My 148 throws a key
Wow, First off, you have a darn good radio there. I had a 148 GTL myself, years ago. I had a Ham Radio Bench Tech max it out for me. ie.
Enabled full coverage from 26-29 MHz, unlocked the clarifier trans./recv. -15Kc to +10Kc from "0" so I could cover the 20Kc jumps on Ch. 3,11,19, and 22..or 23... I think...(It's been awhile..) Peaked the trans. finals to 25W RMS / 30W Peak on SSB and a flat 12W dead key on AM. And I could modulate 120% with a stock mic. (I eventually switched to a K40 Speech Processor.) Add that to a Siltronix 3 Step Variable Linear @ 5 / 10 / 20X Step, with a 30W Max. Rf. Input, coupled to a pair of Avanti Co Phase (center loaded) Hippo's..(1500W max input rating for each one)...Well enough bragging...You brought back memories...Hope I don't get fired by FixYa...
Anyways,
I believe your audio signal is getting lost in, or before, the circuit that changes and mixes your audio signal to the Rf Pre Amp Output signal.
I assume you know electronics, have a oscilliscope, Audio Freq. Gen. and the proper "SAMS FACTS MANUAL" or a comparable circuit block / component diagram with the standard TSD test points data ref. values. If you don't have the above, take it to a shop. Don't take a chance on the hit and miss R&R parts until you get it working.
OK Here we go:
If your PA works, you can eliminate alot of diagnosis time in testing, and block circuit diagnosis too, providing I understand your above statement correctly.
If your words "throws a key to my bench radio" means you have a steady RF Output, and the PA function works properly, Do This: Does the Mic. Modulation register a reading when selected? If so, your audio input amp./mixer circuit is ok. Therefore, the problem has to be between the PA / CB splitter circuit and the Audio / Rf Mix / Conversion circuit, which is the only circuit ( except for a buffering circuit) between there and the Rf. Pre Amp Circuit.
That is where you need to focus your attention and your tests.