Uploaded December 2024 | Updated September 2026, 1 week ago

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Birt from the "Hey Birt!" channel donated an HP 3438A digital multimeter. Let's repair it and figure out how its dual ramp A to D converter works.

HP 3438A Repair Part 2: youtu.be/Ko1Dukv1Byo

My HP 3438A Service Manual Scan: archive.org/details/hp-3438-a-digital-multimeter-operating-and-service-manual/mode/2up

Viewer Fredrik tells me that the HP 3435A manual has much better troobleshooting info and is almost the same board: keysight.com/us/en/assets/9018-08104/service-manuals/9018-08104.pdf

HP 3478A Multislope II DVM repair: youtu.be/p1r5ze8Xcfw

"Hey Birt!" Channel: youtube.com/@HeyBirt

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"Elevator Music" Credit: Crinoline Dreams by Kevin MacLeod

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00:00 Intro to the HP 3438A DVM
01:32 Observing our donated unit's faults
02:44 I think the switches are the culprit
04:19 Tear down
07:58 The switch springs fight back
09:19 Cleaning the power switch
10:37 Dealing with the interconnected latching switches
12:50 Repairing a latch defect
15:00 Another latch defect
16:34 Cleaning the front/back rotary switch
16:57 Reassembly and test
18:03 The Nanoprocessor
18:16 Calibration
20:44 The ohms range won't calibrate
23:07 The proprietary HP hybrid might be defective
25:14 More calibration
29:03 Detailed look at the dual slope A to D converter
32:49 Nicely calibrated instrument works again (mostly)