Realistic TRC-421A distortion
 

https://youtu.be/le70OrKchHs
This is a car mount CB transciever that I've had for about 6 months, the handset was broken so I used it as a monitoring rig. Tuned to channel 1 and listening 24/7.
Today I recieved a new Radio Shack handset so I can TX properly from this transciever. TX from it sounds fine on my friends' radios but now I notice RX is super distorted and unintelligible.
The previous owner has installed a 0-400 Hz lowpass filter on B+.

Ok so turns out the problem lies with the built-in speaker. Sounds fine with aux headphones connected.
Even when squelched the built-in speaker makes a really quiet static noise when the radio is on. Could it have been damaged from constantly running?

Does the earphone output come from the same final stage as the speaker or an earlier point? Is it possible the problem is the final output amplifier stage?

Yes, earphones are driven from the same stage as the speaker.

I've isolated the distortion problem to a failure of the built-in speaker. I took it out of the radio and attached it to a hi-fi amplifier. Any audio you play with mid-range volume gets obliterated, but not anything very loud or very quiet.

The speaker must be replaced. How do I prevent this failure from occurring again?

Basically, no worries with new speaker, unless leaving it 24/7 at maximum volume (I think)... must be ok and last "forever" with normal use.

Agree. Failure probably due to paper getting very old and deteriorating.