Replacement Fan for Tektronix 2230 Oscilloscope

Folks,

I recently acquired a second hand Tektronix 2230 'scope. It powers up fine and reports no errors, the sound of the scope starts with a very high pitched whine and reduces after about fifteen minutes to around about the ambient sound of a TV CRT. I notice that the fan does not run continuously, in fact it doesn't run at all. The oscilloscope also makes a periodic knocking sound, about 2 beats per second. After approx. 30 mins. the scope will turn itself off. After about 5 mins you can turn it back on.

I took apart the scope and removed the fan. The fan turns freely but there is more friction than I imagined there would be. The fan is a 12 V DC Buhler 69-11-52, it has four terminations marked 0,1,2,3. The service manual quotes B9965EPL as the part number.

It is fed by another component also by Buhler, this has no part number marked and I was unsuccessful finding it mentioned in the sevice manual. It has 6 terminations, four to the fan and two from the power supply PCB board.

The thermal shutdown board has a pin soldered into it at one end which is designed to make physical contact with the metal chassis of the cooling fan. when everything is in place.

I can post photos tomorrow, my camera is not here with me today.

I have a few questions that I hope some people may know the answer to:

1) Should the fan run constantly when on?
2) What do you think, would this be a PWM controlled fan?
3) What could I replace the fan with?
4) What do people think about the chassis making contact with the thermal board, how would I emulate that with the replacement, any ideas?

I would be most grateful if anybody could help me with this.