The older style controller that used analog components was indeed repairable, which is the main reason that units made now are digital. Harder to troubleshoot and six months later you can't purchase the controller chip if you wanted to. Also, they would be programmed and the only source of the programmed controllers is the manufacturer. And by the way, they don't sell just the controller chip.
We had a similar failure on a Sears microwave oven, in that the failure was an opto-isolator that drove the power switch, but the only recourse was to replace the whole circuit board at a cost equal to the price of a cheaper replacement oven. Not as many features, but would you pay $250 to repair a microwave oven?
This controller sounds very alike the technology that I saw in the Luigjes Hottop solar boiler which I installed in my home (in the last century ;-). Including the single sides PCB and dual comparators. Only this one controlled the pump via a relay contact, so there was no high current through the electronics.
Its operation was extremely simple and I doubt that even today you'd find a digital controller because the analog controller was so simple. Maybe just for calibration purpose it would be made digital (and to allow the boiler temp to be displayed so the user would have a nice display to look at instead of the two LEDs on the original controller.
The function of the entire controller was executed by the two comparators:
1. Is the temp of the solar panel a set amount of degrees higher than the water in the boiler? If yes, switch the pump on.
2. Has the temp of the solar panel reached close to boiling? If yes then switch the pump off.
The first comparison guarantees that only energy (hot water) will flow into the boiler, no hot water will cool down in the panel without the sun shining. The second will avoid the water getting pressurized from steam production and the boiler be damaged. The solar panel may get (very) how without water in it (NOTE that there was a quantity of air in the system that normally filled the solar panel when the pump was off - that protected the system so it could not freeze or boil). When the pump engaged, the panel was filled with water and the air moved to either the return-flow pipe or the designated container in systems with large height difference between boiler and panel, like in my system.
The calibration required for the controller were two potmeters to set the temp difference between boiler water and panel before the pump turned on and the max temp of the panel before the pump was turned off again.
The system was very simple and reliable and I enjoyed it for many years until I sold the house. I still plan to go back and ask if it is still working - I left a spare controller just in case.
I had a similar problem with the controller for a double oven. When the manufaturer's repair guy came out he told us the control unit was no longer produced. We would have to reaplce the over. This was a $2K+ proposition. Oh, and by the way, the new units were slightly larger, so the hole in the cabinetry would have to be enlarged, if possible. Sounds like a way to make work for the contractors. I took the unit out and confirmed the problem. I isolated the problem to some power transistors. Rather than fixing it myself from that point I found that there were several companies that would refurbish the units. One was a short drive from home. So, I took it there and they fixed it. It has worked ever since.
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