HP ZR2740w monitor and weird EMI problem - Page 1
Forum section: electronics / repair

author: Muikku
author_rank: Newbie
timestamp: April 27, 2020, 07:55:31 pm
content: I learnt from internet that this monitor model has had a load of problems with blacking out display etc. This one had similar things and I thought I would take it apart to see if it has bad caps or something. Taking the display apart made it work, the picture quality is great and it seems to be stable for long periods of time.Freeze spray doesn't seem to do anything (main board, T-con board or PSU). Also pressing and knocking boards (main board, psu board and T-Con) with insulated screwdriver does nothing. Both of these tests were to find bad joints or components.Measuring PSU outputs gave me 11.45V on the 12V line, it seems to be stable, adjusting backlight doesn't affect it. 5V line was 5.05V (just with a cheap multimeter)Knocking away more, backplate over the T-con started giving me some fails, took it away and knocked the board without issues appearing. Then touching the screen with freeze spray can gave the problem and I found it is 100% repeatable.I've got non grounded outlets all over the house. I can only get picture to the monitor from one of my computers, and I think this problem has something to do with floating voltage by SMPS because there is (depending on computer) 60 to 90V ac floating voltage.Any ideas where to tackle this?Youtube link to video of the problem:https://youtu.be/ZPUvxc2OzzUPictures of the boards:https://www.dropbox.com/sh/3a1u1gvj1asgz8g/AADWVxGQ-zrZAUceJuOR5ykra?dl=0Thank you for your inputs-Muikku

author: wraper
author_rank: Unknown
timestamp: April 27, 2020, 08:11:17 pm
content: Did you try changing DVI/DP cable? Does it happen with both inputs? Such problem is often caused by bad cable and image will go blank from any transient.

author: Muikku
author_rank: Newbie
timestamp: April 27, 2020, 08:20:01 pm
content: Quote from: wraper on April 27, 2020, 08:11:17 pmStart with the easiest.. DVI seems to have problems (no picture at all) and changing the DP cable to known good one fixed this issue!I will assemble the display next and see if it is fine again! Thank you

author: Manul
author_rank: Super Contributor
timestamp: April 27, 2020, 08:23:45 pm
content: A lot of power supplies designed to work with earth ground have filter capacitors from neutral and live going to ground. If this ground does not exist, the device ground floats on half mains voltage. Thats one thing. Another thing is that there will be some capacitance from "HOT" side to "COLD" side anyway. Through flyback transformer and additional filter capacitors. It is best to have ground. But if you can not have it, try first to remove filter capacitors which go to ground on the power supply input. This may make it a bit "better".

author: EHT
author_rank: Frequent Contributor
timestamp: April 28, 2020, 10:24:04 pm
content: Oh this reminds me.. I have a couple of these still to fix. One of them has a fault where the display periodically stops updating and then fades to black in a pattern of horizontal black lines in about 1 second, then reverts to normal. Not sure if this is likely to be the mainboard or the T-Con.I can't find the schematic for this monitor but I think if i had a schematic for a similar one split into PSU / Main / T-con that would be useful.However, I don't mean to hijack the thread.. will post separately