Re: Troubleshooting a 9845C
If the mainframe is the problem, it’s most likely caused by bad ROM’s and then you have to replace them by a ROM board.
Van: VintHPcom@groups.io <VintHPcom@groups.io> Namens Rik Bos
The 9845C uses the same graphic subsystem as the 9845B option 200 so it is possible to use a 98780A monitor instead of the color 98770A monitor.
The color monitor is a very complex peace of work with 3 independent power supplies build in. Because of its complexity and power hunger it has 2 fans one on the outside one on the inside.
It is also very sensitive for overheating, if the power transistors won’t get enough cooling they will be destroyed. Most of the time this will result in a short and destroy the driver transistors also.
First thing to do is building a turnon fixture to start the 9845C without a monitor to narrow down the problem. https://hp9845.net/9845/tutorials/testrom/images/turn-on-fixture-schematics.png
If the system doesn’t beep it’s the monitor.
BE VERY CAREFULL LETHAL VOLTAGES EVEN WHEN THE UNIT HAS BEEN SWITCHED OFF!
The big capacitors keep the power for some time, it takes them a few minutes to discharge fully.
Monitor fault finding, first download the service manual for the 98770A monitor and the diagrams. (I’ve printed the diagrams on A3 that works for me)
Next remove the outer casing of the monitor, place the monitor on the 9845C and connect both to the main power.
Switch on the 9845C and look at the power leds on the left side of the monitor, just only the rightest LED should light, all other should be off.
If more led’s are lighting, remove main power, take a small break go to the toilet etc..
Remove the cover of the power and remove the power supply board, check the power transistors (separate board), rectifier board and capacitor board for the obvious.
You should probably replace all the ELCO’s for new ones, after 30-40 years they’re worn out and developing high ESR. (Maybe do this first before initial power on.)
When you replace a power transistor also check the drivers on the main board, If the power transistors keep dying probably one of the small driver transformers has gone bad.
Hope it helps
I recently, after a long search and much patience, managed to acquire an option 200 power supply for my option 200 9845C. I finally had a chance to power it up, after verifying voltages on the supply.