Power inverter

[chrisktx]

More of an avionics question. Plane has Narco 12E and 12D Nav/coms. Static during transmission and reception, in the air. Com between pilots is fine. Garmin 340 audio panel (I've tried two). No problems on the ground. Tried new antenna, properly mounted, static wicks (half are new) in good shape, radios checked twice by The Radio Shop in Louisiana. Throttle settings, shutting down alternators, no changes. Just two headsets plugged in.

Both Nav/Com are 14V... I had an 'eureka' moment looking at pictures of them: maybe the power supply is bad? There must be some kind of step down onboard .
A Google search reveals King KA-39, or possibly Narco MP-11. Will verify what is onboard in a few days once back at the airport. Maybe it is as easy as a bad contact...

Any of you have issues with the inverters? What is a good replacement. KGS looks Cadillac but a little on the high end. I suspect something more modern will 'clean up' the power and make the radios happier...at least I want to factor that out. Maybe something upstream, the ships alternator controller bundle. Bad power in makes bad power out...

Any thoughts on 28V to 14V inverters and how they would effect the radios?

[cessnadriver]

Hello. I was checking on a converter myself. Saw this one on Aircraft Spruce website. LONE STAR STEP DOWN 24V TO 12V CONVERTER, although it's $300 the 12 volt output is 10 amps continuous 13.5 peak? Also it's TSO approved. Hope this helps. BILLS

[chrisktx]

Also looking at both Condensers/Noise Filters. From what I read, they're located near the cowl flaps. They're up in the heat, weather, vibration, ground to the airframe and might cause an issue...especially if they're 1974 vintage. Parts catalogue 'Fuselage Electrical Equip Installation' leaves a lot to be desired (Figure 134)
Cessna part 0770038. Lone Star also makes a filter for this.....the chase continues...

[DrDave]

Greetings:

If I am understanding you correctly you have static in two separate radios during flight but not on the ground. You didn't say if the engines were running on the ground or what equipment was on at the same time. The best place to start is to determine what kind of noise you are hearing and where it is coming from. It sounds like you have ruled out audio panel. However, there is another quick test to conduct. Is the noise coming through the cabin speaker as well as the headsets?

A scope meter or simple o-scope would give an idea of what the noise looks like. This would also help locate the noise.

I think the simple thing to do at this point is to test tx and receive on the ground with the engine off. It's more that we want the mags off. If the radios are clean in this configuration I would start the rear motor. The rear mags being further from the radios is the purpose of the rear engine start. Try single mag operation and see if you can hear a difference in the headsets. An open shield on a mag p-lead can give you fits until it is corrected.

The other question being is the noise a recent occurrence or always been present?

An independent data point could also be established if there is any beacon or strobe noise in the headsets with everything else off. This would give an indication of ground loops in the airframe.

Everything can be fixed. There's no such thing as "they are just that way."

IF you will follow a logical troubleshooting path and IF you will report your findings I'd be glad to help.

Dave

[chrisktx]

Both coms work fine, on the ground, engines on. Throttle changes, every combination of alternator on/off. Speaker is too weak to hear with engines running. With the engines off it is difficult to tell, I will listen more carefully to the speaker next time.

Static really only occurs while transmitting and receiving in the air, its almost like the squelch is stuck during those moments, but it is not.... and the squelch operates normally on both radios. I replaced the Com1 antenna. (In this plane it's over the copilots head) Nice and clean, no paint, grounded to airframe. Wiggled the base/wires in flight to make sure ground was solid.

I did find the 28V-14V Voltage Converter, on the avionics tray, copilots side. It's a Aircraft Radio Corp P-528A PN 41010. Someone has been in it before so I carefully took it apart. Visually in good condition but 'I think' one of the leads to the fuse inside it had loosened. I'm not 100% sure . Tightened it up, hit it with DeOxit. Cleaned the base of the tray and the feet of the converter itself.
Hopefully will run the plane around the patch tomorrow and see if that's the smoking gun we were looking for

btw: I looked at the Condenser Noise Filter 0770038 near each cowl flap motor, mentioned in my second post. Correct me if I'm wrong, but it looks like they're only there to prevent static for the very short period of time the cowl flaps are in operation... if that's true, I'm factoring that out

No beacon or strobe noise in headsets, engines off.

[DrDave]

Good job so far. The cap by the cowl flap motor is likely for the motor. The cowl flap motors put a lot of DC hash on the line while they are operating. If they make noise in the headset while operating you may have luck central point grounding the ground lead. The flap system ground is located near the switch on the panel. I was able to completely eliminate headset noise from a flap system by lifting the local ground and moving it to a central ground bus. You can pull the capacitor in question and measure it. Foil wrapped caps can dry out and go bad. Unfortunately there is no printed spec for the value of these caps.

Back to the radio noise. My favorite antenna cable is RG400. That would be the ideal for all your antennas. You could try reaching up and disconnecting the antenna while in flight. Do no transmit on your radio with the antenna disconnected. See if this changes the receive static. Let's also disconnect one antenna and see how the other radio sounds.

I'm assuming you have a good volt meter. While we are at it we should check some voltages. Clip your test lead on the avionics bus and then pick a good ground for the other lead and go flying. You are looking for 28.4-28.8 volts. The next thing to do is set the meter to millivolts AC. This will read AC hash on the line. You will to see that between 50-150 millivolts AC. The reason you are seeing AC on the line is that the rectifier in the alternator is not 100% efficient. A bad diode in the alternator will cause you to lose a phase of the three phase alternator. The alternator will current limit. It will make proper voltage until you put a load on it. This can induce all sorts of interference on the line directly into your radios.

While you are a it lets listen to the VOR audio. It would be nice if you had a VOR station that broadcasted HIWAS. This would help separate out Nav from comm or tell us that the problem is affecting the entire radio. This test would also shed some light on the antenna circuit for the comm radio.

Report your findings.

Dave

[chrisktx]

Well finally got it figured it. Replaced four ignition harness. Found a lead with a shade tree repair, under the cap on the mag side. Problem solved!

[DrDave]

Very interesting. Looks like we were fairly close back in March when we started this. I'm glad it's resolved.

Dave