My original HP-25, ailing

[Duane Hess]

Peter:

Not sure if the behavior is the same as a 27 I have. An ultra-cheap, known defective item. When powered on all digits lit at once and "shimmied." Or could be referred to as vibrating. Don't know what baubling means, but recall a VERY faint left to right intensity ripple.

Kept looking at it and noticed a pattern I could not really see. Was too close to a window. Each segment of all digits shimmied VERY rapidly simultaneously. Hence, it appeared the entire display was lit with shimmering 8's. In a significantly dimmer location I saw the pattern. Within the shimmer all digits were performing a count (up/down-forget) in unison. All 0's, then 1',s, etc. to all 9's (or the reverse direction) and continually repeated. The count image had an incredibly short duration. i.e. a ghost-like flash amungst all the shimmering.

The display going completely blank didn't happen to me.

Since sold as known defective, I wasn't surprised. Opening it up, it looked like most components were resoldered on the power supply portion of the circuit board. i.e. if you look at the PCB from the front, most every component to the left of the power pack hole was re-done.

For whatever reason, I was checking continuity in that area. (this was a good 5-7 years ago, forget reasoning; if any) I noticed a trace was weird. i.e. measured a value (close to zero) between 2 points. Then had largely varying values elsewhere. I was measuring the trace itself, so should have been small everywhere. Held one probe stationary and slid the other along the trace, watching the ohmmeter. As approaching an area which looked like a crack occured, about 1/4" of the tracing fell off the PCB.

Please reference the link:
https://www.keesvandersanden.nl/calculat...inside.php

At the top are pictures of the front/back of the PCB. Look at the left image. The tracing that fell out was the long one, immediately left of the battery "hole." (don't recall if I checked the 27's behavior at this point)

Don't recall how many connections the trace has. By checking all the images (both sides) you can see it goes to one of the 3 holes at the very bottom of the PCB. The one closest to the middle of the calc. It goes to a chip at the top of the PCB. Believe the left chip labeled 7551LX (upside down) in the row/column driver picture. As I recall it was the 3rd or 4th pin from the left (right?) on the top edge of the chip.

Running a copper wire between those 2 points resulted in the 27 working 100%.

Moral of the story?
- ..... dern if I know whether there is similarity to your situation. Doesn't really sound the same. Mine had no functionality at all.
- never trust a cracked trace you might replace the wrong thing?? I have no electronics training. Usually can only handle the obviously broken situations.
- I don't know, that's just what happened to me.

For what its worth.