Return of the 15C with Bad Zebra?



#5

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?


#6

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.


#7

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?


#8

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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