It appears that there is sort of a judgment call on the fan motor used in the 8640B signal generator.
Looking in the 8640A manual that I have (08640-90114) the motor is definitely called out and illustrated in the schematic as of the brushless design.
However, there are differences seen in the 8640B documentation. I referred to a 2-volume manual set for the 8640B number 08640-90215 dated May 91 and also noted as a “Mark-up Copy” that applies to serials 2520A through 2923A plus any included updates for later serials.
Referring to volume 1, there is a call-out in the parts list on page 6-117 of 2 DC style motors:
For serial numbers 2939A and below:
3160-0453 “FAN-TBAX 23-CFM 12VDC .25KV-DIEL” Mfr code 28480. Mfr part number 3160-0453
(Mfr code 28480 is HP)
For serial numbers 2939A and above:
3160-0497 “”FAN-TBAX 23-CFM 12VDC” Mfr code 11039, Mfr part number 612
(Any reference for Mfr code 11039 cannot be found)
However, on page 8B-123 titled “Service Sheet 19” there is a description of the -5.2V supply & fan driver assembly A18 where it describes the brushless motor & associated driver.
However in volume 2 if you go to Service Sheet 19 you will find a DC motor being driven by a simple DC controller.
As for the part number for a brushless motor, I could not find it in either volume.
It’s not that I don’t believe that a brushless motor design was used in the 8640B since there is a hint of the description of one but it also appears that HP once used a DC motor design as well. It looks like a judgment call on this one.