HP 42S Calculator problem - Keeps switching off after turn-on

[David L. Jones]

I have a HP 42S calculator and it switches off imediately after
switching on. In some cases the display will flash for a split second,
in other cases it doesn't show anything. If I press the ON key fast
enough I can keep the display on until I release the key.

The self test (ON-LOG) works just fine, and passes OK (it does not
switch off in this mode) . But at the end it just switches off. ON-ROOT
(the reset function) works and very briefly displays "Machine Reset"

I can adjust the contrast, and even get it to do calculations if I
press a key fast enough at power on, but it switches back off again as
before.

Here is an inside shot:
http://alternatezone.com/stuff/HP42SinsideCrop.jpg
As you can see there aren't many components to go faulty...

I have soldered the metal tabs to the PCB in case they weren't making
contact, no difference. I have run the unit from a DC supply in case
the batteries internal resistance is shot, no difference. I have
replaced the top tantalum cap, no difference. The bottom cap and diode
seem to be part of the dot martix display refresh/contast, remove the
cap and the display dims, remove the diode and full contrast. It's not
the SRAM connections, because as I said it passes all internal memory
self tests.

Anyone know anything about the 42S and can shed some light on this?

Thanks
Dave

 

[[email protected]]

Have you checked the battery (ies). Tha is typically what happens when
the battery(ies) are headed to the great recycle bin.

H. R.(Bob) Hofmann

 

[David L. Jones]

Have you checked the battery (ies). Tha is typically what happens when
the battery(ies) are headed to the great recycle bin.

H. R.(Bob) Hofmann

Yes, I have changed them, and as I mentioned, I have even ran it from a
bench supply to eliminate any possibility of the batteries.

Dave