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HP 21 not powering on - repair/troubleshooting advice
01-27-2021, 01:43 AM (This post was last modified: 01-27-2021 01:44 AM by BobVA.)
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RE: HP 21 not powering on - repair/troubleshooting advice
Not sure how much you've researched this, but most importantly do NOT try to power up either a 21 or 25 (or any "Woodstock" series) with just the AC adapter (i.e. without a working battery installed). Best practice is to charge the batteries outside the calculator and never connect the AC adapter. (Sounds like you're already familiar with that.)

If you have a good, charged pack the first thing to check is for corrosion on the battery contacts in the calculator. A little light cleaning with an emery board or fine sandpaper should clean them up.

Next point of failure is the bad connection between that contact and the actual PC board. If the calculator works intermittently as you insert / wiggle the pack, that's the likely fault, but correcting it will require disassembly. Instructions can be found here. You'll need soldering and electronics skills to fix this, or do any further troubleshooting.

If you get a weird display (one bright digit, digits blinking on and off, etc) it's probably a destroyed ACT chip, usually caused by someone powering it up with just the AC adapter. A blank display is also a symptom of this, but it may also be just a power issue (above).

There is a replacement ACT chip available. See this thread which also has a lot of Woodstock troubleshooting information.
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RE: HP 21 not powering on - repair/troubleshooting advice - BobVA - 01-27-2021 01:43 AM

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