HP Forum Archive 20

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Return of the 15C with Bad Zebra? Message #1 Posted by aj04062 on 21 Dec 2011, 9:24 p.m. I pulled an old 15C out of the box today to see if it was still malfunctioning after a brief time of normal function. When I tried to use it, the same malfunction occurred. Reference this old THREAD Here is a picture of what happens. Any key that is pressed results in this display. If you press a key twice, it will "stick". I have left the batteries out to try again in a few weeks. Last time I did this, it worked normally when I installed batteries. Thoughts?

Re: Return of the 15C with Bad Zebra? Message #2 Posted by uhmgawa on 22 Dec 2011, 5:01 p.m., in response to message #1 by aj04062 Quote: I pulled an old 15C out of the box today to see if it was still malfunctioning after a brief time of normal function. When I tried to use it, the same malfunction occurred.. Any key that is pressed results in this display. The voyager lcd controller will mindlessly render whatever display data has been sent to it long after the NUT cpu has gone off into the weeds. It is autonomous in the sense it generates its own scan clock and as long as it hasn't been disabled it will persistently render the last captured display data. The dropout voltage for the display registers is spectacularly low and your best bet to determine weather the cpu is repeatedly writing that display data is to remove the button cells and short the corresponding terminals to discharge the internal bypass capacitors. Quote: If you press a key twice, it will "stick". Not sure I quite follow what you mean by "stick"? Quote: I have left the batteries out to try again in a few weeks. Last time I did this, it worked normally when I installed batteries. Waiting for weeks isn't necessary to dropout the voltage rail, try the above. You may also have an intermittent related to temperature or handling of the unit. If you are comfortable to open the unit, further debug is possible particularly if you have access to an oscilloscope.

Re: Return of the 15C with Bad Zebra? Message #3 Posted by aj04062 on 22 Dec 2011, 7:59 p.m., in response to message #2 by uhmgawa If I press any key once you will see the displayed "text". After you release the key, the screen goes blank. If I press quickly press any key, the display with remain as shown. If I press any key again, it will go blank. I tried shorting the terminals. Nothing changed. I have no problem pulling the the unit apart and do have a scope. I know 4.5V would be an appropriate voltage setting on a bench power supply, what should the max current be? What do I need to look at with the scope?

Re: Return of the 15C with Bad Zebra? Message #4 Posted by uhmgawa on 23 Dec 2011, 1:52 p.m., in response to message #3 by aj04062 Quote: If I press any key once you will see the displayed "text". After you release the key, the screen goes blank. If I press quickly press any key, the display with remain as shown. If I press any key again, it will go blank. Is the 15c to your knowledge actually in its "on" mode when you see the above behavior? Or is it "off" and pressing any key causes the above? Other than being illegible text, are the driven lcd segments solidly polarized or do you see any indication of weak partial drive? The latter would point a finger at the lcd controller or contamination of the interconnect between it and the lcd glass. Quote: I tried shorting the terminals. Nothing changed. I have no problem pulling the the unit apart and do have a scope. In the event you haven't opened one up before, do so in something like a deep tray to capture the esd bonding springs which are carefully designed to disappear as soon as being exposed to light. Concerning springs, once you have it open do a quick check of the button cell positive spring terminal to verify the pcb end is solidly under tension between the pcb and the cell stack well -- the spring tension is divided in two parts due to the cell well. I've seen a loose spring cause all sorts of trouble and it could also frustrate the attempt to discharge the power rail as above. Quote: I know 4.5V would be an appropriate voltage setting on a bench power supply, what should the max current be? When the cpu is active and the LC tank is driven, somewhere around 1ma is typical. Note you'll need to hold a key down to keep the cpu active as it will enter poweroff in under 100ms when it is idle and no keys are down. Quote: What do I need to look at with the scope? Take a look at the LC tank and verify it reliably becomes active (~880KHz) upon key press and becomes inactive when the key is released. IIRC the inductor is situated horizontally just under the cell well on that vintage voyager. Before that I'd get a jeweler's loupe and have a look at the solder joints of all package leads checking for fillet stress fractures, and shorts due to whiskers or contamination. You can also try poking at the leads with a toothpick to see whether that alters the behavior. Mild flexing/torque of the case/pcb (back removed) may also be useful. At least this is what I'd hope is the source of the trouble, and (if I follow correctly) the intermittent nature of the problem would seem to indicate some type of dependency on temperature, humidity, or stress. Unfortunately except for the cpu, the only source of replacement ram/rom/display devices are another 15c. And the only conclusive approach to debug a suspect device is to sniff the ISA bus to capture the transactions which is somewhat involved electrically and mechanically. FWIW I've disassembled dozens of voyagers and every problem I've encountered has been due to an interconnect of some sort.

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