# Repairing a Hewlett-Packard 7035B X-Y Recorder, Part 2

**URL:** <https://scanalyst.fourmilab.ch/t/repairing-a-hewlett-packard-7035b-x-y-recorder-part-2/1588>\
**Category:** Continuity\
**Tags:** repair, hewlett-packard, chart-recorder, servomechanisms\
**Created:** [24 June 2022 15:14 UTC](https://scanalyst.fourmilab.ch/t/repairing-a-hewlett-packard-7035b-x-y-recorder-part-2/1588 "2022-06-24T15:14:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![johnwalker](https://scanalyst.fourmilab.ch/user_avatar/scanalyst.fourmilab.ch/johnwalker/32/17415_2.png) [@johnwalker](https://scanalyst.fourmilab.ch/u/johnwalker)\
**Post date:** [24 June 2022 15:14 UTC](https://scanalyst.fourmilab.ch/t/repairing-a-hewlett-packard-7035b-x-y-recorder-part-2/1588/1 "2022-06-24T15:14:49Z")

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[![](https://scanalyst.fourmilab.ch/uploads/default/original/2X/9/98b906beee5d3fb3606ad067bf81a9a46d91694f.jpeg "HP 7035B X-Y Recorder Restoration Part 2: Back to its Former Glory.") ](https://www.youtube.com/watch?v=LkEpuHeNd0s)

The first round of repairs left a “wobble” in one of the axes which led down a rabbit hole of neon bulbs, photoresistors, balky mechanical parts, and fussy tension adjustments. Unlike pen plotters from the 1970s through the present, this is a _completely analogue_ device: it uses servomechanisms to make the pen motion proportional to the voltage applied to each axis. Engineers acquainted only with digital circuits may wonder that it was possible to get something like this to work at all, but it was or, more precisely, “just barely”. Everything has to be just right, and a little binding in the mechanism or drift in the electronics can wreck everything, as the intrepid restoration team discovers.

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<div class="post-metadata">

**Author:** ![johnwalker](https://scanalyst.fourmilab.ch/user_avatar/scanalyst.fourmilab.ch/johnwalker/32/17415_2.png) [@johnwalker](https://scanalyst.fourmilab.ch/u/johnwalker)\
**Post date:** [24 June 2022 15:21 UTC](https://scanalyst.fourmilab.ch/t/repairing-a-hewlett-packard-7035b-x-y-recorder-part-2/1588/2 "2022-06-24T15:21:36Z")

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Here is thirty minutes of the repaired plotter doing its thing—plotting [Lissajous patterns](https://en.wikipedia.org/wiki/Lissajous_curve) from signal generators driving its inputs.

[![](https://scanalyst.fourmilab.ch/uploads/default/original/2X/1/141f607cae0c944ed729223f5a15622ab97ba5a1.jpeg "30 min of Analog Plotter Therapy") ](https://www.youtube.com/watch?v=tWBqoiX3KKU)