Monitor just died on me.

[iankellogg]

diode that measures properly should be fine. I am not sure how to tell someone to fix this type of monitor at this point. Maybe start probing across each capacitor your repalced with your meter on continuity mode. Try to see if there is any short on any of those caps. I am not saying the caps are bad but generally when something is shorted you can see it at a capacitor.

[HarrieD]

I followed the flowchart and found a defective diode D13 (from the +24V circuit) also died. It's a BY299. I replaced it with a BY255 which I got on stock but this didn't even fix the issue.

I didn't mention this however: when the monitor is "cold" (I guess all the caps are discharged) the monitor tries to power up when switching on the power. I hear the screen getting static for just a second and then the power supply goes "down".

I will order a full set of replacement diodes and transistors. Maybe that will help.

O, before I forget. The flowchart states that there should be around 7/8 Volts on pins 15 and 16 of IC1 (TEA2164). Mine reads about 6 Volts.

[HarrieD]

Was reading the flowchart to add the IC name for the previous post and noticed this in the manual:



The voltage on the yoke is about 180 Volts. Probably there lies the issue. Will check that this evening after work. I'll keep you posted.

[iankellogg]

If i am following correctly it sounds like your monitor is going into high voltage shutdown. The first switching mode power supply should be generating 140V. if there isn't enough load on the system that could cause it to ride high but something is certainly going on with that. The yoke should have B+ across it.

[HarrieD]

What's B+ ?

[iankellogg]

B+ is what the industry calls the main power supply rail. This is what feeds the horizontal output transistor. according to what you just posted the B+ should be 146-149V. If it is going to 180V then either it is unloaded or somehow the primary supply reference got messed up. Generally it is not a good idea to mess around with the voltage feedback of the primary supply so it is likely something else is going on.


Usually when i am stuck like this I will try to isolate the power supply anyway I can.
So often what I do is remove the HOT, find the B+ at a good spot. usually a test point or the filter cap for the B+. then solder a 10-20W load onto it. Like a lightbulb or something else. If the supply starts working when the load is there then you will know the supply is fine and you need to find out why the HOT isn't running.

[HarrieD]

Thanks for the tip.

Did some measuring yesterday. Yoke on: 149Volt is now about 3Volts. Yoke off: 163Volts.

When measuring behind diode D2 in both situations: 6Volts. And that's strange, this should be 320VDC. Maybe a floating ground???

Also replaced the NTC, this was 27 ohms and dropped to 17 ohms after heating it up with my soldering iron. Will check it out this evening. (Again)...

[iankellogg]

I am going to make some assumptions with that information provided.

Your input power supply is fine, I think it is going into underload foldback. This is very different than a short and is caused by having too little load, this means most likely the HOT is not doing its job.

I'm not sure the reason for replacing the NTC but by its own name the resistance should drop when it is heated. (Negative Temperature Coefficient)

I am not sure if D2 is the same as as the schematic but D2 is listed as being one of the input bridge rectifier diodes. On the annode side it should be 120VAC. The cathode side should be 170VDC but that should be with respect to neutral, ground only if the monitor is properly grounded. IF the monitor is not grounded you have no reference to measure with respect to the frame. Also worth noting, if you are trying to measure D2 with respect to the "ground" that is marked on the PCB that won't work since that ground is electrically isolated from mains input.

I need to try to think how to tell you to test certain things on the monitor. What multimeter do you have so I know what features it has.

[HarrieD]

I forgot to mention the NTC, this indeed will decrease in value once warmed up. The value should be around 7 Ohms (according to the manual) but was 27. So i replaced it with one i use in my pinball machines to avoid the circuitbreaker issue.

FYI: i know my electronics, analog and digital, but i'm lacking the experience to troubleshoot this issue. If i'm measuring a value I use a decent multimeter and a scope, unfortunately no fluke but these kind of issues makes me second guess why i don't own a fluke multimeter.

D2 is the same as the D2 on the schematic. Over here we have 240Volts AC. I will do a measurement tonight, got so little time unfortunately.

Thanks for your patience!

[iankellogg]

Well hell if you have a scope that makes it a ton easier, I am so use to people not owning them that I am always trying to figure out ways to measure stuff with the wrong tool.


I wouldn't be too concerned with the NTC. It is just acting as a current limit to charge up those filter caps. If the value isn't quite low enough it will heat up and keep going lower (until it pops!)

I have to say the power supply is one goofy flyback circuit. Can't say I have ever seen anything like this before.

Anyways, i think the power supply is just fine so lets move on. You can probe fairly freely on the board since it has an isolated power supply.

use your scope and probe the base of Q2 and the base of Q3. Ideally you would see 15.7khz at both.

The driving chip is a TDA2593 https://www.tu-chemnitz.de/etit/zentral/ddr/ds/TDA2593.pdf
Check to see if you are getting the 14uS pulse into Q2 and go from there.

[HarrieD]

The grounds are seperated too. Now i know why i don't get a 320Volts reading. Will dust off the scope.

[HarrieD]

Couldn't it be the case that the monitor is in "power save" if it doesn't receive any input? I get output on the vga interface if set to vga modus, but i can't test the rgb output else than with a scope. Will do this of course, but can it be that the monitor is functioning correctly and we are thinking just wrong??
Doesn't explain the broken Q3 however, that thing was broken as hell.

Anywayz, will check the 320 Volts DC and will work my way down. I agree with your idea that the power supply shall be okay, just want to rule that one out.

[iankellogg]

The monitor shouldn't have a power save feature, just going from the circuit it has a PLL to keep the HV flyback running without an input that is also feed into the input flyback for sync.

To test the power supply you are going to want to wire in a 10-20W (120V) lightbulb. i have used CFL's in the past to do this, just get one that has a real power rating of around 10-20W which is equivalent to about 60W incandescent.

Wire the lightbulb across C12 to load it down. I usually remove the HOT as a starting point so you aren't powering up the HV flyback and hurting yourself.

Pretty easy test because if the lightbulb lights up, your power supply is working.  To test with the scope just look at the waveform and make sure you are getting fairly smooth 146-149V DC at the cap and then move your scope ground over to the AC mains ground, grab that at C79, and probe the base of Q1. that should be around 15khz with probably a low duty cycle.


i think it might be worth chatting live for this since it takes a lot of words to describe everything.

[HarrieD]

Will surely try that lightbulb load. This weekend was a little hectic and a tried the quick and dirty method where i replaced IC1 and IC2, and a couple of diodes. After i replaced the ic's the power supply tries to switch on when the yoke isn't connected. I'm really not sure if this behaviour was present before, I'm pretty sure the power supply even didn't want to startup when the yoke was disconnected, but not for 100%.

Thanks for your live chat offer, we need to schedule that because of the time difference. What do you use, skype? Need to fix a camera for that.
But first I will check that lightbulb thingie and poke around with the scope. Will post some screenshots.

[HarrieD]

Hi Ian,

Nice to see your appearance on John's channel a couple of weeks ago! Now I know who i'm talking to.

Just connected the scope to the base of Q1 (anode of D7). I connected the ground of the probe to C7 to ground the scope and bam! all lights went out in my house. I'm stuck and real close to giving up. I really don't understand this schematic.

Got any ideas?