HP L1940T Monitor Button Faults - Pressing by themselves

[paul_g_787]

So I gave my father my old monitors some years back when I bought two new larger ones. 2x HP L1940T monitors. I also have a third one on top of my server rack which is occasionally used.
All 3 owned since new and never abused or stored anywhere and are in immaculate condition like new.

Beginning of December one of the two my father uses on his PC started randomly "pressing the menu button on it's own" and also showing the message "OSD Lockout" randomly (which is activated by holding the Menu button for a few seconds).
After a couple weeks it then started "pressing the power button on it's own" as well
.
I took the button PCB out thinking the buttons might be stuck and I also checked them with my multimeter and they working fine are not getting stuck down. I gave them a clean with contact cleaner to be safe.
No obvious problems elsewhere on the PCBs that stood out, but I never had time to look in more depth during December as I was working constantly thanks to panto season!

So I swapped the monitor with my server rack one and all was good temporarily.

Last night now the original server rack monitor has started having the same fault. Same model Monitor bought at the same time. The third monitor is still alright for now.

Tried the monitors on another PC and with different leads and the fault is with the monitor. It happens regardless of using DVI or VGA input also.

Anyone have any clues as to what might be causing this fault and where I could start looking?  The monitors otherwise work fine. Just unusable in this state.

A quick google found this page but is it archived. If anyone knows how to view it?
https://h30434.www3.hp.com/t5/contentarchivals/contentarchivedpage/message-uid/5081431

Service manual with schematic:
https://www.manualslib.com/download/918483/Hp-L1940t.html

[Swake]

Did you try without the keyboard PCB connected?

Try to find out if the issue is on the board with the keys or on the main board. Swap the keys board with the one from the good monitor to see if that solves the issue. And put the bad one in the good monitor to confirm the issue.

[DavidAlfa]

Very common, moisture eventually damages them after several years, repaired a lot of them.
Get a bag of buttons at AliExpress, replace them all.
It's 100% a button fault!

[paul_g_787]

Very common, moisture eventually damages them after several years, repaired a lot of them.
Get a bag of buttons at AliExpress, replace them all.
It's 100% a button fault!

Good call. I took out the button PCB and tested the switches with my multimeter in resistance mode and the menu and power switches are both activating on their own.
The other buttons all show overload OL on the meter when not pressed and short to GND when pressed, but the menu and power show 300Ω - 600Ω fluctuating up and down when not pressed, so the main board is obviously interpreting this as an input.
Interestingly the menu button is showing worse on my meter than the power button, and the menu button was the first to play up.

I have found a PCB for sale on eBay here that is the same but this could also be faulty due to age.
https://www.ebay.co.uk/itm/111385816015

I would like to find a pack of switches as you said and change them. Will start searching, I have attached a picture of the PCB switches also showing the board model no.

[Swake]

HP made by Benq. Yeah times change...

[paul_g_787]

HP made by Benq. Yeah times change...

The power supply is BenQ as well. Made 2006

[Swake]

Closing in

[Swake]

Exist in many sizes. This shop on Ali has them:
https://www.aliexpress.com/item/1005003751805886.html?search_direct=true&spm=a2g0o.productlist.0.0

[Original]

My lenovo LT2323p had the exact same problem, same pcb layout, same buttons.  Replacing them fixed it.
Temporarily, till new parts arrive, you could cut the two molded sides of the switch, disassemble the internal spring an clean in with toothpaste and wash it with IPA.

[DavidAlfa]

Alternatively you can "patch" the issue by soldering strong pullup resistors to VCC for each button, ex. 100ohms.
Buttons are rarely pressed, power consumption won't be a problem, neither the extra current for the contacts.

[paul_g_787]

Left the monitor a few days without the front panel PCB connected and the issue has gone away so definitely the buttons. Still need to order some.