Help me repair Tek 2440 - Display Flicker when running CTE Calibration

[chihaxinh]

Hi Everybody

I have on Tek 2440 have problem , when running in 5 10 min have gain error and runing selftest fail . I open case and force cooling CCD and DAC module by other fan, everything ok when running 1 hour or more, selftest Pass.

Inside case not have must dusty so heatsink very clear. I checking power rail it's all OK , voltage value and voltage rip OK.

So i runing coldstart to carlibration Tektronix 2440 . Runing ATTEN and TRIGER pass but CTE not pass.
NVRam in 2440 using static cmos HM622556LP-12 , and external Lithium battery ( Checking 3.7V OK - Battery not dry )
So I attach picture and video clip , when runing CTE calibration , display flicker and allway report overload in 50R input ATTEN . I Don't know why CTE Calib auto switch to 50R ATTEN input



All information reference you can see in topic
https://www.eevblog.com/forum/metrology/tek2440-mini-teardown-nvram-replacement-and-calibration/

I concern register memory in NVRam wave form capture have error.

Many thank so much!

Edit : Display Flicker only when runing CTE Calibration mode ( You can see in video clip )

[LazyJack]

Ah, the overload explains a lot.
CTE calibration switches the 50 termination on, as the official calibration procedure uses a pulse generator with 50ohms output. If the 50ohms overload comes on, then your input voltage is too high. You should lower the amplitude or insert a attennuator. The calibration requires a fairly low signal level 120mV or something like that. It needs to be 6div in amplitude in whatever the scope sets up the volts/div.
Other than centering the signals, you should not change any other setting that the scope sets up for the cal. The flickering is probably because of the overdrive.