HP 37E - Battery charging issue

[jebem]

Hi, John,

The Spice series power supply schematics are published at the end of this Kees page.

If the machine works fine using charged batteries or alkaline batteries, then the switching DC-DC power supply is fully operational.

These machines are NOT designed to work from the charger adapter alone (having the battery pack removed).
It is the battery that works as the regulator device for the input DC voltage to the switching power supply.

The charger adapter just supplies AC voltage that is rectified by diode D1 and:
- Applied thru resistor R1 current limiter (8.2 ohm) to the battery when the machine is powered off.
- Applied directly to the battery when the machine is powered on.

The charger nominal output voltage is 9VAC, so reading 10VAC is fine.
These chargers presents a high internal impedance, so it is normal to see this kind of 10VAC high voltage dropping to 2 or 3 Volts under load.

So, I would investigate the obvious suspects here:
- Check all the power supply PCB traces and switches for good continuity.
- Check all the charger adapter wiring and connector for good continuity.
- Check the rectifier diode D1 and Resistor R1.

Applying AC voltage from the charger to the machine while having the machine in powered off state, the DC voltage at the battery terminals should be around 2.4VDC for a good working battery that is not completely depleted.
You should be able to see a DC voltage increase when connecting the charger.

I would never power on these Spice series while charging the battery, to minimize the chances of destroying the internal IC's.