Well I just saw that Sr Javier made it back to the scene and needs a more positive answer.
SO . . . . I pulled a radar IF amplifier strip assembly off my $100 an hour spot on the bench and cobbled up that regulator circuit to evaluate its operation at the very low voltage levels.
Indeed it does start getting dodgy at about 7 V, where its stops having a linear change and starts having a cramped /compressed variance until it stops.
That must relate back to the reason for the 4.7 V zener shown in the initial schematic . . . .to make an abrupt limiting knee on adjustment in the low voltage end of the spectrum.
This action / evaluation was being ascertained by using " Breadus Boardamus ".
My circuit designs should be regarded as experimental. They are unbuilt but have been tested by Spice simulation.
Back in MY initial knowledge base spectrum spread . . . . . initially in the 1946'ish era . . . . . Spice was a condiment !
Now in your situation,with a parts availability / obtainability aspect . . . . . . . . is it easy to get a LM317 there ?
It would also be able to use a pass power transistor as a preregulator between your present switch mode power supply to INITIALLY get down below that 40VDC LM 317 voltage input threshold .
Am i correct that a max of 1 amp is the power supply's current supply specification ?
Not that It will help you but I have 3 LM783 high voltage regulators on hand in my stock that work with up to 125 VDC input and will regulate down to that desired low adjustable voltage that you want.
I find that mailing one to you from Texas to Columbia will cost 20 times the initial price of one.
The Lm 783 uses a higher voltage Mosfet instead of a conventional bipolar transistor for its HV pass transistor
http://www.ti.com/lit/ds/symlink/tl783.pdf
(Refer to Page 12 Fig 20)
Or if you can supply a model number AND brand of that switch mode power supply of yours, we might research its design to see about modifying its output to get it down to just below that 40 V specification.
Am I correct, that a max of 1 amp is the power supply's current supply specification ?
By inspecting your main power supplies schematic/ design, the voltage limit may be as easy as changing the voltage division ratio of two resistors, or their associated zener diode, associated with the units optical isolators feedback circuitry.
73's de Edd