Electronics > Repair
HP 8568B NO DISPLAY (HP 85662A display issue)
andreal1967:
I am absolutely devastated. After a lot of work, i managed to have an hp8568b working, but after moving the display unit on the table i lost completely the image. The ugly thing is that i don't' see anymore the dim light of the filament of the CRT near the neck of the tube and this is very sad.
So or i have a problem with the filament supply, or i have lost the tube. I cannot find anywhere which are the pins of the crt that goes to the filament, because my first measure is to check if the filament is interrupted.
I don't' want to put the tester near high voltage , so , i just want to measure the continuity of the filament. Can anyone help me to find which pins are filament related?
perieanuo:
remove the tube connector, discharge everything in tube but i doubt your dmm cannot handle the values, then the filament is readable by ohmmeter.
or try to guess looking at the traces the filament pins, the voltage there is small and won't kill your dmm
maybe this https://www.manualslib.com/manual/3450554/Hp-8563a.html?page=207#manual page 246
andreal1967:
yes, it's what i want to do, the problem is that i cannot figure which pins of the tube are the filament. In the schematics i cannot figure anything about the filament pins!
perieanuo:
Pins 1 and 14
on page 414 you have the troubleshooting for display&power, see also page 422 for intensity problems
take care, heater has -2.5 KV voltage on top of small heater AC supply from A17 crt display driver board, see page 431 (connector J7 - pins 1, 2 or on the tube neck connector W9 pins 1 and 14 like I said before).
visual inspection may help, if you removetube neck connector or J7&J3 you can check the heater resistance. you can always discharge tube voltages using some power resistor and some gloves with care
andreal1967:
thanks a lot i am ready to measure the filament....
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