Hi SK,
I'm pleased to hear you tried the mike first. Just didn't catch any mention of that in the thread.
A great many breakdowns fall into the 'routine' category. Statistically, the same small list of things tend to break the most. From one brand/model to another, the list will change, but the same principle tends to apply.
Troubleshooting one of those with suggestions from forum posts can be surprisingly easy. Just takes somebody with experience on that radio, asking the right questions, mostly.
Then you have that "one in twenty" or a hundred. A failure that just doesn't happen over and over in dozens of that model. An "oddball" problem. This is when a basic tool like an oscilloscope pays its way. Simply tracing your audio signal, from the detector, to the volume control, past the squelch circuit and then into the audio power chip is pretty fast and efficient that way. It's been long enough since I had to do that without a 'scope that I've forgotten how. Reminds me of using a dime for a screwdriver. Awkward and error-prone.
That being said, there's one more "low-tech" test I haven't suggested. All that it does is narrow down where to look.
Think about the loud "HUM" you get from a guitar amplifier if you pull the plug out of the guitar with the volume turned up. Touch the tip of the guitar plug with a finger, and you get the same loud hum.
Turn on the radio, and turn the volume full up. Take a screwdriver with a thin tip, touch it to the clockwise lug of the volume control. Lay one finger along the screwdriver shaft. It works better if you don't have your other hand in contact with the radio chassis.
You should get an "AC" hum out the speaker, fairly loud. Now do the same to the volume control's center lug. You should hear the exact same sound and level. If not, the volume control is shot. Bridging the center lug to the Clockwise lug with the screwdriver tip will get you FULL volume channel chatter if that's all that is wrong.
If you can get NOTHING from this test, the problem is downstream from the volume control.
If you get a loud hum, and the volume control is okay, the problem is upstream from the volume control.
The "shotgun" approach has two main drawbacks. Statistically speaking, you'll probaby replace a bunch of parts that are not bad before "pin the tail on the donkey" gets you back your receive audio. And every time a part gets replaced, the odds that some small accident, like a solder bridge will occur. This is what's called "muddying the water", adding new defects to the old one. Seen it a lot.
Many kinds of radio breakdown don't call for all the 'high-tech' tools and toys to track down. Some of them do.
73