HP15C displays gibberish. Is it broken?

[Marcovecchio]

I just saw this same pattern on a 15C I was fixing yesterday!

The calculator was working, but some LCD segments were failing, so I dismantled it to clean and reassemble. However, I use a bench power supply to work on my calculator repairs, and I accidentally inverted the power supply terminals when trying to power up the 15c!

I went into despair, because the calculator drew 2 amps from the PSU, with the wiring inverted! I quickly unmade the inversion, connecting the terminals correctly, and this is the moment I saw this very same pattern you posted.

I went into despair a second time, I thought the calculator was fried forever, but miraculously, after 10 or 15 minutes, it started working ok again, and I noticed a big resistor-like component was hot. I think it's not a resistor, but an inductor, used to generate calculator clock.

So, I suspect this pattern happens when the calculator has no clock on the processor. In my case, perhaps the voltage inversion made the oscillator circuitry overheat / fail temporarily, but after some minutes, it went to nominal temperatures again and operation was restored.

I will make a test in my spare time, with a 12c in my stash: I'll remove some components from the oscillator circuitry and see if I see the same pattern. If so, my theory is correct.

Eduardo