I agree with the comment that they'd rather sell you a new circuit board than troubleshoot to component level, mostly because the current avionics techs don't know how.
Electrolytics eventually dry up. No one repairing and restoring old radios will ever disagree with that. It's the first place one looks when a radio starts acting up that has many years of service. Sometimes they ooze their guts out and damage the traces around them on a board, but it's usually repairable.
I've got all sorts of vintages of radios and gear in regular service, and there's nothing to "wear out" in a radio physically other than migration of the conductive material inside transistors that are pushed to their limits in the RF power amplifier sections, and that takes typically 30 years or more. Well, that and switches and knobs...
The KX-155 is a well built radio for its day. Opening one up, the layout is excellent and the Achilles heel of the radio is the vacuum fluorescent display. Those die with some regularity and are getting harder to find. The manufacturer stopped making them long ago. Additionally many of the older radios don't meet the RoHS requirements that changed all the components they were built with, and mixing lead-based solder with the newer non-lead variety isn't advised in most cases.
I love our 155s but they're at a price point now where at current shop time rates, they're not worth repairing, usually. The audio quality is excellent from them, since they were engineered at the peak of the analog engineering age. There were engineers on staff who could tell you exactly how something sounded if a specific R/C circuit was applied, and had the component values to impedance match with best audio cut-off memorized. And by best, I mean, tailored to the audio job the device was going to do.
Garmin radios sound flat and lifeless to me, like their audio sections were simulated on a computer and built from that. And they probably were.
