Re: HP-97 Blues... any advice? Message #5 Posted by Tony Duell (UK) on 18 Sept 2003, 5:41 p.m., in response to message #1 by Michael Meyer
I had a Topcat in for repair that did this (it wasn't a 97, it was a 95C, but the circuitry is much the same).
There's a circuit in the Topcats to load the power supply down if the input voltage tries to exceed the correct +ve output voltage -- this protects the machine if it's connected to the AC adapter with no battery pack in place, so you don't get IC failures like in the Woodstocks.
Anyway, if the power converter (on the CPU board) isn't running, this circuit will try to pull the inpjut voltage right down to 0V, and gets hot in the process.
I would start there. The service manual, with scehamtics, is on the MoHPC CD-ROMs. THe power converter is a simple discrete component circuit based round 2 transistors (oscillator and regulator) and a transformer. Most of the times I've had problems with this circuit (not just in Topcats), the oscillator transsitor is open. A 2N3904 is a suitable replacement.
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