QRD's were very common and now somewhat of a dinosaur.
We have tons of Sorensen stuff around here and lots of manuals
in the file cabinet. I'll look and see what is there. I don't think
we own anything smaller than 40-2, but I think the manuals
cover several models. Most of ours PWM the input to the power
transformer and my two biggest problems are open capacitors
in the control circuits and poor connections at the edge connectors.
I see every one of our supplies about once every two years because
it is reportedly 'acting strange'.