On Mar 1, 2021, at 6:53 PM, Rhys Lowry <email@example.com> wrote:
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).
12.43 V was within the spec and probably didn't need adjustment immediately. Did you look at the schematic before you turned the potentiometer? Doing so without understanding what you were adjusting or being able to measure was not a good idea. What were the voltages of the other supply rails?
I hope that you did not do this while it was connected to the logic boards, as the version of the power supply you have does not have the crowbar circuit modification that would have protected the rest of the unit from a short circuit in one of the power supply's voltage regulating pass transistors.
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?
Have you identified the failed component(s)?
As to the original problem, when you cleaned the unit out, are you sure you reconnected all of the cables correctly?