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The HP97 (certainly, maybe other Topcats too -- I know for a fact the 95C has this) has a circuit to load the AC adapter should the 'battery' voltage attempt to exceed the main +ve supply rail voltage. This is designed to protect the machine if the battery pack goes open-circuit, preventing the sort of damage that's all too common in Woodstocks.
If the DC-DC converter circuit fails (this is often due to a failure of the oscillator transisotor on the CPU board), then this circuit will attempt to pull the battery voltage down to zero. I've seen one draw over 1A from my bench PSU when this happened.
As others have mentioned, the HP97 service manual is on the MoHPC CD-ROMs. And my experience suggests a 2N3904 is a suitable transistor to use in the DC-DC converter.