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Full Version: Disaster: help for a couple of HP97 needed!
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I received today a couple of HP97 units. I was told that they were functioning. With the excitement of the new toys, I just plugged them with the enclosed power supplies.

Well, the first briefly lights up and then dies. Same for the second. What a pity!

Then I realized: s**t!!! I have used the 110v adapter in my 230v house!!! I may have burned both machines. With the proper 230-to-110 transformer, now they don't work at all.

Before opening them, is there anyone that knows what are the likely damages? Any schematics? Any fuse in there to change?

Your help would be greatly appreciated!!

Are the batteries good or original?

Well the good thing is the standard HP power transformers are not good for a lot of current and there is some over-voltage protection built in. The DC input comes in through a 4.7 ohm 2 watt resistor which limits the current draw from the AC adapter, then there is a tab transistor that pulls down the input voltage through a 4.7ohm 2 watt resistor. On one of mine with no battery pack (worst case) the input voltage when plugged into 110 is about 11.5 V with the calculator turned off. When I turn it on the transistor pulls this down to about 4.5 V when I check the voltage drop across the resistor it is 0.633 Volts so I=E/R gives us 135 mA current which I would think the transistor would easily be able to handle even if the input voltage was higher. If you had a battery pack installed even better as the battery pack would pull down the voltage too. In summary I think it should survive even twice the input voltage.

The first thing I would check would be to see is the power transformer survived, unloaded and connected to 110VAC the output of the transformer should be about 12.5 VAC.

Forgot to add that if AC out from transformer is good then if there is a battery pack remove it and turn it on, display should come on and the calculator work except for printing and the card reader, and the display will likely flicker as the DC input is not filtered. The service guide says it is ok to operate like this but I would not recommend it for anything more than to see if it works.

Edit 2
I just hooked up two transformers with their outputs in serial so that I measure 24VAC across the free terminals and connected that to the AC input of one of my 97s (not my good one) and as expected it worked just fine.

..and one last thing there is a service guide for the 97 on the MoHPC DVD set.

Edited: 18 Jan 2013, 10:10 p.m.


As I have the DVD of the hpmuseum, I'll check the service manual when I return home on Sunday.

With the proper 230-to-110 transformer, now they don't work at all.

Hi Jose, what do you mean, that the calculators you bought have been used with a double voltage 220/110 input adapter and they were set on the 110 V input and the third time you fed them with adapter properly set....or the culculator are "died" even supplying them with a third (only 220 V), supposed workin' adapter?

Edited: 19 Jan 2013, 7:41 a.m.

I guess he used just a plug adaptor for US-American mains plugs first, no "110V adaptor" though he wrote the latter.


You're right. I first tested it with the original power supply, and then realized that I needed to add a voltage converter transformer. Too late!

With the proper 230-to-110 transformer, now they don't work at all.

my interpretation is that you are still trying to power the calculators through the plugpacks that you had connected to 230 volts? if you connect a 110v plugpack to a 230v supply, you will simply burn out the primary winding of the plugpack... your calculators are probably fine, the plugpacks are toasted.

check between the 110v pins of the (disconnected) plugpack with a meter set to OHMS. if the pins show open circuit, the plugpack is dead. cut off the low voltage lead, and splice it onto a new plugpack that has a suitable output voltage.


if you connect a 110v plugpack to a 230v supply, you will simply burn out the primary winding of the plugpack...

Actually I don't understand it: if you feed the primary with a double voltage source you simply 'll get a double voltage on the secondary and maybe you 'll get the circuit beyond damaged....if the secondary should not be affected, this way, by a short circuit

Agree that maybe Jose's "calculators are probably fine, the plugpacks are toasted" (>>>>>>to be repaired).

Edited: 19 Jan 2013, 1:21 p.m.

hi aurelio, i agree with robert here: applying double voltage to the 110VAC wall adapter will immediately saturate the iron core of the transformer, thus leading to a short circuit on the primary side which either damages the primary winding or a primary (thermal) fuse. the fuse is sometimes wound inside the primary winding, so not sure as to whether it is replaceable. anyway, your toys should still be in good shape!

Edited: 20 Jan 2013, 5:49 p.m.

Hi Hans!
Thank-you for your post, as I suppose the problem on the primary could be inducted really by a short circuit on the secondary coil.
I have somewhere an adapter with this issue and I would like to open it since a long time to check and if possible to repair. Actually it happened that I recovered only the flat cord (calculator side) as spare for another "sick" one.
By the way, unfortunately the toys are not mine, they belong to Jose Gonzalez Divasson, who opened this thread