HP-71b does not turn on when using batteries

[hpcalcuser83]

I have a 71b that will not turn when using batteries. When I use an AC adapter it will power on fine. I checked the battery terminals and they are fine. I checked the DC voltage with a multimeter and it is fine until I place the last of the 4 batteries in such that it touches the terminal (4 batteries in line will measure 6 VDC until I insert the last battery an it touches the terminal - then the voltage measures 4 VDC or less). Anyone ever experience this?
-Gil

[rprosperi]

(12-19-2016 09:11 AM)hpcalcuser83 Wrote:  I have a 71b that will not turn when using batteries. When I use an AC adapter it will power on fine. I checked the battery terminals and they are fine. I checked the DC voltage with a multimeter and it is fine until I place the last of the 4 batteries in such that it touches the terminal (4 batteries in line will measure 6 VDC until I insert the last battery an it touches the terminal - then the voltage measures 4 VDC or less). Anyone ever experience this?
-Gil

Is there any evidence of prior battery spill in the battery compartment? If so, there may be internal damage and/or a short that you can't see.

Are all 4 batteries known good, tested separately (voltage new s/b ~ 1.6V)?

[cruff]

If there were a short or other cause of excessive current draw in the unit, I would expect that when powered by the AC adapter for it to behave similarly, as the AC adapter, through the bridge rectifier and regulator transistor, is unlikely to be able to provide more current than the batteries can through their blocking diode. The only difference is that the rectified AC voltage before regulation would be higher, which might be enough head room to allow it to run.

The service manual has some power supply diagnostics, do you have a DMM to perform some basic measurements? When active the unit should only draw about 10 mA, with no modules plugged in. If you have anything else plugged in (RAM, ROM, card reader, etc.), remove it and see if it behaves or not.

[hpcalcuser83]

The 71b used to work with batteries. The battery operation was intermittent. I would move the batteries around a little bit and the 71b would power on and operate fine - until the batteries moved around. I got my DMM out and found that the battery terminals that have the spring-look were corroded and not providing enough contact space to the batteries. I replaced the spring-look terminals with better ones from a donor 71b. After this, I found that the batteries and terminals were providing the correct voltages after each battery (1.5VDC, 3.0 VDC, 4.5VDC, then 6.0 VDC - until I allowed the final '+' to connect to the final terminal - then the voltage went down to 4VDC or less). I am referencing the 71b service manual on the theory of operation and trouble-shooting process. I am thinking that I have a bad CR2, Schottky diode. The HP part number listed in the parts list is 1901-0999. Does any know what the commercial part number is for this diode? I will begin searching the internet in hopes of finding a suitable replacement part.
-Gil

[cruff]

(12-19-2016 04:12 PM)hpcalcuser83 Wrote:  I am thinking that I have a bad CR2, Schottky diode. The HP part number listed in the parts list is 1901-0999. Does any know what the commercial part number is for this diode

The HP part number cross reference document suggests the equivalent is JEDEC SD103C. You should be able to use any Schottky diode with a small voltage drop that fits and can support a reasonable current.