Very similar to how my TDS2012B died.
Same main board for 100 and 200 MHz models.....yes 100 can be hacked to 200 MHz.
The service manual specifies one of the checks is the probe comp output and yes when its just 1V and 5KHz you have a problem !
First, of course check the PSU.
Second, carefully inspect the top side of the main PCB. Tek used an open top design for ventilation and contaminates can fall onto the PCB and over time corrode IC pins.....that's what happened to mine with one pin on U900 corroded through !
So very very fine to repair...yes I've tried and as the chip is 100 pin and programmable I haven't replaced it yet.
Last time I looked it was a $30 IC.