The Museum of HP Calculators

HP Forum Archive 15

[ Return to Index | Top of Index ]

HP 34c needs help
Message #1 Posted by David Derrer on 10 Oct 2005, 3:03 p.m.

This great forum has inspired me to try to repair my 34c, which I have had since new around 1981. So far, thanks to all the helpful posts, I have managed to rebuild the battery, open the case, and repair the lost digit in the display. However, I have made no progress dealing with Pr Error every time it is turned on, and STO ENTER returns Error 9. Clearing Error 9 returns 0.0000. It seems to have no memory at all - I cannot program or use storage registers (the stack does work). This is a solderless unit, and I have double checked chip positions and locations (I'm sure I am the only one to have opened this one). I have twice cleaned all the flex PCB connections. One other oddity - I cannot enter decimal point from the keyboard, although I can enter Pi. It does display decimal points correctly resulting from division of integers.

Thanks to all who have shared their knowledge,

David Derrer

Re: HP 34c needs help
Message #2 Posted by Tony Duell on 11 Oct 2005, 7:01 a.m.,
in response to message #1 by David Derrer

In the -C spice series, the memory (8 pin) chip closest to the display gets a separate power connection from the other chips -- it gets power all the time to maintain the continuous memory.

I _think_ the power pin is pin 8, I can check this on the schematic.

With the machine switched off (but a battery pack fitted), you should measure a little less than the battery voltage between the power pin of that chip and ground (-ve side of the battery). With the machine on, you should see the same voltage there (or a _little_ less) than on the power pins of the other memory chips.

If either/both of those voltages is incorrect, you have a problem on the flexiprint, or on the power converter PCB (open-circuit diode?). If they're both OK, I would, alas, suspect that memory chip.

Re: HP 34c needs help
Message #3 Posted by David Derrer on 11 Oct 2005, 12:18 p.m.,
in response to message #2 by Tony Duell

Thanks for the help Tony. I am not reading any voltage between pin 8 for any of the three 8 pin ICs and battery (-). Battery is at 2.68 volts. Yet it turns on, and seems to work fine for ordinary calculations and built-in functions (except decimal point entry). So the CPU must be getting power, and the power PCB must be at least partially working. I wonder if this suggests a problem on the flex, or is the power for the CPU separate on the power PCB from the power for the memory chips?

Thanks again, David

Re: HP 34c needs help
Message #4 Posted by Tony Duell on 12 Oct 2005, 7:41 a.m.,
in response to message #3 by David Derrer

The most likely explanation is that I've mis-remembered which is the power pin for that IC. I will check tonight (the schematics are at home...)

Re: HP 34c needs help
Message #5 Posted by Tony Duell on 13 Oct 2005, 6:37 a.m.,
in response to message #4 by Tony Duell

I checked the schematic last night. Pin 8 _is_ the power pin, and pin 7 is ground. If you're not getting any voltage between pin 8 and the -ve side of the battery on any of the memory chips, then I don't see how the thing can possibly work.

Just to check the obvious. There must be a battery pack in the calculator for this test. Pin 8 is the right hand side one nearest the display (to the right of the notch on the IC package). The machine should be turned on to get voltage on the front 2 chips.

Re: HP 34c needs help
Message #6 Posted by David Derrer on 13 Oct 2005, 9:13 p.m.,
in response to message #5 by Tony Duell

Thanks again Tony. I think I haven't cleaned it adequately. After another round of cleaning, I checked continuity on the flex, and power on the little bit of flex attached to the power PCB (with battery connected). Both were okay. I then reassembled and did your test again on pin 8 (yes, with battery connected and power switch on). While applying a fair amount of pressure to the ICs, I briefly registered voltage (about 6.5v I think) on the number 8 pins. I'm going to try cleaning again, this time I've gotten some deoxIT. I'm also considering increasing the pressure that the plastic backing piece applies to the chips by adding a tiny strip of electrical tape where it lays over the pins. The underlying foam has a pretty good impression after 25 years. I also wonder if the slightly higher voltage of alkaline batteries would make a difference. Regards, David

Re: HP 34c needs help
Message #7 Posted by Vassils Prevelakis on 13 Oct 2005, 5:03 a.m.,
in response to message #3 by David Derrer

I would suggest that you purchase the HP Calculator Schematics CD-Rom from

These are schematics that Tony Duell made by tracing the PCBs of the various computers in the collection. He then graciously donated the collection to the HPCC club. The earning from the sale of this CDROM go to HPCC, Tony is not earning a penny from this operation.

So I think purchasing this CDROM is worthwhile to (a) support HPCC and (b) get a better understanding of what is going on with your calculator.

Tony graciously gave me a copy of the 9915 schematics to help me debug by (now working) 9915 computers. Eventually I purchased the CDROM to get access to the other schematics.


[ Return to Index | Top of Index ]

Go back to the main exhibit hall