Keithley 2400 Display not working

[Remdawg]

VFD display on my Keithley 2400 is not turning on, and there were no visible signs of damage.
I get 5v on the display board supply header and IC's but there is nothing on the 37V supply header for the display, nor is there any high voltage supply going into the display driver IC's.

The manual also mentions that the VFD is supplied with 60VDC and 5VAC directly from the digital board,
which are supplied by U625 but this does not exist on either the digital board or the analog board
(as far as I can tell, it is neither mentioned nor shown anywhere else in the service manual or on the physical board.)


I read correct voltages on TP5 - TP9 on the digital board.

[TiN]

Photos of the screen? Is getter shiny?
On 2400 and 2002 and few others DC-AC/DC module is on front panel pcb, not the digital board.

[Remdawg]

The getter looks fine, I can see the DC-AC/DC module you mention are you aware if it uses 5v and or 37v as the input voltage. As I did not find a 37v dc input to the display board I started trying to find the supply on the digital board.

[TiN]

Yep, glass looks good. It's using 5V input.

[floobydust]

Looking at Model 2001, U625 takes 5VDC in and generates 5VAC for the filament and 60VDC for the VFD drivers.
The 37VDC rail seems to be for the HV output stage, the display does not appear to use it. P1005 pin 9 is 60V not 37V? is that a typo in the manual.
I would look for signs of life with the DC-DC converter and display MCU.

[openloop]

+60V for the driver chip and AC for the filament are made on the display board by T901 from +5V.

T901 primary's middle tap is connected to +5V. other ends of the primary are connected to collectors of Q901 and Q902.
Those two guys pull it back and forth, controlled by a separate feedback winding.