1980s_john
Experienced Member
Hi,
I bought this monitor back in 2015 as faulty, and it has been sat on my shelf until a few weeks ago. The service manual is on-line, make sure when downloading your copy has page 2 with the connector pin-outs and signals as some versions out there have this page missing.
The monitor comes apart easily, I cleaned the covers as these were dirty, and cleaned most of the PCBs and the tube. I didn't remove the main PCB for cleaning as this is hard to do so still has a bit of dust lurking there. I started with the PSU as this is a common failure point, this comes out easily. I checked the electrolytic caps in circuit with an LCR meter, here's list of rated value and the measured as follows (those marked x I replaced):
Primary:
C605 220uF 400V = 202uF
C611 220uF 16V = 2.6uF x
C610 470uF 16V = 407uF x
C609 4.7uF 50V = 6uF Rs=160 ohm x
C617 220uF 16V = 1.7uF x
C616 470uF 16V = 407uF x
C620 22uF 50V = 20uF Rs=30 ohm x
C615 4.7uF 50V = 5.3uF Rs=70 ohm x
Secondary:
C658 100uF
C662 100uF
C657 100uF = 280uF (all three in parallel, looked OK)
C621 22uF 50V = 19uF Rs=36 ohm x
C664 1uF 160V = 1uF Rs=26 ohm x (but could have left in as measured OK when removed)
C660 0.47uF 50V = 520nF Rs=150 ohm x (again measured OK when removed)
C652 1000uF = 970uF
Following were paralleled up by some inductors, I left as is as Rs measured low.
C651 330uF = 380uF
C653 100uF = 515uF
C654 100uF = 515uF
Having replaced the caps marked x (with good brands eg Panasonic and Rubycon) I then removed the metal shielding around the rear. These shields had some surface rust so I sanded them down to a nice grey finish. I reassembled everything and the monitor powered on OK. There is no raster until a signal is applied, for testing I used the following:
Video leads - bought a 9 pin male to 9 pin male straight through cable (not shielded), and a short 9 pin female to 15 pin VGA adapter lead (wired so that RGB, V & H sync and ground were connected through).
Video tester - Blackstar Orion TV pattern generator. This outputs RGB at TTL or 0.7V, and syncs at TTL levels (or sync on green) at a fixed 15.75kHz horizontal and 50Hz vertical rates for UK TVs or monitors (plus has a PAL composite output not needed here). Outputs various patterns such as dots and colour bars. The NEC monitor auto-adjusts between TTL and 0.7V levels (clever).
PC with VGA card set to 640x480 60Hz, running Nokia monitor test software.
When hooked up the picture looked good, apart from being too bright even with brightness set to minimum (so black appeared grey) and would wash out to all white when increased. I did some reading and looked through the service manual and thought the issue was the CRT G2 or screen voltage not being set correctly. I eventually found the screen voltage control was on the side of the flyback transformer (LOPT) under the focus control. I adjusted it (with monitor unplugged) and eventually got it so that with brightness control set to minimum black was black and rest of picture slightly dim, and can now set brightness and contrast normally. The screen control was very sensitive, I reckon only had to turn it 5 degrees from its original setting (anticlockwise to reduce brightness, clockwise to increase). Maybe the previous owner with a failing PSU tweaked it up, I don't know.
This is a great monitor when working, luckily I didn't have to delve in to any of the other PCBs (there's the main board, two boards for signal input, PCB neck board, front control board and of course PSU board) as although the monitor is designed to be taken apart it is quite a job to do so.
There was one feature I haven't got to work (so may be a fault) which is the text mode which with a colour input is meant to switch the display to a monochrome white, amber and green (I think). This control didn't do anything, maybe I need a CGA input rather than VGA, but not an issue as I don't need the feature.
Regards,
John
I bought this monitor back in 2015 as faulty, and it has been sat on my shelf until a few weeks ago. The service manual is on-line, make sure when downloading your copy has page 2 with the connector pin-outs and signals as some versions out there have this page missing.
The monitor comes apart easily, I cleaned the covers as these were dirty, and cleaned most of the PCBs and the tube. I didn't remove the main PCB for cleaning as this is hard to do so still has a bit of dust lurking there. I started with the PSU as this is a common failure point, this comes out easily. I checked the electrolytic caps in circuit with an LCR meter, here's list of rated value and the measured as follows (those marked x I replaced):
Primary:
C605 220uF 400V = 202uF
C611 220uF 16V = 2.6uF x
C610 470uF 16V = 407uF x
C609 4.7uF 50V = 6uF Rs=160 ohm x
C617 220uF 16V = 1.7uF x
C616 470uF 16V = 407uF x
C620 22uF 50V = 20uF Rs=30 ohm x
C615 4.7uF 50V = 5.3uF Rs=70 ohm x
Secondary:
C658 100uF
C662 100uF
C657 100uF = 280uF (all three in parallel, looked OK)
C621 22uF 50V = 19uF Rs=36 ohm x
C664 1uF 160V = 1uF Rs=26 ohm x (but could have left in as measured OK when removed)
C660 0.47uF 50V = 520nF Rs=150 ohm x (again measured OK when removed)
C652 1000uF = 970uF
Following were paralleled up by some inductors, I left as is as Rs measured low.
C651 330uF = 380uF
C653 100uF = 515uF
C654 100uF = 515uF
Having replaced the caps marked x (with good brands eg Panasonic and Rubycon) I then removed the metal shielding around the rear. These shields had some surface rust so I sanded them down to a nice grey finish. I reassembled everything and the monitor powered on OK. There is no raster until a signal is applied, for testing I used the following:
Video leads - bought a 9 pin male to 9 pin male straight through cable (not shielded), and a short 9 pin female to 15 pin VGA adapter lead (wired so that RGB, V & H sync and ground were connected through).
Video tester - Blackstar Orion TV pattern generator. This outputs RGB at TTL or 0.7V, and syncs at TTL levels (or sync on green) at a fixed 15.75kHz horizontal and 50Hz vertical rates for UK TVs or monitors (plus has a PAL composite output not needed here). Outputs various patterns such as dots and colour bars. The NEC monitor auto-adjusts between TTL and 0.7V levels (clever).
PC with VGA card set to 640x480 60Hz, running Nokia monitor test software.
When hooked up the picture looked good, apart from being too bright even with brightness set to minimum (so black appeared grey) and would wash out to all white when increased. I did some reading and looked through the service manual and thought the issue was the CRT G2 or screen voltage not being set correctly. I eventually found the screen voltage control was on the side of the flyback transformer (LOPT) under the focus control. I adjusted it (with monitor unplugged) and eventually got it so that with brightness control set to minimum black was black and rest of picture slightly dim, and can now set brightness and contrast normally. The screen control was very sensitive, I reckon only had to turn it 5 degrees from its original setting (anticlockwise to reduce brightness, clockwise to increase). Maybe the previous owner with a failing PSU tweaked it up, I don't know.
This is a great monitor when working, luckily I didn't have to delve in to any of the other PCBs (there's the main board, two boards for signal input, PCB neck board, front control board and of course PSU board) as although the monitor is designed to be taken apart it is quite a job to do so.
There was one feature I haven't got to work (so may be a fault) which is the text mode which with a colour input is meant to switch the display to a monochrome white, amber and green (I think). This control didn't do anything, maybe I need a CGA input rather than VGA, but not an issue as I don't need the feature.
Regards,
John
