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hpcalcuser83 · 12-12-2015, 01:39 AM

I have an HP-71B calculator that is draining batteries after a couple of weeks. i do not have any extra RAM or ROM modules installed. The calculator self test passes. Anyone know how to correct this problem? Thanks.

-Gil

**rprosperi** · 12-12-2015, 01:49 AM

(12-12-2015 01:39 AM)hpcalcuser83 Wrote: I have an HP-71B calculator that is draining batteries after a couple of weeks. i do not have any extra RAM or ROM modules installed. The calculator self test passes. Anyone know how to correct this problem? Thanks.

-Gil

Are you using long-running programs (several minutes of run-time)?

Are you using HP-IL and if so, do you leave the loop powered up all the time (e.g. you can OFF IO when not needing the loop to save power)?

TASP · 12-12-2015, 02:21 AM

Not trying to be a twerp, but make sure we've ruled out some things.

Replacing all 4 batteries at same time with 4 matching fresh new ones ??

No battery corrody crud in the compartment ??

Terminals bright and shiny ??

You're the only one with access to device, and use of it ??

No modifications to your 71 ?? (just read an article about hardwiring an ultrasonic range finder to one, LOL)

I can't recall how the on board clock/alarms work, is it like the 41 with the ability to 'wake up' and do things at specific times ?? If so, is it ??

{Chuckle} do you like playing with the on board beeper a bunch (I know I did) ??

hpcalcuser83 · 12-12-2015, 02:57 AM

I do not have the HP-IL interface installed. I juat tried to put in new batteries into my calculator and I noticed that one of the battery terminals (not the ones with the springs) is missing. It was not like this before. I was able to power on this calculator a few months ago. I am not sure where the battery terminal went. It does not appear to be inside the calculator. Maybe this was the reason for the battery drain. Well, now I need to repair/replace the battery terminal. Anyone have any tips on how to do this? Thanks.

-Gil

**rprosperi** · 12-12-2015, 03:21 AM

(12-12-2015 02:57 AM)hpcalcuser83 Wrote: I do not have the HP-IL interface installed. I juat tried to put in new batteries into my calculator and I noticed that one of the battery terminals (not the ones with the springs) is missing. It was not like this before. I was able to power on this calculator a few months ago. I am not sure where the battery terminal went. It does not appear to be inside the calculator. Maybe this was the reason for the battery drain. Well, now I need to repair/replace the battery terminal. Anyone have any tips on how to do this? Thanks.

-Gil

So this machine has the foam-springs with metal terminals glued-on?

Assuming so, there is nothing special about the terminals that were there, they can be replaced with any conductive material, as long as it is securely mounted/adhered to the foam to ensure good contact, even when moving and jostling the machine about. It may be good to lay small strips of rubber bands across the batteries (perpendicular to the battery length) so that the door will press on the bands, which in-turn hold the batteries tightly in place.

Is it possible the battery contact stuck to a battery and was lost when you disposed of the last batteries you replaced? If not likely, make very sure that it did not find it's way into the machine interior as it could easily short connections on the board.

The loose contact could well have contributed to shorter battery life; you will have to fix that securely before you can expect longer battery life.

hpcalcuser83 · 12-12-2015, 03:08 PM

I have checked for the missing terminal inside the caculator and did not hear or find anything inside. Maybe it was attached to the older battery. I have attached a photo showing the missing terminal. I am not sure I understand where the mentioned foam should be. I do not have any foam in the battery compartment area. So I just need to add some conductive material (battery terminal from another device) at the location of the missing terminal? It seems that this terminal should be connected to a GND connection.

-Gil

Dave Frederickson · 12-12-2015, 04:23 PM

Later model 71's and those repaired by HP had the spring contacts replaced by a pieces of foam and simple, stamped metal contacts. When you stated that it was not a spring contact, Bob probably thought you were referring to the foam contacts.

The missing contact is normally connected by a wire inside the calculator. If you can't find a replacement contact, I think you could fashion something out of a piece of tin can. However, the calculator still needs to be opened to solder the wire.

The two PC boards inside the 71 are connected by a delicate flex cable that is prone to cracking so if you decide to undertake the repair then proceed with caution.

Further instructions can be found in the 71B Service Manual.

Dave

hpcalcuser83 · 12-12-2015, 05:02 PM

Thanks. I will review the service manual and decide if I want to undertake this task.

-Gil

**rprosperi** · 12-12-2015, 05:50 PM

(12-12-2015 04:23 PM)Dave Frederickson Wrote: When you stated that it was not a spring contact, Bob probably thought you were referring to the foam contacts.

Exactly right. I thought you were referring to the other side.

Loss of that contact is very unusual, and highly unlikely that it came out stuck to the battery, unless the battery had spilled corrosive material; as Dave noted that terminal is wired to the board inside.

Very strange, never seen that type of terminal damage; just about all others, yes, but that one, no.

If you do undertake Dave's suggestion, we'd be interested to hear the verdict on what happened.

Paul Berger (Canada) · 12-12-2015, 10:17 PM

(12-12-2015 05:02 PM)hpcalcuser83 Wrote: Thanks. I will review the service manual and decide if I want to undertake this task.

-Gil

The positive and negative terminals are a u shaped piece of metal on the end of a wire attached to the memory and I/O board they clip over the end of the battery box from underneath, I think it would be pretty hard for one to completely fall out, and it could not easily be pushed down, the battery box is almost touching the keyboard PCB, the only thing left would be if it snapped in half where it is bent, in which case the part iside the battery box would be easily lost and the other half with the wire attached would be behind the wall of the battery box. If you nee a replacement send me a PM I have a parts machine with both of those contacts intact.

Paul.

hpcalcuser83 · 12-13-2015, 04:24 AM

Yes, it is just as Paul described. I opened up the calculator and saw that the battery terminal was broken in half right at the edge of the battery compartment. It looks like the PO flexed the terminal too many times and it finally broke. I fastened some aluminum foil to extend the battery terminal and closed the calculator back up. I tried to turn it on and it would not power on. I removed the battery and connected the AC adapter and the calculator operated fine. I removed the adapter and inserted the batteries. The calculator would not power on. I measured the voltage between all four batteries and got a reading of about 1.7 VDC. This did not seem right so removed all the batteries and measured the voltage as each battery was re-inserted. Everything went fine until I installed the last battery. When the last battery is not making contact with the battery terminal the voltage is about 5.7VDC, but when the battery is making contact then the voltage drops to 1.7 VDC. It looks like some circuit is loading down the battery. This is most likely the source of battery drain. Tomorrow, I will insert a new set of batteries in the calculator and see it will power up.

Dave Frederickson · 12-18-2015, 05:57 PM

Look familiar?

[Image: s-l1600.jpg]

On eBay for parts.

hpcalcuser83 · 12-19-2015, 04:06 PM

Yup. Except i do not have the broken terminal. i am going to continue trouble-shooting my low voltage when batteries are inserted problem. Does any have a schematic of the AC/Battery circuit for the HP-71B?

Dave Frederickson · 12-19-2015, 04:09 PM

(12-19-2015 04:06 PM)hpcalcuser83 Wrote: Does any have a schematic of the AC/Battery circuit for the HP-71B?

That would be on p.7-5/7-6 of the above Service Manual.

jebem · 12-20-2015, 07:37 AM

(12-19-2015 04:06 PM)hpcalcuser83 Wrote: Yup. Except i do not have the broken terminal. i am going to continue trouble-shooting my low voltage when batteries are inserted problem.

I would not overlook the obvious things to do in first place.
Assuming battery corrosion from acid leaking, I would dismantle the machine as much as possible and then clean all parts to make sure no acid is causing the symptoms you describe.

I use warm water and hand washing dish detergent, scraping with a soft brush.
Then I repeat the operation on the electronics PCB only (keep the LCD out of this procedure) with Isopropyl alcohol and et id dry for at least 24 hours before applying power supply.

brickviking · 12-22-2015, 07:54 AM

(12-20-2015 07:37 AM)jebem Wrote:
(12-19-2015 04:06 PM)hpcalcuser83 Wrote: Yup. Except i do not have the broken terminal. i am going to continue trouble-shooting my low voltage when batteries are inserted problem.

I would not overlook the obvious things to do in first place.
Assuming battery corrosion from acid leaking, I would dismantle the machine as much as possible and then clean all parts to make sure no acid is causing the symptoms you describe.

I use warm water and hand washing dish detergent, scraping with a soft brush.
Then I repeat the operation on the electronics PCB only (keep the LCD out of this procedure) with Isopropyl alcohol and et id dry for at least 24 hours before applying power supply.

I'd also like to add the following: examine all the traces nice and close up if you happen to have the right equipment to do so. A lit jeweller's loupe or lit magnifying glass is possibly the best device, but that's rather expensive and hard to get. These days, some digital cameras have a good macro function, and you can often use that to take photos of the affected circuits. You'll need a good light on the board when you take photos of it this way. To be totally thorough, you could take the same photo using different lighting angles, that may possibly pick up fractures in the copper traces.

Regards, BrickViking

(Post 49)

hpcalcuser83 · 01-05-2016, 06:14 AM

I cleaned the circuit board around CR3, C2, R22, VR7, Q1, CR2, C8, C1. Inserted new batteries and the calculator would not power on. The voltage at C8 is +2.3 VDC. When I removed the batteries and used the AC adapter the voltage at C8 is +5.7 VDC and the calculaor functions perfectly. I am going to read through the power supply circuit theory of operation and see if I can identify which component(s) are not operating correctly.