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  #1  
Old 03-19-2018, 02:13 PM
Shibby Shibby is offline
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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.
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Old 03-19-2018, 03:36 PM
Chip Chester Chip Chester is offline
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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/
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  #3  
Old 03-19-2018, 04:02 PM
Shibby Shibby is offline
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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.

Last edited by Shibby; 03-19-2018 at 04:10 PM.
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  #4  
Old 03-19-2018, 05:26 PM
zeno zeno is offline
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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
LFOD !
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  #5  
Old 03-20-2018, 11:39 AM
Shibby Shibby is offline
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Quote:
Originally Posted by zeno View Post
Unplug the degauss coil !!
Measure the AC amps at turn on. It should be < 1 amp.
If over its over current
Measure amps where the degauss coil connects?

Quote:
Originally Posted by zeno View Post
Measure the HV at turn on OR usually the 200V supply to
the 3 video outputs. If high its over voltage.
I don't have a KV capable tester so i'll try the 200v supply.
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Old 03-20-2018, 06:31 PM
zeno zeno is offline
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Quote:
Originally Posted by Shibby View Post
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.
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  #7  
Old 03-20-2018, 07:01 PM
walterbeers walterbeers is offline
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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.
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Old 03-21-2018, 09:46 AM
Shibby Shibby is offline
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Quote:
Originally Posted by walterbeers View Post
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.

Quote:
Originally Posted by zeno View Post
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 will be doing this tonight and will report back. Sorry I have been distracted by some reel to reel goodness
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Old 03-23-2018, 10:57 AM
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The first thing you need to do is verify that the low voltage power supply is regulating properly. Does it have a switching power supply, or a linear voltage regulator?

If it's a switching power supply, is the power supply shutting down, or just the horizontal sweep/HV?

If it's a linear power supply, it's common for the regulator IC to short causing excessive B+ and HV shutdown. You can do a quick test for this by running it at reduced line voltage using a variac.

I don't suppose it has any large ICs in sockets? NEC had problems with IC sockets in the 80's that would cause all kinds of trouble. The solution was to get rid of the socket and solder the IC directly to the board.

A few good pictures of the inside might prompt some more suggestions.
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Old 09-21-2018, 11:52 AM
Shibby Shibby is offline
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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.

Last edited by Shibby; 09-21-2018 at 12:07 PM.
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  #11  
Old 09-21-2018, 12:12 PM
Shibby Shibby is offline
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Quote:
Originally Posted by walterbeers View Post
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 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?
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Old 09-21-2018, 12:15 PM
Shibby Shibby is offline
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Quote:
Originally Posted by andy View Post
The first thing you need to do is verify that the low voltage power supply is regulating properly. Does it have a switching power supply, or a linear voltage regulator?

If it's a switching power supply, is the power supply shutting down, or just the horizontal sweep/HV?

If it's a linear power supply, it's common for the regulator IC to short causing excessive B+ and HV shutdown. You can do a quick test for this by running it at reduced line voltage using a variac.

I don't suppose it has any large ICs in sockets? NEC had problems with IC sockets in the 80's that would cause all kinds of trouble. The solution was to get rid of the socket and solder the IC directly to the board.

A few good pictures of the inside might prompt some more suggestions.
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.
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  #13  
Old 09-21-2018, 12:34 PM
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Electronic M Electronic M is online now
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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.
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Old 09-21-2018, 02:47 PM
Shibby Shibby is offline
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Quote:
Originally Posted by Electronic M View Post
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

Last edited by Shibby; 09-21-2018 at 03:12 PM.
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  #15  
Old 09-21-2018, 04:57 PM
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Quote:
Originally Posted by Shibby View Post
Thanks, a link to a page that only says "500 Internal Server Error" is just what I always wanted.
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