Flaky Apple Cinema Display 30" touch button repair. - Page 1
Forum section: electronics / repair

author: jchw4
author_rank: Regular Contributor
timestamp: November 21, 2021, 06:35:12 am
content: My old Apple Cinema Display 30" has developed strange issue: when turned off (and probably cooled, usually at night) it randomly swtiches on and off.Searching the Internet did not help much except that the issue is common with this model.It does not happen every day, but sometimes when it's off, it starts to periodically turn on and off. The delay between changing states could be a few seconds and down to half a second (visibly).Checking it later with a scope revealed that delays could be even shorter, but monitor just cannot react that fast.I traced it to the touch on/off/brightness panel on the side. Basically the sensor chip starts randomly signalling that power button was pressed.The control panel was origically glued with the adhesive. I disconnected it and attached with a masking tape so I could probe it.It was quickly revealed that the behavior depends on the distance between the case and the panel. The more the distance, the less flakiness and vice versa.(I keep it about an inch from the aluminum case and it basically never exhibits this behavior. It just looks bad.)Sensor board schematics is attached. I measured a few capacitors in-place and resistors are what's marked on them.It's based on QT240 chiphttps://media.digikey.com/pdf/Data%20Sheets/Atmel%20PDFs/QT240-ISSG.pdf. I actually went ahead and replaced the chip, but nothing changed.I checked the power, but it seems to be OK.

author: jchw4
author_rank: Regular Contributor
timestamp: November 21, 2021, 06:37:53 am
content: The trace always looks like at the first photo. The yellow trace is the power button signal at J1(4), the blue trace is +4V from U1 to U2 measured at C3.So there is a fast pulse which is followed by the button action.Other shots are these pulses zoomed in.Could somebody suggest any ideas here?

author: Rasz
author_rank: Super Contributor
timestamp: November 21, 2021, 06:42:20 pm
content: Have you probed 5V? and brightness pins too? If those also double trigger its something external, but if only on/off double triggers I would compare components between 3 capacitive switch channels."larger values of Cs require higher stability to ensure reliable sensing. "you might try lower value of C9

author: jchw4
author_rank: Regular Contributor
timestamp: November 22, 2021, 04:24:30 am
content: Quote from: Rasz on November 21, 2021, 06:42:20 pmActually screenshots starting from 7 are +5V (it's noticeable if you count grid). More 5V captures attached.I did not check the brightness pins probably because I never saw brightness being flaky.I actually disassembled the monitor to check whether it has a ferrite choke on the wires to the panel, and it does. (I cannot find photos though.)What I don't understand is "if it's something external, why the 5V and 4V traces are identical"? At least I don't see any difference in traces (1-6 vs 7-16).QuoteThis is what I was thinking about, but it would be nice to know the reason for the behavior.

author: tooki
author_rank: Super Contributor
timestamp: November 22, 2021, 03:40:08 pm
content: Capacitive sensing is sensitive to power quality. This is noticeable, for example, with phone and tablet touchscreens becoming erratic when used with very “dirty” USB chargers. Perhaps a similar effect is happening here, be it due to noise on the AC (getting coupled through to DC or to the capacitive sensor) or noise in the DC power supply.

author: jmelson
author_rank: Super Contributor
timestamp: November 22, 2021, 04:29:43 pm
content: I had a Dell LCD monitor that started acting up. It had been used at a trade show and a tiny amount of a drink had been splashed on it.The drink had made its way to the button board and caused a bunch of corrosion under capacitors across the button switches. I unsoldered the caps, cleaned the board thoroughly, and replaced the caps with a small-value new part. I think I used 1000 pF. The switches were tarnished a bit but not contaminated inside. It works fine now. The specific switch in this case was the input selector button.Jon

author: Rasz
author_rank: Super Contributor
timestamp: November 22, 2021, 07:36:23 pm
content: Quote from: jchw4 on November 22, 2021, 04:24:30 amslap bigger C1 C3 in there? maybe standby 5V is getting tired in the monitor.

author: jchw4
author_rank: Regular Contributor
timestamp: May 26, 2022, 10:59:59 am
content: Quote from: Rasz on November 22, 2021, 07:36:23 pmYou were right.I should probably share the repair.The correct repair would be to replace electrolytic capacitors inside, but disassembly is painful so I left it for later times.I could not see the glitch on my scope so I had to guess.My first idea was to put RLC filter on the power line.The input power is 5V. Regulator outputs 4V. The board consumes about 1.5mA of inactive current, and about 2.5mA when button is pressed.The regulator needs some voltage on top of 4V, say 0.5V, so I could put about 200 Ohm. I also wanted the largest L that could fit the case.I had 100mH/20mA inductor that measures 230 Ohm. Ok, no separate resistor and 230 is kind of scary. The largest C that I had was 100uF ceramic.The result is below. No more flakiness(The ground wire was isolated before I pushed it in, there is black heatshrink tube that I moved on top of the connection after taking this photo.)

author: dnotq
author_rank: Contributor
timestamp: September 10, 2022, 05:13:28 am
content: @jchw4 Thanks for the thread, this put me on the correct path to fix my 30" Cinema display. My repair thread is here if you care to check it out:https://www.eevblog.com/forum/repair/lt3435-switching-reg-with-2v-ripple-on-output/QuoteThat is what I did, replaced the three electrolytic caps on the output of the 5V switcher. The monitor *is* a PITA to open, however you might want to consider it because the 5V supply also directly feeds the 3.3V regulator, as well as providing 5V for the USB ports and other electronics.The 5V also goes up to the "T-CON" Control PCB, where on my monitor one of the SMD fuses was blown; I assume it was probably due to the monitor turning itself on-and-off rapidly due to the 5V noise feeding into the button panel. I only knew the monitor had a flickering problem after finding and fixing the fuse.Matthew