9825A Troubleshooting #VintageHPComputers #HP9825

Rhys Lowry

Hi all, I'm a newish vintage electronics repair enthusiast having recapped and done basic troubleshooting and the such in the past. No expert, although I aspire to be. After picking up this HP 9825A last week from a local recycler who thankfully didn't throw it out, I gave it a nice cleaning before powering up (it was full of broken glass..) Upon powering up, the fan spun and the speaker gave out a single tone that was on as long as the calculator (computer?) was on.

I decided to check the voltages coming from the power supply board and it was partially off where the +12V rail was 12.43V, which I figured might have made the logic upset so I twisted one of the potentiometers on the power board while it was off. Upon turning it back on, one of the power diodes (possibly one of the voltage regulators) gave some magic smoke (I didn't see exactly where the smoke came from).

I'm planning on getting some replacement parts from Mouser and other sources but my question to you all is what should/shouldn't be replaced on this to give a better shot at this thing working? Any idea on what the original problem might have been?

Any help is appreciated.

Thanks! Rhys Lowry, student @ UMO studying EE

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