Hi Jens,
Two things come to mind, both around the LPF selection circuits.
1 - There is diode failure such that only one LPF is selected
2 - The HV generating circuit has a failure like Gordon Gibby had.
First, verify the proper operation of the LPF selection changing frequency in receive mode. Then, verify that the correct filter is selected.
If receiving works, verify that the HV voltage is higher than the peak voltage of the output. For 40 watts into 50 ohms, Vpeak = Squartroot(2*50*power) = 63.4 volts.
Be sure to do all tests with a dummy load connected.
73
Evan
AC9TU
