Tek CSA803A Fan power source question.

[JimS]

I'll start with the background to what is going on.
I decided to resurrect my CSA803A after being on a shelf for just over 10 years.
At first it had 4 timing errors on power up test.
Removed the sampling head's I put in, removed the covers & looked around.
Power it up again, with out the heads and regular and extended diag's passed.

I noticed (how could I not) the not exactly quiet Pabst fan in it.
So I decided a good project would be to find a replacement fan with the same CFM.
The original Pabst, from what I could find, is 6300RPM and around 105CFM & a quite high dba.
I settled on a 140mm Noctua industrial fan, 2000 RPM and 105CFM and 20+ dbm quieter.
So with the fan a grill and a 120/140 adapter away I went.
(The fan exhausts the air from the rear of the scope)

The retrofit went well and I decided to check the voltage at the fan.
What I discovered is why I'm making this post.
First test I saw the voltage jumping all around on my Fluke DMM.
Removed the power supply & the regulator PCB and looked it over.
The power source to the fan comes from the regulator PCB.
From a +5V regulator transistor's input, to the power supply.
There is a small ceramic bypass cap, then through a fuse and to the fans plus input.
So far so good, but on the fan's minus side things get interesting.
It goes though a diode to a 10 ohm resistor and from there I lost the trace.
I did not want to dissemble the power supply so I took the following scope measurements.
This is the first time I have run into this type of signal on a DC fans power lead, no less than the negative lead.
I would think that if you wanted to control the fans speed you just vary the DC voltage, not do what I saw.
Thoughts, Ideas??