Spent a night over the THM to make it working again. My first idea was to provide -8V to Vee pin of the LCD through a brand new dc-dc interter (TMA 0512s with a resistor divider), but in this way no more contrast adjust.
Looking again and again I found that pin 3 of U607 is connected to an IC within the signal acquisition board. That is a MAX529 (serial DAC) that is used to define the signal that, amplified from U607, regulates the LCD contrast.
MAX529 Vee should be -5V but it was something continuously changing from -1V and -3V.
Ok, let's use the new inverter to put the -5V on IC Vee. I don't want to cut the track, so try to add the -5V in parallel directly on IC pin. Seems to work but, after a while, started again as before.
I then decided to check where the Vee come from. Found !!! it's the ouput stage of the U604. To be precise, the electrolytic highligted in the attached picture (left lower side). It is a 150uf 10v low ESR.
At its ends I should find -5V, but it is not so. Let me try to desolder it and check, as the so variable voltage is a typical behaviour of a broken electrolytic.
As soon as I put the soldering iron tip on the pcb, I felt a typical smell and found electrolyte liquid around the component.
Replaced it with a new capacitor, a 220uf 15V (I have not spare low ESR 150uf capacitors), but it should not be a so crytical component at that stage of the inverter so, let's try.
Go back to measure output voltage and ... well done! I have -4,8V on MAX529 Vee.
Magically the LCD screen started to work, but the better thing is that also contrast regulation is working like a charm.
Spent an hour more to test all functionalities and to be sure that the repair is stable, then close it.
I wanted to share with all my experience, as someone else could find it useful to make his/her THM revive.
mostorer