Hey all!
Just figured the error; the former owner tried to TB the unit and was a little careless with solder....... the cap of the Vref was connected to the V+
Some of the changes that I found when comparing the PCB with the schematic you can find:
The input section was replaced by
this
All 0.05 uF caps were 0.047's green caps
All 22k's were replaced by 24k's
Vref circuit was quite the same (10k, 10 uF = tant), trimmer was replaced by a 25k trimpot with 12k and 7.5k on the outer lugs, the 1M following the wiper was made 470k and the 3M9 was replaced by a 2M2 resistor direct to the speed potmeter (removing the 4.7k)
Another 10k was added on the output in series with the signal
Maybe this can help other persons in the future!
I used
this as a reference
I probed it and I lose the signal after the first IC; I guess it is due to the fact that the voltage on the 1M is very very low. 