Hello again,
Ok that might help.
As i said before, this could have happened due to a mod or that they all became defective due to a problem that caused them to exhibit this behavior, but let's proceed as if it was a mod because that is the most probable.
The purpose of a mod like this is to limit the range of the pot and/or get finer resolution on the adjustment range. The simplest way to get this to happen is to increase (or add) the resistor on one terminal of the pot so that adjusting it only allows the total resistance on that side to go only so low. For example if we had a 5k pot and got adjustment from 0 to 5v but then added a 5k fixed resistor to the bottom terminal we might only get an adjustment from 2.5v to 5v.
So the first thing to check for is the two resistors that connect to the voltage adjustment pot. That pot on your schematic is labeled "W5".
You can also note that the "top" resistor is W6A, and there is no "bottom" resistor. However, they may have added a bottom resistor so you should check for that.
To see how the top and bottom resistors are added you can look at the current adjustment pot which is W6 on this schematic.
Note the 'bottom' resistor is 1K adjustable, and the 'top' resistor is a 4.7k adjustable in series with a 3.9k fixed resistor. The top resistor usually limits the top adjustment range and the bottom resistor usually limits the bottom adjustment range but take note the functionality could be reversed.
So look for the pot W5 or equivalent and try to find the top and bottom resistors. On the schematic there is no bottom resistor, but they may have added one in series with the bottom terminal of the pot so look for that. On your supply they may actually have one anyway even if not shown on the schematic.
If you find a resistor addition modification then it becomes obvious that they added it and it has to be either shorted out or reduced in value. I would try reducing the value first and see what the effect is on the operation. To reduce the value by 1/2 just parallel it with another resistor of equal value, and that should be a pretty good test.
There are other ways to get a limit on the low end, but let's hope they did it the simplest and most common way first.
Even if the power supply is different than the schematic the idea is the same on almost all power supplies like this.
So to recap, first find the voltage adjust pot, then find the bottom resistor, parallel with the same value resistor, test to see if the bottom end range has moved down lower so you can adjust below 5v then. If you can, the test was a success, if not, you have to take that resistor off and parallel the top resistor and test again.