The International Cessna 170 Association

Latest update.

-With the intercom breaker pulled I still get the tone so I believe the issue is with the radio.
-The tone/squeal is not there when I disconnect the new antenna coax cable from the antenna pigtail off the back of the radio
-the new coax shielding has continuity with ground and the inner wire does not
-the shielding of the antenna pigtail off the back of the radio has continuity with ground and so does the inner wire

It doesn’t seem right to me that the inner wire of the antenna coax pigtail off the back of the radio and the shielding are grounded, but I’m not sure how radios are setup. Other than connecting the new coax I didn’t do anything with this coax from the radio. Any thoughts?

Possibly you’re seeing 5K ohms to ground ?

Some easy to check items are USB charging units plugged into cigar lighters, cheap versions of these sometimes cause interference. The other thing are certain LED lights.

lowNslow wrote:Some easy to check items are USB charging units plugged into cigar lighters, cheap versions of these sometimes cause interference. The other thing are certain LED lights.

Good things to check. I did check the USB charger with no change to the tone. I also get the tone with all the circuit breakers pulled except the radio (so no LED effect). The only time I don't get the tone is when I disconnected the new antenna coax cable at the junction shown in my last picture.

DS

Don't transmit without an antenna. It's not good for the radio. If you want to test the radio without the new coax and antenna you have to replace it with a 50 ohm "dummy load". A BNC coaxial terminator like this one would work. Many available, that's just one example. They're usually only rated for 0.5 to 1 watt, but should be ok for a one-second keying of the transmitter.

Measuring the dc resistance of the RF output of the radio doesn't mean much. Measuring the dc resistance of most RF components doesn't mean much. They can work perfectly well at 100MHz but be short or open circuits at dc. Transformers and inductors have low dc resistance, and according to this schematic of the KX170B that I found on the internet, that is what the meter will see if you put it on the radio's comm antenna connector.

Double-check the other stuff you worked on, too.

I’m not sure how, but the squeal/tone is gone when I transmit. Don’t know why. Only thing I really did was ring out the new coax to make sure it was fine and it reconnect it.

However, now I found a new issue. I have a Zeftronics voltage regulator that cuts out when I press the push to talk button. Start the plane, run up to 1400 RPM and it starts charging fine. If I press the PTT switch on either side the generator stops charging. Reset the breaker and it charges again until I press the PTT again. I have something mixed up.

Doug

Here's a guess: Somewhere in your wiring, 12V power is shorted to the push-to-talk signal. The PTT keys the mic by grounding the mic key input, but it sounds like it's also accidentally grounding a power wire. Not enough of a short to blow a fuse, just enough to cause a low-voltage transient that trips the regulator. It was kind of happening before, causing the squeal, but it's gotten worse.

First place I'd look is at the long rear connector of the KX170B. The PTT and switched power pins are directly across from each other in the connector. Maybe one is touching the other? Maybe a pin isn't inserted in the connector shell correctly?

Does the generator drop out if you switch off the com side of the radio and then key the mic? It won't transmit but does the generator drop?

It's also possible a mic key solder tab on a microphone jack is touching something it shouldn't.

That's the first thing I'd look at.

Figured it out!

I had a friend of my A&P come help me troubleshoot my push to talk issue today and he solved the puzzle! My new wiring was all correct, except I had routed the new coax cable along the field wire. He clipped a couple zip ties and separated the coax by an inch or two and at a 90 degree angle to the field wire. Problem solved. It ended up being RF interference causing the tone (squeal) in my radio and also a voltage spike in the field wire that was shutting off my regulator. I learned about running this type of wire at 90 to other wires a couple decades ago, but that had been long lost in my memory.

I was ecstatic to find out nothing was wired incorrectly or broken and it just took a couple zip ties to fix it.

Thank you to all who helped me troubleshoot this issue!

Good find! Glad it was easy and cheap!