Tektronix 11801C E5322 T1331 Errors

[metalhead777]

Hello everyone,
I'm currently trying to fix a Tektronix 11801C Digital Sampling Oscilloscope.
When I tried to boot it, during extended diagnostics the error "E5322" occurred.
This translates to an error in (following down the rabbit hole):
E: Subsystem - "Executive"
5: Block - "Subsystem Comm"
3: Area - "Mainframe Comm"
2: Routine - "Time Base"
2: Failure identity - got no information on this.
according to the Error Index Code. I read this from the Scope directly, there are no tables available.
The address where the error occurred is "800C00", it expected "FFFE" and got "0001".

Through an online search for the error code I found this:
https://xellers.wordpress.com/2014/02/26/fixing-a-50ghz-sampling-scope-tektronix-csa803/
It suggests to replace U500 and U511 (battery packed NVRAMs) with alternative parts, because the communication with said NVRAMs (Hitachi HM62256B series) has an issue, which may be caused by a dead backup battery. The NVRAMs are mounted on top of a DS1213 Smart Socket, which provides battery backup for the RAM during a WRITE-process, if the supply voltage fails.
I measured the battery power and got 2.9 V and 2.7 V, so this doesn't seem to be the source of this error. I still replaced the DS1213 and NVRAMs with the suggested replacement part to see if I got lucky, and the NVRAMs themselves would have been faulty.
The error still occurred, as I expected, but after one or two reboots, the error "T1331" in the "Time Base" subsystem occurred. According to the service manual, this points to a static RAM battery failure:

Sockets for devices U500 and U511 provide power for the NVRAM on the A5 Time Base/Controller board. If the diagnostics consistently report a Static RAM Battery (T1331) failure over multiple power-ons, then the batteries should be tested. If either one of the battery voltages measures less than 2.1V, as measured on pin 28 (Vcc), follow the instructions for battery replacement and disposal earlier in this section. (It is recommended that both batteries be replaced at the same time.)

Batteries were okay, so I switched the old DS1213 with NVRAM on top back in, still got the same error.
Also from the service manual:

Turning the instrument off while Extended Diagnostics is executing one of the NVRAM memory tests may cause failure of the NVRAM battery test. If the diagnostics report an NVRAM battery failure, then exit the diagnostics. This will rewrite the confidence words into the NVRAM. Turn the instrument off for at least one hour. Then, turn the instrument back on. If the diagnostics still indicate an NVRAM battery failure, then the battery should be tested.

So I exited the diagnostics, in case I interrupted any NVRAM memory testing. I waited for about a day, and then turned the 11801C back on, and got "E5322", again.
So I'm pretty much where I started. And I didn't find any other solution online.

This leads to my question:
Does anybody have an idea how to fix this or what else to try? My search didn't get me any schematics or extended trouble shooting routines, and Tektronix appareantly doesn't want to give them away.
The service manual wasn't helpful, the programmer manual either.

Thanks in advance for any help

[Testtech]

Go to Ko4bb.com, Manuals, Tektronix, 11801 Factory Diagnosis Manual.

This should have enough to get you going on the right path.

[metalhead777]

Thank you, this looks promising
I'll post again if I got lucky.