[Drake] TR-7 SSB Modulation Disappeared ???

[w0ng at comcast.net]

Hello all....  With the terrific help I received with my last question (intermittent digital display) I'm hoping some of you might suggest a solution to a curious thing that just occurred to my TR-7.

I've searched all of the archives and advice that has been published on the internet but haven't found anything that addresses my specific problem.

I've been using a Heil BM-5 boom/headset mic with my TR-7 (while it is on the workbench) with no issues. Good modulation waveform, good modulated output power.

Tonight, for no apparent reason, the ALC lamp now will no longer "glow." SSB & AM modulation is gone no matter how much the mic gain control is advanced. One minute it was fine, the next minute there is no modulation unless I literally scream into the mic, then I get about 15-20 watts output but nowhere near 100% modulated.

On CW, the ALC lamp works (glows) just fine. On CW, output is great on all bands except 10m (only ~50 watts).  I checked the output of the mic on my scope and it looks normal. 

As I begin again to study the the manuals hoping something comes to me, I'd appreciate any suggestions as to where to look first and any solutions that have worked for others.

My longtime SK Elmer (Sid, WB8FYI) used to tell me that "you don't learn nothin' about troubleshooting from a radio that works good." Looks like my first TR-7 will be a good one to learn somethin' from with your alls help, hi, hi.  Thanks & 73, Bill, w0ng