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@ -17,11 +17,18 @@ It seems that U008 was the culprit. I have a feeling that the failure mode had s
To save the meter from an early demise, I replaced all the capacitors, and indeed some seemed to be leaking already, I was lucky to replace them before the corrosion got worse.
![LeakyCap](3458-WorkLog/imgF.png)
Another healthy upgrade I did was to update the firmware to the latest version, which improved the accuracy of the meter to some extent due to the various mathematical updates to said firmware.
Another healthy upgrade I did was to update the firmware to the latest version, which improved the accuracy of the meter to some extent due to the various mathematical updates to said firmware. In the service note [here](/3458-WorkLog/SN12C.pdf) one can find the fixes listed. But as a quick overview on some fixes from Rev 4 to Rev 9 are:
- Revision 8,X: Period measurement correction
- Revision 7,X: Resets line frequency to 60Hz
- Revision 6,X: ACV adjustment corrected
- Revision 5,X: False self-test failure at elevated temperatures
- Revision 5,X: DCV measurements on the 10 V range were shifted by 0.13 uV (within specifications but slightly biased)
- Revision 5,X: Sending “AZERO ONCE” does not always cause the 3458A to immediately make the autozero measurement.
## Final bugs
Now that the meter seems to be working, I tried ACAL with the meter after letting it run, and now it shows me an error I have never seen before. error 204 flatness dac convergence 199. The error occurs while the meter is warming up and the ACAL step is always 100V. This immediately made me suspect the HV attenuation part, mainly the DAC. Cooling the board slightly, and seeing which component cooling makes it work, made me suspect U302, the LT318 op-amp behind the DAC. However, after replacing it with a different one, the problem still seemed to persist deterring me from assuming the fault is U302.
![amprep](3458-WorkLog/imgC.png)
Now, the resistors for this amp seemed to be affecting the pass or fail, replacing them and retrying indeed made the error disappear. Interestingly, looking at the circuit in question I would not intuitively expect this to fix the fault.
Now, the resistors for this amp seemed to be affecting the pass or fail, replacing them and retrying indeed made the error disappear. Interestingly, looking at the circuit in question I would not intuitively expect this to fix the fault. Interestingly, in the firmware issues listed above one finds an imporvement
![resrep](3458-WorkLog/imgD.png)
## Calibration
@ -35,10 +42,12 @@ Now the big question with any 3458A project is whether the U180 ASIC at the hear
## Improving
If one looks at semiconductor ageing there are a few things we can do to improve the stability in the long term.
Firstly, it is possible to lower the current passing trough the reference devices on the die, this in turn will decrease the drift experienced from electromigration. This is the movement of material caused by electons bumping into then. Thus this could be lessened by decreasing the amount of electrons troughout the device. This coming at the tradeoff of extra noise.
Firstly, it is possible to lower the current passing trough the reference devices on the die, this in turn will decrease the drift experienced from electromigration. This is the movement of material caused by electons bumping into then. Thus this could be lessened by decreasing the amount of electrons troughout the device by decreasing the amount of current. This coming at the tradeoff of extra noise.
Secondly and majorly is chemical reactions over time, though stable a chip still exists upon a mixure of chemicals all slowly reacting on oneother. Thus [Arherrius' equation](https://en.wikipedia.org/wiki/Arrhenius_equation) comes into play, where reaction rates increase as temperature rises. Emperically this has been tested to double the drift for every 10degreesC.
Thus it is interesting lower the setpoint of the LTZ1000 voltage reference setpoint. Well known on the 3458A is the voltage reference being operated at an extremely high oven temperature. As such I have added a 200K resistor to the voltage reference board at designator X411, decreasing the temperature to about 75degC.
Thus it is interesting lower the setpoint of the LTZ1000 voltage reference setpoint. Well known on the 3458A is the voltage reference being operated at an extremely high oven temperature. As such I have added a 200K resistor to the voltage reference board at designator X411, decreasing the temperature to about 75degC, giving me a bit more overhead for high ambient temperatures than [traditionally is done](https://xdevs.com/fix/hp3458a/#opt002).
![ltz-mod](3458-WorkLog/LTZ-MOD.png)
## Mechanical touchups
Unfortunately this unit came without pushrods making use slightly more difficult. After unsuccesfully trying to buy some parts from the maufacturer. I have designed 3D printable pushrods for the DMM to enable hobbyists to repair such missing parts.
Unfortunately this unit came without pushrods making use slightly more difficult. After unsuccesfully trying to buy some parts from the manufacturer. I have designed 3D printable pushrods for the DMM to enable hobbyists to repair such missing parts.