Hi Andy,
That sounds like an irksome situation! I hope you had a nice walk and some relaxing time away from it for a bit.
I’m thinking, if your LTP stage was all sweet-as ,yesterday, and it’s not today, and your’re suddenly getting just 1V PP there, how about catastrophic failure of a passive component , specifically a coupling capacitor?
Perhaps you’ve transiently exceeded a voltage somewhere and now you have a big old leaky thing in hiding.
Alternatively, wildly thinking outside the normal range of options .... is this beast in a building outside? With drastically lower temps over the last few days, what if you’ve got some condensation in there?
These ideas don’t need circuit-specific knowledge , I know , but just putting them out there !
