Tektronix 465 soft sizzling noise + fan not running

[paulvo]

As my Tektronix 465 started making some soft sizzling noise, I opened it to inspect.
I noticed the classic dirt in the HV section, and behind the blue plastic back cover, which I cleaned out.
While switching it on, I have the impression that the sizzling noise is not coming from the HV section, but from somewhere more inside. Could it be somewhere inside the CRT compartment itself ?
Another thing I noticed is the fan did not run. Is it supposed to immediately start running after power on ? When I try to rotate it manually, it gives the same physical resistance as say a stepping motor. I expected a free and loose run like for instance a PC cooling fan.
Could that cause an issue inside the CRT section ?
So far the 465 is still fully functional, albeit with a dirty horizontal position potmeter... 
But my understanding is that sizzling noises and error-free operation do not combine .
(Oh yes, as a preventive measure, I replaced the 2 parallel tantalums 22µF in the 5V power supply section with a 100µF/50V electrolytic).
Paul

[james_s]

The fan should definitely be running any time the scope is on. Could the sizzling noise be coming from the fan motor? I've never had the fan in mine apart but I think it should spin freely. Not having the fan working is definitely going to be a problem.

[Alfons]

With the old devices: measure all voltages and especially the ripple. Only then can you go to troubleshooting.

[paulvo]

Some feedback : I measured on the collector of Q1698 about +14,68 V (instead of +5,4V), and behind the 280 ohms resistors R1693 and R1694 I measured 4,5 V (instead of +1,6V).
So I removed the PCB with the fan-motor still attached to it. A tiny 1,3mm hexagonal key is required to remove the fan.

I removed the PNP transistor Q1698 to check it out, but nothing seemed to be wrong with it.
So I put it back into the socket, and applied 15VDC to the supply pins.
The motor started to run !
After that, I just mounted the PCB with the fan-motor back into the Tektronix 465, attached the fan again, and it started running immediately after turn on.
After a while I can hear the sound vary, so the temp sensor is also working fine.
No more sizzling noise is produced. 

Conclusion: there must have been some bad contact of the transistor pins in its socket.