Update readme.md
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@ -6,8 +6,7 @@ This unit has clearly seen better days as the pushrods are gone and it looks pre
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On inspection, there are some initial problems that are quite obvious, namely burnt resistors on the main A1 DC measurement board. These have been burned to a crisp, probably due to overvoltage. Looking at the schematics, these are two 5K resistors along the DC input path. Looking at the components, they appear to be CRF65 resistors, very similar to the CRF65. Luckily I had some resistors on hand that could function as a suitable replacement with some artistic creativity. Some measuring around the input section for any other bad devices also made me inclined to believe all was in order.
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Selftest afterwards reveals seemingly no issue with the DCV section, excellent!
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## AC trouble
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Great! Having sorted out the DCV section so easily, I can now move on to the next error the meter displays on power up; Error 202, Hardware failure - AC board. My clue here is that I don't hear any clicking relays and the fault seems to happen almost instantly. This leads me to suspect that the fault is gitial in nature. Troubleshooting reveals that fuse F701 has blown and the meter now has no 14V supply rail on the AC board. Fixing this by inserting one of the spare fuses allowed me to move on, but the error still persisted.
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@ -37,7 +36,7 @@ Another healthy upgrade I did was to update the firmware to the latest version,
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- Revision 5,X: DCV measurements on the 10 V range were shifted by 0.13 uV (within specifications but slightly biased)
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- Revision 5,X: Sending “AZERO ONCE” does not always cause the 3458A to immediately make the autozero measurement.
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## U180 drift evaluation
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Now the big question with any 3458A project is whether the U180 ASIC at the heart of the meter is still in good condition. This requires an extensive check, powering the meter and monitoring the calibration constants from the ACAL DCV. For this, a toolkit was written. And although this is a rather short data capture, several captures were made. All with similar results in terms of the drift observed.
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@ -57,3 +56,4 @@ Thus it is interesting lower the setpoint of the LTZ1000 voltage reference setpo
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## Mechanical touchups
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Unfortunately this unit came without pushrods making use slightly more difficult. After unsuccesfully trying to buy some parts from the manufacturer I have decided to design some #D printable ones. To enable hobbyists to repair such missing parts. These will be added in this article later as I finish up the 3D models.
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