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Old 03-19-2018, 01:13 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, 02:36 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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Old 03-19-2018, 03:02 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 03:10 PM.
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Old 03-19-2018, 04:26 PM
zeno zeno is online now
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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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Old 03-20-2018, 10:39 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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  #6  
Old 03-20-2018, 05:31 PM
zeno zeno is online now
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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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Old 03-20-2018, 06:01 PM
walterbeers walterbeers is online now
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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, 08:46 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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  #9  
Old 03-23-2018, 09:57 AM
andy's Avatar
andy andy is offline
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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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