That was my intention, but I was a little too blatant about it.
While I'm at it, I might as well show you what I came up with. I didn't try to sim it. It looks like a series current controller, I assumed the transistor type (NPN). Don't know much about these really. And I probably screwed up transposing the circuit.
There are legit reasons for retro e: the mfg no longer exist or is not willing to provide service info and also to learn and understand a concept, but it is wrong to copy (I would neeever do that!).
There is no resistive connection of the collector of TR3 to V+ or another potential of the circuit. Could the diode connected to the collector of TR3 be an SCR (R5 than should go to the gate) and TR3 is used to turn off the SCR by commutation? E