Kenwood TS-140

[Eva2256]

gnif: Post was edited to spam, account banned, content removed.

[Stephen_Pejak]

Well, because you don't have any problems outputing higher power with different input suggests the problem is not in power supply nor is it in the main amplifier. Couple of things could potentially cause this. I will assume that the amplifier uses some kind of MOSFETs but the story is similiar for BJTs(because you didn't mention anything about distortion if you have BJTs, I will assume that you are getting propper biasing in case it's A class). So there are couple of things that the signal passes through, when it is on USB it has to pass through DAC and some filtering so you might consider checking it. After it  goes through DAC and filters it goes either to some communication chip that connects it with main preamplifier IC(sometimes integrated in main amplifier IC) or it goes directly to it. All in all those systems aren't easy to deal with, and they sometimes tend to be very complicated. But the fact that you are getting good output with one input and bad output at the other, again, suggests that the problem is somewhere before the amplifier circuit. Also make sure you check outputs of voltage regulators because they can make ICs malfunction if they are oscilating.

[CaptDon]

45 watts on AM and FM is the 'carrier' power. I bet your modulation is nearly zero in those modes. SSB output depends on Mic Gain and a control called either 'power' or 'carrier' (my TS-690 has both) The problem is more than likely low mic gain or an incorrect/broken mic.

[Wallace Gasiewicz]

Captdon has a good point 
Here are some suggestions: 
Check and see if the power out is variable with the slide control, if it is then  it is the Carrier only.  If this is the case, Problem is in the audio portion of the radio   
 Check the Mic connection and especially your Mic  
If you have an audio generator put a one KHz signal into the Mic input pin and key the radio (push SEND button) and see if you get SSB output from the radio.  If no output then: 

Look for the mic amp and see if there is audio signal 
See if the audio signal gets to the Balanced Mixer   
If audio and IF get to Balanced mixer, then look at the mixer.