HP 8568B NO DISPLAY (HP 85662A display issue) - Page 1
Forum section: electronics / repair

author: andreal1967
author_rank: Contributor
timestamp: August 29, 2024, 12:46:41 pm
content: 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?

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 29, 2024, 01:18:55 pm
content: 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 dmmmaybe thishttps://www.manualslib.com/manual/3450554/Hp-8563a.html?page=207#manualpage 246

author: andreal1967
author_rank: Contributor
timestamp: August 29, 2024, 02:27:21 pm
content: 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!

author: perieanuo
author_rank: Frequent Contributor
timestamp: August 30, 2024, 11:54:57 am
content: Pins 1 and 14on page 414 you have the troubleshooting for display&power, see also page 422 for intensity problemstake 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

author: andreal1967
author_rank: Contributor
timestamp: August 30, 2024, 01:27:05 pm
content: thanks a lot i am ready to measure the filament....

author: Runco990
author_rank: Frequent Contributor
timestamp: September 01, 2024, 05:46:32 pm
content: Check the tantalums on the display HV board. I had one short on me, same symptom. Shorted tantalums are getting pretty common.The CRT doesn't generally just die.

author: andreal1967
author_rank: Contributor
timestamp: September 02, 2024, 06:01:02 am
content: thanks a lot... i measured the filament and it was ok... then i removed and checked all of the connectors of the hv board and cleaned the contacts.. and tada... the display works again.I have just to figure which is the trimmer for moving vertically the display picture because the bottom line of the characters are too low.. i am reading the manual of the 85662a but i cannot find the page with the regolations of the image on the display!

author: perieanuo
author_rank: Frequent Contributor
timestamp: September 03, 2024, 11:12:52 am
content: page 43 see A17R77 Y POSN, so board A17 something marked R77 (see Table 2.2 with details on those adjustments), also page 421 explains this, search for R77

author: andreal1967
author_rank: Contributor
timestamp: September 03, 2024, 11:20:11 am
content: alignment done!!