I checked the Link and the schematic, so for Japan where mains voltage is 100V you have to set the voltage switch to 115V.
That doesn't present any problem, what happens with a lower input voltage is that the voltage at the transformer secondary will be a little bit lower, so for example for a 12V secondary, you might have 11V or 10V instead of the 12V. Then it goes into a full wave rectifier and the DC voltage will be 1,414 times higher than that and it feeds the input of the voltage regulators.
Voltage regulators in this schematic have a good margin in the voltage they can accept at the input
for example (approximate values):
So with 2x12VAC at the tx secondaries, when connected in series you have 24VAC, after rectification that will be 33,94VDC that will feed the regulator input to regulate to 24VDC
if you have 10VAC instead of 12VAC (with Japanese 100V mains), then in series you have 20VAC,
after rectification that's 28,28VDC which is fine for the regulator to be able to output 24VDC.
Actually it's even better as it will run cooler as it doesn't have to dissipate as much heat.
Rule of thumb, in Japan when using foreign equipment use the 115V or 110V input.
In the case of the JLM power plant, use the switch at 115V, measure the ouput rails, if it outputs 24VDC and 48VDC (for phantom power) all is well.
You will not damage a transformer or PSU by feeding a lower input voltage (100V into a 115V unit), only the opposite, for example feeding 230V into a 115V transformer.
