Re: 9825A Troubleshooting #VintageHPComputers #HP9825

Paul Berger

On 2021-03-02 2:23 p.m., Rhys Lowry wrote:
I was measuring the 12V rail when it happened and was confused as to why it wasn’t changing the voltage when I turned the pot. As for dead components, the 2N4398 is shorted and it looks like some solder mask was blown off the back. The 2N3055 still tests fine with a 1.2V drop across it. As the now dead diode connected to pins 1 and two of the card, I searched downstream for any damage. I’m sorry to say I did leave it in while I was testing. Lesson learned. I did not see any obvious damage to the main boards (I didn’t have the front section attached). Following both the rails from the blown mask pin and the diode rail was inconclusive. There may be damage I can not see but I can not test it until I get the power supply board up and working again.

The pins I am referencing are the first, second and fourth from the bottom of the picture of the card connector. Blown mask can be seen in the other.

The 2N4398 is the pass transistor for the +5V supply if it failed short (mine did too) it will put +20V in places it really should not be.  I am afraid the damaged trace would suggest the fuse did not blow quickly and damage could be extensive.  When repairing your power supply I would recommend disconnecting the power connectors from the circuit boards until you are confident all the voltages are correct.

You may want to make sure the fuse in the power supply is the correct rating, I had the +5V regulator fail on a  9825 much the same as yours did and did not have any damage to the circuit traces on the board.

When the 5V regulator failed on my power supply it took out CR5 as well that is the big stud diode 1N6096  I have spares of 2N4398 and 1N6096 left over from repairing by power supply if you need them.

Lastly I have most parts for a 9825 if you find you need some, the only thing I don't have is a good print head for the strip printer, mine got damaged when the +5V regulator failed and tehre was a big black splotch on the paper under the print head.


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