NEC DM-2710 Professional Video Monitor Repair
 

So I have had this Monitor/TV for several months now trying to get it up and running. It was (from what I understand) going into high voltage shutdown as I could hear and feel high voltage as I hit the power button but it would instantly shut off. After taking a look at the high voltage section of the power supply it was obvious that it was a cap issue as they were leaking out of the bottoms on essentially every cap that was original to the set. Others had been replaced before and had the telltale markings on the tops of those caps.

I got all of the original electro caps replaced and powered the unit up. It was the same deal as before but It was happening much quicker. I guess that sort of makes sense with nice fresh caps? I went ahead and tested the hot transistor and it was fine. I also checked the Film caps that I assumed were ok but they were testing out of spec with my capacitance tester. I went ahead and ordered what I needed to get all of those replaced and finished the job last night. Well it seems there is no difference except that it is happening seemingly even quicker now.

Looking through the nearly impossible to find Service manual for this set and testing voltages with my limited comprehension of how everything works it seems as if my B+ voltages are coming in a bit too high which compounds as it moves through the high voltage circuit causing the unit to go into high voltage protection/shutdown. As of right now I am not sure what would be the best way to proceed other than to test each and every component one by one then replacing what is "bad" then crossing my fingers. I would much rather have a good plan of attack instead of a shot in the dark for the sake of my sanity.

Is there anyone out there that might be able to steer me in the right direction to get this beauty up and running again? I mean the thing has EGA, VGA, BNC RGB, S-Video....this thing needs to work again.

P.S. I am currently trying to upload the service manual for reference. Will update with a url when its done.

Are you sure it's the caps that are leaking, and not just glue holding them in place?
That glue is nasty stuff, as it turns conductive over time, and will mess with your tuned circuits.

Here's the beginning of one of many deep dives on the subject:
https://www.reddit.com/r/audiorepair...e_in_80s_amps/

I would say I am about 99% positive. When this monitor was built they would have been using the silastic stuff which I can see on various places on the board. It is also generally white which the stuff under the caps was not though I suppose it could have broke down and changed color like described but other items in the vicinity have the same stuff that looks perfectly fine. This monitor was built in 92 which in my experience puts it right in with the other electrolytic cap disasters of that era. Either way those caps were replaced and the board thoroughly scrubbed with alcohol. Another thing was that you could see that the rubber plug in alot of the caps were pushed out. Keeping that in mind I will do a quick once over to see if there are any places that might be causing higher voltage due to what you have shown in that post. Could certainly cause what I am describing.

I think my biggest issue is i am not very good at spotting the begining and end of circuite to be able to trace the voltages backwards and see where the faulty component is. I guess its a lack of general understand of a circuit that confuses me.

You have to look at whether its over voltage, over current or
no vert sweep causing the HVSD. That will narrow things down.
Keep in mind most of these tests are best done with an analog meter.
On a digital meter numbers are flashing by in the few ms it runs.

Unplug the degauss coil !!
Measure the AC amps at turn on. It should be < 1 amp.
If over its over current

Measure the HV at turn on OR usually the 200V supply to
the 3 video outputs. If high its over voltage.

No vert sweep is best looked at with a scope. It would probably pass the
previous tests.

With that & a CLEAR schematic you can take it further.

good luck
Zeno:smoke:
LFOD !

Measure amps where the degauss coil connects?

I don't have a KV capable tester so i'll try the 200v supply.

Measure total amps at AC input. Reason to unplug the DGS is when
you turn on it will draw a LOT of current for a second & it will go
into HVSD before it drops.

I would assume that the 200v supply is running too high, causing the monitor to shut down. One problem I have run into in the past is that there may be a B+ adjustment on the PC board, (usually a very small trimmer pot) also glued into place and position by this nasty glue stuff. The idea of gluing it was to keep anyone from tampering with it, thus causing the B+ to run to high. Most times the only fix is to replace the pot, and adjusting it to the proper voltage.

I believe the only trimmer that was covered like this was for the HV adjustment which I guess could actually be an adjustment for the B+ if I understand B+ correctly. Per the service manual when testing power issues I am supposed to turn the 3K pot fully counter clockwise then measure HV and adjust from there. Well of course in the process of pulling the cap and goo off the pot it ripped off the board. I bought and installed a new one but it doesnt seem to be making a difference. I may not have actually measured B+ though while adjusting the trimmer. Either way no matter where that trimmer is it still wont produce constant HV.

I will be doing this tonight and will report back. Sorry I have been distracted by some reel to reel goodness :)

...

Well I am finally reviving this project after several months because well....i need it out of my kitchen and for it to be put in the game room where it belongs. I headed over to Ebay to pick up a flyback ring tester. I almost picked up a new antek blue ring tester but came across a new out of the box Heathkit ST-5235. I went with this tester because im a sucker for vintage gear and it had a few extra features that seemed useful. The standout one being the high voltage with both a 10kv and 40kv scale. I wasnt able to find the 990 MOhm probe for a reasonable price but managed to pick up the 1090 MOhm probe for a steal. I know the reading will be off but nothing a little math cant fix.

The reason I want to eliminate the flyback as the issue before I do anything else is because this part seems nonexistant anywhere. I have actually been unable to find a single reference to it online anywhere. This seems to apply to this set in general as well. I cant imagine they made more than a few thousand of these things that were imported here. anyways if the flyback is bad I am unsure if I will even be able to source a replacement so I'd rather know now than to go and do a bunch of other troubleshooting work only to find out later it was a waste because I cant get a replacement. I was also wondering if anybody might be able to help me with being able to find one. I'm sure most of you guys know a few things that I dont't.

Part# 47105388
This is an NEC part number

I have found several references to numbers that are close but none to this number. The closest that I have found is 47105380.

Anyways I will let you guys know what I find. I really want this thing to come back to life. It's too awesome not to.

Here is a picture from the service manual if it helps in any way.
https://cdn1.imggmi.com/uploads/2018...0b89b-full.png

I did find this trimmer pot and it was indeed capped and filled in. Removing this cap also removed the trimmer pot in quite spectacular fashion :). I sourced a new one from digikey and installed it with no change in results either way I set it. I believe this trim pot is used to dial in the HV? Does that sound right?

These really do sound like very good suggestions but I believe at this level it is where it begins to start going over my head. Unfortunately I do not have a variac and I am unsure what type of power supply it is. Is there something simple I can look for to help identify the type? As far as the IC's go I dont believe that any are socketed but I will go back over it and check.

On a linear supply, the AC cord connects to a large 60Hz power transformer the secondaries of which are rectified.

On a switch mode supply, the AC cord connects to a diode bridge and some LARGE capacitors...Then to a switching transistor a small high-frequency transformer and some additional smaller rectification and filtering.

The idea of switch mode is the higher freq the smaller and cheaper the trans, and regulation and protection is built in by design.

Here you go :) (PWC-3529)
https://i.postimg.cc/ncp41dql/Power.png

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

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...

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:smoke:
LFOD !

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.

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.

...

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

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

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

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.

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.

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

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?

The transformer will be wound with heavy wire & not much so you
will get very low ohms.
Unfortunately the ZOOM doesnt blow things up enuf so I can see it.
The hoz lin & pincushion usually will be in series with the yoke. Follow the hoz
windings of the yoke from the collector of the hoz out transistor through. Watch for a yoke coupling cap in series with it. Also look for any diodes off the C of the hoz output. If any lift one end & do diode test with your meter.

Yes I did do some monitors inc NEC. I am NO expert in them. In fact I hated
monitors. I was doing piece work for a cat & told him " no more monitors ,
projos, or VCR / TV combos". All a PIA.
Keep in mind monitors were fixed BOARD LEVEL ONLY as far as a MFG goes.
Even getting a manual set you back $50 - $75. Add to that you could not
buy parts ONLY boards. If you get this going you will have something to
be very proud of.........

73 Zeno:smoke:
LFOD !