Radio troubleshooting--looking for advice

I need some troubleshooting advice/education. I’ve owned my RV-4 for 15 years and the radios have always been good. A few months back I noticed was intermittently scratchy and faded (one transmission would be clear, the next faded and quiet) yet, according to others, I was transmitting loud and clear. Problem existed on both my primary KX125 and backup KX175B. Because the problem existed on separate radios with separate antennas I suspected the intercom—an old 2-place GCA 400—as the common failure point…and also started having some intermittent ICS comms with passengers. Cleaned all contacts, only marginal improvement.

Last weekend, I noticed that if I turned up the volume pretty high on the radios—almost max—both radios were loud and clear. I just turned down the volume on my headset and was good to go.

Ignorant question: The radio issues existed in both ICS and Isolate modes. If the radios independently seemed to work fine at high volume, how would the ICS affect that (i.e. be to blame)? In other words, is the problem likely to be from the ICS or something else? Any recommendations on how to better troubleshoot? I feel like I've tried multiple variables but haven't come any closer to determining the root cause.

Jacks maybe

Hi Indy

In this type of audio panel/intercom and given the age, and since we are looking for a common point I'd look at all the jacks, in paticular the headphone jacks. Not just if they are still mounted tightly but the contacts which could simply be weak, as in not as springy, corroded, solder joints. You may simply want to replace the headset jacks if they get flown a lot and you remove the headsets each time. They will get weak and loose their coating over time. That will clean up the solder joint obviously as well.

Hope that helps as a starting point.

I thought about that. I visually checked them...but that doesn't mean I know what I'm looking for. Another ignorant question: if it were the headphone contacts, why would the radios be clear at high volume? Is there resistance across audio jacks whereby higher volume basically just overpowers that resistance?

headphones are the resistance

The headphones are part of the resistance when connected to the audio jacks but poor connections create additional resistance. An easy test is to jiggle the headphone jacks.
What you are describing is pretty common in musical equipment (electric guitar as an example) where a device that is often plugged/unplugged eventually gets worn out. You won't neccessarilly see anything if it's just the spring tension of the jacks themselves.
It's also just a starting point. It may go deeper than than but we are eliminating the low hanging fruit.

Have a look for an earthing failure, just try moving any earth connections. That seems to be the cause of a lot of hard to find electrical problems.

Get some spray electronic control cleaner, open up the radios and intercom, and spray all the volume controls.