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

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 wrote:
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

wrote:
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