I still have it and I actually found a pair of thermistors for it the other day..
I think they are the ones I took off the collectors when I took the system apart.
If it's running all the time, one of your thermistors might be bad or maybe
disconnected (open). The wires on one could be shorted together.
Try reading the DC voltage across them.
You also want to take a look at all the wiring.
Make sure nothing has come apart up on the roof..
If the installer used a wire nut, or tape, it might have rotted off..
If the tank and the collectors are near the same temp, the voltages should be close to equal.
It's very easy to check thermistors with a VOM, since their resistance varies with temperature.
The fact that it's running, means the relay is being energized by the control circuit.
So, most likely the board is still good.
On the slide switch, there is a thin slot, where you can see inside the switch.
All the contacts & the sliding bar are visible. If the contacts aren't shinny,
try sliding the switch back and forth about 200 times. That should clean the contacts.
If the contacts have gone bad, you might be able remove it and solder in some wires..
Which can be connected to a new switch..
Or, just short out the 'Auto' contacts and just unplug it for OFF..
If you have a techic nerd or Ham Radio friend, he might be able to help.