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HP 15-C has odd flashing display
Message #1 Posted by dougo51 on 19 Oct 2010, 10:57 a.m.

Inserted new batteries. Calc only comes on after holding the "on" key for about 5 secs. Display comes up as follows and is flashing. Note that 3 characters look like incomplete zeroes but obviously I can only post them here as complete characters. Note my use of "space" between characters:

"083spacespace0d,088space," Again, all zeroes are partial as if lcd is failing but it was working fine.

While flashing, if I hit other keys, calculator goes off.

Any help to get this old dog (bought in 1985) working again is much appreciated.

      
Re: HP 15-C has odd flashing display
Message #2 Posted by Eric Smith on 19 Oct 2010, 1:10 p.m.,
in response to message #1 by dougo51

Try holding the y^x key down then pressing the ON key. This forces a hardware reset, but doesn't change the content of memory. The content of the display should change, but not necessarily to anything sensible given what you've described.

Then turn it off. With it off, try holding down the minus key while pressing the ON key. That should clear the memory, resulting in a display of "Pr Error".

If that doesn't work, remove the batteries, leave them out for a few days, and try again.

I've never seen a 15C behaving as badly as you describe, so it's possible that it has a hardware fault.

            
Re: HP 15-C has odd flashing display
Message #3 Posted by uhmgawa on 19 Oct 2010, 11:47 p.m.,
in response to message #2 by Eric Smith

Quote:
I've never seen a 15C behaving as badly as you describe, so it's possible that it has a hardware fault.

I've seen some similar weird behavior trying to come out of power down recently with a pre 28xx s/n 12c in which I was probing around. It required removal of the batteries and short of the electrolytic cap to sort itself out. Not that the OP was doing the same, but it seems either the NUT or R2D2 can get into a locked up state, perhaps more conventionally via non fatal ESD.

            
Re: HP 15-C has odd flashing display
Message #4 Posted by dougo51 on 20 Oct 2010, 3:28 p.m.,
in response to message #2 by Eric Smith

Thank you for your help and suggestions. Unfortunately, it had no impact. Flashing characters remain exactly the same regardless of what I do including the y^x suggestion. Also, batteries were out for a few days until I got around to replacing them so I doubt that suggestion will work but will try. I had followed ops manual instruction to short battery terminals briefly, but that had no effect either. Someone asked aboout s/n. It is: 2840A87206 with USA printed to left of s/n. How about using a previous cure all before power up/down became so popular--several blows with a hammer?

                  
Re: HP 15-C has odd flashing display
Message #5 Posted by uhmgawa on 20 Oct 2010, 4:32 p.m.,
in response to message #4 by dougo51

Quote:
Someone asked aboout s/n. It is: 2840A87206 with USA printed to left of s/n.

That's the package reduced vintage with the former memory-use only r2d2 integrated in the cpu. I don't have one here but I'd speculate the external ISA bus still carries all activity. You could scope it to see if the cpu is executing bus cycles coming out of sleep. It should fall back into sleep (after a few seconds IIRC) where bus activity is quiesced, notably the bus clocks are idled.

The R2D2 has AFAICT a separate RC oscillator which clocks the display drive scan. So if you are seeing random junk on the display it could be the cpu hasn't set the display registers to a known state coming out of sleep/power-up and it is just rendering random power up data.

I'd still take a look at the LC oscillator and verify that is stable. Other issues I've noticed is a stuck key on the same row as the on key can cause startup problems (f/g/sto/rcl). Although in that case the firmware bails on power-up and shouldn't cause the failure you've described.

Don't discount physical damage in terms of an open lqfp package to pcb trace. I've seen some marginal reflow solder jobs on voyagers that I'd never expect to be associated with the name HP. So it could be an intermittent (or a contamination short).

Quote:
How about using a previous cure all before power up/down became so popular--several blows with a hammer?

Resist the temptation. If all attempts at resurrection fail someone will still buy it from you on EEEbay for a price which will leave you with an incredible story to tell.

      
Re: HP 15-C has odd flashing display
Message #6 Posted by uhmgawa on 19 Oct 2010, 11:44 p.m.,
in response to message #1 by dougo51

Quote:
Inserted new batteries. Calc only comes on after holding the "on" key for about 5 secs. Display comes up as follows and is flashing. Note that 3 characters look like incomplete zeroes but obviously I can only post them here as complete characters. Note my use of "space" between characters:

"083spacespace0d,088space," Again, all zeroes are partial as if lcd is failing but it was working fine.

While flashing, if I hit other keys, calculator goes off.

Any help to get this old dog (bought in 1985) working again is much appreciated.


If you're still having issues and are up for some investigation I'd scope the LC oscillator tank. If the clock isn't restarting reliably when the NUT tries to come out of power down it could lead to such anomalies.

What might the serial number of this unit be?

Edited: 20 Oct 2010, 12:29 a.m.


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