something has not gone bad up in the end caps where you cannot see. Sooner or later you will find the problem...but it WILL be the last thing you check! Yes, look the wiring over really good where you can see it. I did just find a break in the glowplug circuit on my diesel generator that was hidden under a piece of heatshrink tubing. Beat my brains out finding the problem...and it was the LAST place I looked! HAHAHA!

[km74]

I found the problem. It was a bad wire on the back of the switch. Now I'm off to post another problem I found today. Thanks for all your help.

Ken
51 170A

[wa4jr]

GOOD for you! You just saved yourself a bundle of money and learned a little bit about your 170!

[zero.one.victor]

Ken,I'm curious as to which switch had the bad wire--was it by any chance an avionics master? My 170 had an avionics master when I bought it,which I promptly bypassed and removed. Just one more thing to fail--for my limited equipment (nav-com, transponder,and intercom) I felt that it was not worth the (potential) trouble. It kinda amuses me how a lot of airplanes have a big,expensive radio stack with dual everything for redundancy,yet if a $10 avionics master switch goes toes-up you're out of the game.
My 2 cents....

[km74]

Yes it was the avionics master switch that the bad wire was on, but I don't think I will bypass mine. I carry a handheld radio for backup and use batteries for my gps.

Ken