Re: 2445A calibration

Max Vlasov
 

Hello Chuck,

Thank you for the great help. Looks like we are moving forward with 2445a, slowly but surely. I've recaptured the photo of the calibration CAL02 step 111:

https://groups.io/g/TekScopes/photo/74577/17?p=Name,,,20,1,0,0

Now it seems that the dots are round. The problem was with the faulty coax (BNC plug was a bit loose).

However, all the CAL02 and CAL03 tests with the external signal generator generate LIMIT error regardless the channel number. I have assumed that the LIMIT is detected by the peak detector circuit in U500 by altering the trigger level signal generated by the DAC.
Also I've checked following your advice whether the CAL/NCAL Voltage pot can be left where it was or placed into the detent position. In both cases LIMIT error is generated....

But good news are that after running CAL03, even though the LEVEL error was generated every time, I don't have a problem any more with AUTO LVL. So, when I press the AUTO LVL on channel 1/2/3/4 the scope automatically finds 50% voltage level of the selected trace. So, now it really works. IMHO, there is nothing wrong with U500. Also checked attenuators for CH1/2 in all the modes and all the V/DIV positions. They work properly, no problem (also I assumed that a film resistor is blown on channel 1, but can't find a fault).
By looking at the vertical trace levels when the external signal is applied, it seems like they are off by 1-2%, which is a small error (calibration should take care of that, but it doesn't).

Looks like the scope is fully functional...

Maybe the problem is with DAC and/or with the demultiplexer and sample&hold? I'll try checking the DAC reference. Also I wonder how symmetrical the DAC calibration should be (when judging by voltage measurements on J119). Also I wonder how precise the main voltage reference must be set (10.000V)? Likely I have omitted an important step somewhere, therefore the FW can't close the calibration loop.
I plan to re-check the power supply, DAC output voltages. I wonder if anybody else had anything similar to that....

Thank you again,

Best regards,

Maxim

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