The BNC goes directly to chassis ground, so, your answer
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is no. There is no weak point between the BNC and the
power line bond wire. Working on telephone station equipment,
I occasionally got caught by the grounding conventions, and
once burned up a probe ground wire on a 2465 without issue.
Have you solved the problem of your cursor vs the 1st and
11th graticule line yet? If not, you are still playing in
the wrong ball park. You cannot pass CAL01 and CAL02 if you
cannot get the cursor and the graticule to match each other.
The cursor is how the calibration routines know how wide the
spacing between the graticule lines is.
Also, it sounds a little to me like you don't understand that
Tek used the opamp as the "pass transistor" for the -1.25 and
+1.36 reference power supplies.
Siggi, Chuck, thank you for your answers,
I'll upgrade the firmware to -09. Also, hope that the calibration procedure will be more forgiving (maybe)....
Also will see what is consuming 11mA. After checking the TL072/082 and LF347N datasheets, found in LF347N the maximum voltage swing vs sink/source current rating, which is -/+12V under -/+10mA. So the operating range is close to its limit but not marginal yet.
Have to check C731 220uF capacitor on A1.
Well, I've made a small LTSpice simulation (see the schematics below) to check the DC operating point and find the currents and voltages:
It's very unfortunate, but it seems like everything is well in spec, so I've changed the U2420 for no reason.. Siggi, thanks a lot for offering the help in opening your scope, but it looks like everything is nominal and designed to be like this (arghhh)...
-1.25V reference and +1.36 runs through A1 and also the front board. All the pots are connected to these two rails. This circuit also includes the hold-off, SWEEP A/B ICs and even the attenuator temperature controls (every single thing measured back by ADC runs off the DAC 1mA REF. Logical, but scattered and fragile).
What I found really weird on the running oscilloscope is the difference between the A5 ground and A1 ground potential is about 0.3mV. 300uV! Also same gradient found on A5, going from 0 to about 0.3mV as measuring between ground traces on the board.
If someone have connected the o-scope to HV or the life AC and got the frontend busted, I could only assume that the ground could get compromised on A1 (somewhere between the BNC probe ground and the AC plug ground).
When you have repaired the o-scopes have you found a weak point of some sort, which can be damaged by the excessive current between BNC and the power socket GND?