Cisco 6851 hook switch issue

[Vesa]

I found cheap IP phone with side car for monitoring Asterisk PBX status. Cisco 6851 side car has 2 x 14 buttons and LCD screen for line statuses. Nice addition to system, you can see directly call progress etc.

But this unit has issue, hook switch is intermitted working. Phone goes off-hook state randomly. First step was to check switch condition and clen it. No help. Switch ground IO line when off-hook and switch is open when on hook. So dirty switch can not cuase this issue and switch is mechanically ok.

By measuring hook line inside the phone, line goes down randomly. Specially when touching it with meter probe. So there needs to be issue with pull-up resistor. With out shematics resistor location is unknown, so I just added 27k pull-up resistor from 3.3v line to hook line. Now phone works correctly. Fix is ugly but who cares, this is inside of device and not visible.

It seems that same issue is quite common with Cisco 68xx, 88xx and 89xx series phones. Propably desing error.

[ChrisW6ATV]

Very cool! I am glad that you found and made an easy fix for that hook problem.

I installed possibly thousands of Cisco 8851 and similar phones until retiring in 2023; as of then, I had not heard about a hook-switch problem with them, with installed phones going back to early 2016. It is good to know, if I ever decide to get a phone "like I used to work with" in the future.

[Vesa]

Hook switch issue is probably related to new production pacthes, but anyway there is lot of forum posts to similar issues. Like this:
https://www.reddit.com/r/Cisco/comments/1hfsbgo/cisco_phone_8811_hookswitch_issue/

May be bad version/production series or something. For a normal customer easy repair case, Cisco just swap the phone to working one. Not so good for second hand shoping, like my case. No official seller, no warranty. But that is the reason for ultra low price..

These 3PCCs are anyway rock solid phones, no any issues with sip connectitivity and phone just works. Not so same with older 3PCC:s like 79xx series, sip mode is not ready for anything. Just use SCCP.

[Vesa]

The hook switch issue situation went bad fastly. Even with the pull up resistor phone jumps constantly on - off on hook states. So new repair round. First items was desolder whole hook switch board and measure the issue. First round: no issues on mainboard or hook switch. But new, tens of minutes measurement revealed the issue location: hook switch board generates somekind partial short to ground. Opening the switch component do not reveal issue, contact are separated and there is air or plastics between contacts. Board is clean and no shorts between track and ground plane. But it still creates contact around 1-2 kohm. So switch is somehow bad, plastic just creates connectivity between contacts.

Switch has two pairs of contacts and second pair seems to work ok, even with high voltage insulation tester. So some soldering work and jumper wires fix this issue. Works now ok without extra pullup.

So issue is on switch component plastic body, there have to be crack and moisture. With several years of use, moisture has created oxidation from contacts and now the first pair of contacts are bad.

Modern phone but issue is from old phones: bad switch.