NEC DM-2710 Professional Video Monitor Repair - Page 2 - Videokarma.org TV - Video

Jon A. · #16 09-21-2018, 04:36 PM

Quote:

Originally Posted by Electronic M

Thanks, a link to a page that only says "500 Internal Server Error" is just what I always wanted.

Had I known that I would have gladly given you those I've found.

Seriously though, it'll probably work later on, we just have to wait the bloody things out. I'd like to be able to reach through the screen though and do a hard reset on the server.

Electronic M · #17 09-21-2018, 06:10 PM

Quote:

Originally Posted by Jon A.

Had I known that I would have gladly given you those I've found.

Seriously though, it'll probably work later on, we just have to wait the bloody things out. I'd like to be able to reach through the screen though and do a hard reset on the server.

Snickers, I asked for that didn't I...

zeno · #18 09-22-2018, 03:58 PM

This set will have a switching supply.
The flyback ( FBT ) is probably good. Normally they short
& no HV will come up or it will go into overcurrent SD or self destruct.
NEC monitor switchers were a large module very well shielded.
A look inside may turn up more bad electrolytics.
If its marked on the PCB you can look for the HV B+ supply & see if
it goes above spec ( typicaly abt 120 VDC ) before SD.

good luck
Zeno

LFOD !

Shibby · #19 11-19-2018, 11:35 AM

Well I was finally able to get my hands on a variac. A pre patent General Radio 20A 260v unit that looked like it was marked for military use for a cool $30. I'm pretty happy with that. Anyways as soon as I got it home I tested the variac on an unsuspecting fan to ensure it wasn't going to somehow kill my monitor even more. I then hooked it up to the monitor and slowly brought up the voltage.

The input lights slowly came on and it locked in on its default input 2. The fans started coming up slowly and I could hear various relays clicking. At about 80 volts I heard what sounded like high voltage but felt nothing on the front of the tube. It still had not yet gone into shutdown. Just past 80v I start to hear an electronic type noise that to me sounded like either arching or some type of short. The sound was coming from the DEF-PWB which I can only assume is the deflection board.

As I continued to ramp up the voltage the noise increased in volume and became more sporadic in that the tone was constantly changing. Then at about 100v the relay clicked and the monitor went into shutdown and the fans spun up to 100% as usual. I went through this process a few times which probably wasn't at all good for it but I couldn't help myself. The same thing happened pretty much every time. I did this with all the lights off at one point to see if I could see any arching of some type but all I could see were a few sparks coming from what I think was the degauss coil.

At one point I revved up the variac to just before where it usually went into shutdown and let it sit there for a few seconds. I then began to see smoke. Keep in mind that while doing this my only view was the bottom side of monitor with the bottom of the HV/PS-PWB and the DEF-PWB in view. There are a few passive components soldered and hot snotted to the bottom side of both of the boards. The smoke began coming from a resistor on the bottom side of the DEF board right around where the noise was coming from on the top side of the DEF board. The resistor was of course very hot but I think it was the hot glue around it that began to smoke and not the resistor itself. Anyways I of course shut it off immediately. From my very limited knowledge of what I am doing it seems to me that there is indeed a short on the Deflection board. To be honest while I did look the board over a good few times while still screwed into the chassis I never really inspected it thoroughly. When I got this monitor the HV Power Supply board was so mangled with leaky caps I thought for sure that was the issue. I had also not seen anything obvious capacitor wise on the deflection board so I didn't pay it much attention. Either way I will be pulling it out this evening and giving it a good once over. Now that I look at what all is on this board and what it does it seems sort of silly that I hadn't paid it more attention but I guess that's how it goes when you are learning. I will do my best to get the diagrams up of this board for you guys today just for reference but it may take a few as my work network and security systems tend to get a bit grumpy.

Anyways other than the obvious of looking for broken/burnt/leaking components any tips for helping to track down a potential short? Any idea as to what type of component might emit that type of noise? Anything I am not thinking of?

Additional Info: I was only able to get this result when slowly ramping up the variac. If I were to set the variac to say 80v and flip the switch the monitor would still go into shutdown. I believe this was still happening when set to as low as 60v. When I would slowly ramp up the variac I could get it up to around 100v before it would shut down. Am I somehow creating this shorting issue by not giving the monitor enough juice fast enough when ramping? Just trying to think through everything.

Shibby · #20 11-19-2018, 11:54 AM

Quote:

Originally Posted by zeno

This set will have a switching supply.
The flyback ( FBT ) is probably good. Normally they short
& no HV will come up or it will go into overcurrent SD or self destruct.
NEC monitor switchers were a large module very well shielded.
A look inside may turn up more bad electrolytics.
If its marked on the PCB you can look for the HV B+ supply & see if
it goes above spec ( typicaly abt 120 VDC ) before SD.

good luck
Zeno

LFOD !

Thanks for that info Zeno. I really do hope that the FBT is still good because apparently this is some kind of top secret experimental government component that doesn't even exist on paper. As you can see from my previous post i'm starting to think I might be in the clear there but only time will tell. Will be on the hunt for more bad components tonight.

I did measure the B+ and from what I saw it was slightly over spec. This is assuming that I was measuring correctly and had the right numbers. It may or may not have been within tolerance though. I don't remember the service manual giving me a +/- tolerance number but I might be misremembering. Either way I intend to repeat my measurements with the variac if needed so I can see how the voltages are behaving before shutdown.

You seem to know a bit about NEC monitors. Do you know anything about this specific model? I cant seem to find a single thing on it and was extremely lucky to find the service manual. Only one site seemed to have it and yes I did have to pay for it. It just seems to be very very uncommon and I'd love to know more about it. If it helps any the guy I got it from took it from a security company that he had worked for when they were throwing it out.

andy · #21 11-19-2018, 12:03 PM

...

Shibby · #22 11-19-2018, 12:31 PM

Quote:

Originally Posted by andy

That sparking was probably static electricity from the HV. There's no other reason there should be any sparking around the degaussing coil!

If that glue is tan or brown, remove every trace of it, and look over the rest of the monitor for more. It turns conductive and corrosive with time.

Its most certainly hot glue and looks normal but will double check that tonight. Thanks!

Shibby · #23 11-19-2018, 02:46 PM

Part Location Diagram
http://i63.tinypic.com/6sun89.jpg

Block Diagram
http://i66.tinypic.com/sg1vz4.jpg

Schematic Diagram
http://i65.tinypic.com/10wtld5.jpg

Solder Side
http://i68.tinypic.com/258nok0.jpg

Shibby · #24 11-20-2018, 10:56 AM

So this is where I get stuck. I tend to be decently good at troubleshooting things as this is what I do every day at work but I am missing the fundamental understanding of how circuits work. Knowing that I am hoping that you guys might be able to steer me in the right direction.

I went ahead and pulled the board last night so I could get a good look at it. After a visual inspection of all of the components nothing seemed too fishy. The smoking resistor I mentioned was indeed burned but seems to have retained its resistance when tested out of series. When in series I get clear continuity between both legs of the resistor. Of course this means that there is continuity between the pads that each leg of the resistor is on.

This is where I get lost. If you look at the solder side photo that I attached in my last post and locate Transformer 5004 (T5004) you will see the 2 pads that I am referencing. Unfortunately this service manual does not reference the "burnt" resistor that also shares these pads but if you look at the picture again right where it says T5004 imagine the resistor being right there. On my board it is labeled as R5095.

So on my board these 2 pads are shared by 3 components. R5095, C5015, and T5004. After pulling both R5095 and C5015 There was still continuity between the pads. This obviously left me with just the transformer. It is hard to see in the solder side diagram/image but on the laft side of the transformer there are 4 legs. 2 on the upper pad and 2 on the lower pad. You can see the holes for these legs as faint white dots on each pad. On the right side of the transformer are 2 legs thought there are 3 individual pads. Only 2 of the pads are used. According to the parts list this is a "Choke Transformer". After doing a bit of research the configuration seems to match that of a common-mode choke configuration with the left side 4 pins being 2 separate inputs and the right side 2 pins being the output.

So I guess here is my question. Should all 4 of the pins on the left side of the transformer have continuity between them? If not I would thing that this is my issue as it is essentially bridging the 2 pads together. If they are supposed to have continuity then what should I be looking at next?

I really have no idea if I am even close to barking up the right tree here so please be patient with me. If it helps this is the NEC part number for the transformer with the description. 60906204 COIL,WIDTH CHOKE. That description may be a typo as nothing came up when I googled width choke.

Anyways.....Any ides here? I truly am lost.

Here is a picture to reference the resistor missing in the service manual

Electronic M · #25 11-20-2018, 11:38 AM

T5004 the side with 4 terminals should have w terminals connected with one trace and two connected with the other. The schematic won't open on my phone so from here I can only make educated guesses. I would assume the resistor is connected across the two traces on the.4 terminal side, and that you are getting a very low resistance across those terminals. A transformer winding is just a coil of wire so assuming those two traces connect to the ends of T5004 there should be a likely low resistance there from the transformer... check the transformers rated resistance on that winding to confirm it is not shorted. Transformers can be wound for any resistance on a winding and anything between .1 and 70k ohm could be possible depending on the circuit.

jr_tech · #26 11-20-2018, 12:21 PM

I can’t open the schematic either on iPad...also no luck on an Android device... could this be the problem?

Quote:

One of the main criticisms people have with TinyPic is that uploaded images or videos, which are not associated with an account, usually get deleted within a few days or weeks [6] resulting in any post or website that has used TinyPic to display a picture not found error message. Images or videos which are associated with an account are not deleted.[7]

(from Wikipedia)

jr

Shibby · #27 11-20-2018, 01:20 PM

Quote:

Originally Posted by jr_tech

I can’t open the schematic either on iPad...also no luck on an Android device... could this be the problem?
(from Wikipedia)

jr

Awesome.... Smh... Let me get these reuploaded for you guys.

Shibby · #28 11-20-2018, 01:34 PM

Ok I went ahead and just created a google drive folder and placed all the pictures i tried sharing before in there. i also went ahead and copied the entire service manual there as well. From here on out anything else I need to add will just get placed in there. I'll also look at putting the link in my avatar for easy reference.

https://drive.google.com/open?id=1lO...R5opTG-QQRkQoo

Electronic M · #29 11-20-2018, 01:44 PM

Yeah c5015 is connected across a winding of T5004 and unless the transformer is removed from circuit it's winding resistance will probably dominate any resistance measurements made across the solder pads of c5015...

Shibby · #30 11-20-2018, 01:52 PM

Quote:

Originally Posted by Electronic M

Yeah c5015 is connected across a winding of T5004 and unless the transformer is removed from circuit it's winding resistance will probably dominate any resistance measurements made across the solder pads of c5015...

Right. I pulled the Transformer off the board and measured the resistance/continuity between the 2 sets of 2 pins on the left side of the transformer and they are shorted. I guess my question is should they be this way? What I am really trying to do here is find out what is causing the noise in that area of the board and why its overheating the resistor in the same area. could the noise also be coming from the resistor? What would be the first logical thing to check as to what the cause of that is?