Other options?
Tr41 and TR42, - else very little gets in the way.
Hows' your 8V TX - get a Red light?
A catastrophic failure of say a diode - would affect AM - but not SSB - UNLESS TR41 blew open - means no signal or any voltage gets to the Driver and Final.
IF R193 is still in there, that would allow you to "hear" SSB signal very weakly on another close-by radio if you threw a dummy load on the back at the antenna jack of THIS radio.
So no signal on SSB usually indicates failed MODE, (blown trace/Burnt switch contact) or TR41 and TR42 are suspect. Carrier but no Modulation - check Mic Amp and Mic routing.
So logic to me, says TR41, TR42 and MODE switch need to be checked out.
Mode switch first, for if SSB doesn't work, the switch is suspect, the SSB signal arrives to the TX MIXER - so the entire strip amplifies signal. - also failed TX power to this strip also affects that "modulation"
You say you get Carrier, just no mudulation - so that rules much of anything else except MODE, TR41 or TR42.
Did you put an external speaker on the PA jack and try to see if the CB/PA switch is working?
IF you get PA, then most of the Mic strip is functioning. (I said MOST)
In SSB mode, no modulation usually means Failed Mic Amp strip. But if it was working then just "quit" I'd look into making sure you're in LOCK (Red Light), and/or the MODE switch didn't fail - blow open - to stop sending power to the Driver and Final - which in themselves can - if they fail - will take out the switch too - when they blow shorted. You get Carrier so it's does not appear as a Driver or Final problem. - But you should check the "center leg" and make sure the Driver and Final are getting power. MODE switch in SSB.
IN the above Mic Amp strip - TR22 and TR21 work together - AND - if any part has failed shorted - they tend to OVERAMPLIFY - as in you can see they have bias but if higher by a volt of so, they will "CL
IP" and the audio disappears - simply because the amps are working now like switches than LINEAR signal amps.so you don't hear ANYTHING. You said a Recap a few years ago, so otherwise the Mic amp if the PA amp works, is still Ground Zero amongst the others...
This effect can be reciprocal - one failed mic amp resistor - the train stops dead in it's tracks...
IT's why I asked about the "RED TX light" if the RED light is on, you're in LOCK so the MB3756 would let you get AM - you say you get carrier - so I can presume you have LOCK and the radio wants to transmit.
In SSB - if the switch went - blown open - you'd wouldn't get any power back there in the SSB modes (Both USB and LSB) so no SSB.
But AM? You might because of the poles - the wires from the switch route thru so if one wire opened the pole could still send power thru the other mode (in this case AM)
If you can put a Speaker on the PA switch and it works and the MIC amp seems to be ok - well, then it narrows this down to TR42 being shorted or TR41 - since you have AM TR41 works - to a degree - and that requires TR42 - but if TR42 is shorted - doesn't matter TR41 is still on, tracking what Bias is being sent thru TR42.