HP 21 not powering on - repair/troubleshooting advice

[Steve Simpkin]

(01-27-2021 04:42 AM)dmh Wrote:  Ok, a bit surprised as visually the terminals looked ok but some light abrasion to the terminals has given the following:

- one powers on and works fine except for the '4' button. It clicks like all the other buttons. Any suggestions?

- one shows some screen activity as the power switch is turned off or on but never stays on. Shows either one '0' on left or right or three '0' across the screen. Is this the ACT chip or could it just be the power switch?

- last one still does nothing

Thanks!

For the one with a non functional key, there is a small hole on the keyboard circuit board under each key. Take a stranded small gauge wire and spread the individual wires to make a fine wire brush. Using this and a small amount of contact cleaner, insert one of the wire strands into the hole behind the malfunctioning key and work it around to clean the key contact.

For the one with the possible intermittent power switch, remove the battery pack and slide the power switch back and forth 100+ times to help clean the contacts. Insert the battery pack and try again. If it still has the same symptoms try sliding the power switch slowly to see if there is a good spot. If this does not help there is probably something wrong in the electronics like a bad ACT chip.

For the last one, examine the battery contacts and look at the PCB trace that runs to each of the rivets that contact the battery terminals. Sometimes a hairline crack in one of these traces can prevent power from getting to the rest of the circuitry. An ohmmeter would be helpful here.