Re: 2445A calibration
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?