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:
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 :)