C174 and D76, D77 and D78 also need to be checked...
The Beep runs thru the appropriate limiter to control the "burst" - if the ALC or AMC sense (or Q39 or Q60 for that matter) is gone or modified, Q39 can ENGAGE before the Beep is emitted when you switch from TX -back to- RX
C158 controls the DURATION length of the beep...
As far as BEEP only on AM? Er - no, it can work on SSB also.
D81 FORCES Q36 to disable (Mute) audio so if that Diode is shorted or if you're having issues with Talkback and audio - D75 if it seems ok, and Q36 isn't "toggling" like it should - people do strange things to try and make the Beep do things it shouldn't - check Q36 back to the Talkback circuit Q42, Talkback switch Q44 and Squelch Q43- even old caps can play games with you..