Well, service quote was approx. 10K ...
I did some reverse engineering:
U201 is a 'MKE02Z64' ARM Cortex-M0 microcontroller
all regular TekVPI signals are routed to this MCU and protected with DA112S diode arrays e.g. U901, U801 etc.
J201 is a 2x5 programming adapter without JTAG but with SWD
U501, U401, and U301 is a 'CY7C655-32' USB Interface IC USB-Serial Dual CH Bridge
the D+/D- is routed to the additional two pins above the regular TekVPI interface
Compared to
https://w140.com/tekwiki/wiki/TekVPI_connectorit does have two additional pins which are used for D+/D- of USB.
Each TekVPI interace has a load switch on its +12V rail (U609, U608, U607, U606, U605, U604, U603 and U602) 'LDRB' in a SOT23-5
The RGB led of the power button as well as the power button are routed individually to J1501.
This confirms, that my initial guess was wrong as the ribbon cable is required for switching on the oscilloscope.
There are two buffers U1501 - LC125A marking and U1502 - AHCT125 marking.
https://www.ti.com/lit/ds/symlink/sn74lvc125a.pdfhttps://www.ti.com/product/de-de/SN74AHCT125The signals of the MCU are routed to this and the input/output then to J1501.
There are three inputs and two outputs from the MCU shifted to J1501.
The output enable signals are all tied to GND.
Pin 7 and 8 of the J1501 is USB
As the original failure is mentioning SPI, I guess that
MISO and IRQ are the outputs
MOSI, CLK and CS are the inputs
I will try to connect a logic probe to the signals and record if there is some traffic as I do not believe that U1501 or U1502 is the issue but instead MCU U201.
I will also try connecting a debugger to U201 but even if it is running, I guess that Tektronix will have locked it.
If it won't run, I would need the binary of the MCU ...
Honestly, I think that this will be the point where the repair will stop as nobody will have that image or?
Or does someone have a spare Probe Interface Board or is willing to try to read out the MCU?