Broken Woodstock Question



#4

I found a dead HP-27, which had corroded batteries that had leaked inside and damaged the circuit board traces. I tried powering it directly on the circuit board to no avail. I disassembled it completely and tried to determine if the OFF/ON slider switch was working, but of course on this model the keyboard is heat-staked to the front cover, so it is not possible to do this directly. Can someone familiar with the innards of this model tell me what points on the back of the keyboard to check with an ohmmeter to see if the OFF/ON switch is working, and if not I can short these points to further test the calculator?

Thanks for any help.

Michael


#5

Hello again Michael!

A really simple test, before you even take the guts out or unscrew the case. Of course this works after the guts are removed and you have cleaned the PCB as much as possible.


1   select resistance on your ohmeter
2 place a probe on the -ve battery contact and the other
probe on the +ve battery contact
3 cycle the on off switch and observe the needle deflect from
high resistance to low resistance.

The deflection indicates that the slide contact is working. Also, use extreme caution when cleaning the keyboard silk screening. Do not rub clean or rub dry the surface of the silkscreened numbers. They are extremely sensitive and the clear laqueur that protects them has probably degraded.

The woodstocks are very problematic. Typically when the batteries started to build up internal resistance and decay they still held a charge. This led the user to plug the calculator into the mains with the adapter while leaving bad batteries in the calculator. This has disasterous effects on the PCB and is unrepairable.

Some times you see someone on ebay state that "it worked when the plugged it in but it doesn't work now" so it must need new batteries. Well, it's too late by then!

Good luck, I have a 21 here that looks mint outside and gave an erratic display. The batteries were welded inside from corrosion and the removeable PCB was pure blue. I soaked this in vinegar, (what else, it was a write off) and got it to work a little bit better, but still erratic.

Also, when you jury rig a power source use two rechargeable batteries at 1.2 volts each. These woodstocks become tempermental with age and two 1.55 volt alkalines may be enought to send the calc into cardiac arrest.

Cheers, Geoff


#6

Hello Geoff,

Thanks again for your very detailed and expert advice. The OFF/ON switch is definitely working at the keyboard level (I figured out that the top two traces are the switch circuit), so that is not the problem. The resistance at the battery terminals is very high with the ON/OFF switch in either position. I powered it both with a proper NiCad battery pack and my bench regulated DC power supply at 2.5 Vdc with no response whatsoever in the LED array. Externally the calculator looks very nice, so it looks like I have my first non-functional pretty collectable. It's a shame because the HP-27 is quite rare, and whenever they appear for sale the asking price is astronomical.

Best Regards, Michael


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