Hi. I've just recently bought and repaired an old Gossen Konstanter psu.
Here is the simplified schematic:

It uses a rather high voltage auxiliary winding for the control circuit, which I don't understand why.
It is around 90V, and uses 2 large resistors, and 2 series 8.2V zeners to get the stabilized 16V.
The first 6W power resistor get so extremely hot, it dissipates around 5.5W. They used the same trick for the fan. It has a large 12W series resistor that dissipates more than 14W! The temperature reaches 300 degrees C. (I took the motor apart, cleaned, lubed and it runs smoothly)
The full schematic:

I did some cleanup for better visibility:

I have a few questions:
What is the role of R103 in the auxiliary power supply?
Why they needed R105, R106? (2x5W power resistors) These also gets extremely hot.
I have a feeling on how the voltage control stuff works, but don't understand exactly.
I know that it compares the output voltage with the set voltage (given by the fine and coarse potentiometers and the 7V reference voltage) somehow. But can't wrap my head around the fact, that the aux psu sits on top of the main psu's output. What reference point should I take in the circuit?