Need help debugging a CRT Monitor (NEC XM29+) - Page 1
Forum section: electronics / repair

author: Fusion916
author_rank: Regular Contributor
timestamp: November 24, 2016, 12:02:29 am
content: Hi all,I have been trying to determine why this monitor won't power up for a week now. I have narrowed it down to the microprocessor not enabling the power gate to the rest of the power supply. However, it seems like this microprocessor is not starting up at all even though its getting power and the 8MHz crystal seems to be working. I have schematics for the entire monitor but it is very short on details on exactly what the microprocessor is doing (for example, what conditions are required to enable the power gate.I have tried to attach the entire pdf but its too big, so instead I took a few screenshots. Maybe you guys can give me some suggestions on what to look at next. I have checked all of the microprocessor pins to see if the voltages match up with what the manual says and almost all of them do except a few (which I dont know the functions of those pins) and also don't know which pins relate to the power switch signal.Please let me know if you need anything else from the schematic to help.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 24, 2016, 12:40:11 am
content: Beats me, but something I'd check is the reset line. What is it doing during power up?

author: Fusion916
author_rank: Regular Contributor
timestamp: November 24, 2016, 12:57:23 am
content: Quote from: Alex Eisenhut on November 24, 2016, 12:40:11 amNothing fancy, just a normal logic 0 to logic 1 transition... The manual says the correct operating condition is 5V, so it must be a resetb line. I even tried toggling it between 0 and 5V after powering it up to see if that brings the microprocessor out of reset but no luck. Any other ideas I can check?

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 24, 2016, 01:59:03 am
content: Looks OK. But what about the manual? I assume you tried everything from the outside of the TV before opening it up and I'm sure you're seeing what you're seeing on the waveforms, but the manual shows:"External equipment has been incorrectly connected.Incorrect input selection.""Correct all connections.Press correct RGB1, RGB 2, VIDEO 1 or VIDEO 2 button."Also,"The monitor turns off in 5 seconds after powering on and then the STANDBY/POWER indicator blinks in one second intervals. It indicates that the powersupply circuit or, one or more fans have been damaged."So before we tell you what to do next, tell us what you did first. If there's VCC, obviously there must be some sort of life in the PSU, is there any of this power light blinking?

author: Fusion916
author_rank: Regular Contributor
timestamp: November 24, 2016, 07:46:09 am
content: Ok let me go back to the begining...I brought the monitor out of storage a week ago. When I plugged it in and turned it on, it had a blank screen and it was obvious HV was not going to the monitor as it did not have that distinct CRT power on sound or static on the screen. I took off the back and saw the CRT heater wasn't even on, so thats when I started taking everything apart.After a few days of messing with it, one thing is clear, the power supply for the CRT (on a completely separate chassis and board) is fine. Manually controlling it (by applying voltage to the relay) completely powers it up and all of the voltages are correct. It's just that the microprocessor seems to not start and the default state is to enable that NPN BJT (which turns the power supply relay off).After narrowing it down to the microprocessor, it just seems it doesn't want to start. I have a hard time believing the actual microprocessor is damaged somehow, but I have no way to verify so I'm left assuming its working and debugging around it. I probed all of the data and clock lines and I don't see any wiggle at all. It seems like it's for some reason not coming out of reset or some pin has some voltage on it to tell the microprocessor to shut down the main power supply (like some kind of fault protection or power management).Do you have any other debugging suggestions?Quote from: Alex Eisenhut on November 24, 2016, 01:59:03 am

author: xavier60
author_rank: Super Contributor
timestamp: November 24, 2016, 10:55:59 am
content: A problem I have seen appear occasionally in TVs is the EEPROM getting corrupted, causing the TV to be totally unresponsive. Maybe the same thing can happen to a monitor also.Affected TVs would become responsive again when the EEPROM's data pin is disconnected.

author: strawberry
author_rank: Super Contributor
timestamp: November 24, 2016, 12:18:01 pm
content: how about corosion of power botton contact surface? sometimes 50 or more ohms are too much for tactile switches

author: Fusion916
author_rank: Regular Contributor
timestamp: November 24, 2016, 02:08:58 pm
content: Quote from: xavier60 on November 24, 2016, 10:55:59 amHow would I debug this as the issue? In this board, we have the uP and the eeprom, on startup, which would communicate with what first? it seems like both the eerpom and the uP have no life but I don't know which is the culprit.

author: Fusion916
author_rank: Regular Contributor
timestamp: November 24, 2016, 02:10:45 pm
content: Quote from: strawberry on November 24, 2016, 12:18:01 pmThe CRT power supply has zero issues. Like I said, I have manually enabled the CRT power supply and it is fine. It's just the onboard microprocessor is not releasing the power supply switch.

author: xavier60
author_rank: Super Contributor
timestamp: November 24, 2016, 04:34:48 pm
content: Quote from: Fusion916 on November 24, 2016, 02:08:58 pmIs there brief activity at the EEPROM at any time? Like when power is first applied. The EEPROM is for storing settings.Also with micro-controllers used in this applications there is usually some system that warns the micro-controllers that power is about to be lost. For example with TVs there is usually an AC_DET line that needs to go high to tell the micro-controllers that mains is present before the micro-controllers will do anything.Also at least one of the micro-controllers pins will be for detecting faults. Usually there are multiple fault detection sources that are ORed together. This pin needs to be low for normal operation.EDIT. Maybe IC2003 is for detecting power fail. One more thing to keep in mind, input pins are often are schmitt trigger and have to be over 4 volts to register as a high level.

author: Fusion916
author_rank: Regular Contributor
timestamp: November 24, 2016, 10:48:58 pm
content: EDIT: It seems like the EEPROM is actually not dead, there is some transaction on power up, more to come shortly....On the clock and data lines there is zero activity on the eeprom on power up. I also thought there may be some input to the microprocessor that is telling it there is some kind of fault and to power down, however I have no idea what pin(s) that would be as the service manual does not say. Where is IC2003? I can sweep any of the processor pins from 0 to 5V to see if that wakes it up. Whichs pins to you recommend looking at?Also, the fans are disconnected right now, so even if its detecting the fans as a fault, I already swepted the EMG pins from 0 to 5V and that doesnt wake up the processor. Also, for the fans to even operate, the 15V rails need to be alive and that means the processor has to enable the main PS at least once and that doesnt even happen so I don't suspect the fans being disconnected is an issue.Quote from: xavier60 on November 24, 2016, 04:34:48 pm

author: Fusion916
author_rank: Regular Contributor
timestamp: November 24, 2016, 11:20:48 pm
content: Ok at power up the EEPROM looks to be (trying) to load some settings on the microprocessor. The top signal is clock and the bottom signal is data.Anything else I should be looking at?

author: xavier60
author_rank: Super Contributor
timestamp: November 25, 2016, 05:27:21 am
content: Try to turn it on with the data pin disconnected.I made a mistake earlier. I think IC1003 is the power fail detector.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: November 25, 2016, 12:11:14 pm
content: Well, absent the service manual, I'd look at the amount of time it spent in storage. Have you made a visual inspection looking for insects or humidity damage?Ask NEC for the service manual. I did for my super old monitor and they responded. They didn't have a manual, but they responded, which means it's worth a try.

author: xavier60
author_rank: Super Contributor
timestamp: November 25, 2016, 07:28:08 pm
content: When the leads corrode on small glass diodes, they can become leaky because of metal/glass seal damage.