Hi everyone - I have a turbo mono nextsation and an ADB 17" mono display.
It was working perfectly - but i had to slide the monitor off the computer to reach something, and stupidly - i did it while the computer was on, and the monitor cable wiggled loose.
monitor went black and hasn't then wouldn't come back on. Ive pulled it apart, de-energized the CRT and removed all the boards. Cant find any obvious damage to anything. Capacitors all seem OK - and the few that could maybe have been sketchy, we removed and tested and they test OK.
The computer still powers on and boots as I can telnet into it. The tube's filament still glows.
I'm just getting zero picture at all.
Where can I go from here? I'm getting a high voltage probe to test the flyback voltage, but I would love any help if anyone has any ideas. I am eying an ADB version of the monitor on ebay - does anyone know if the CRT boards are compatible between them?
Is there a circuit diagram of these monitors available anywhere?
The tubes are exactly the same in the n4000a, with the only difference being the sound board and possibly the monitor mount. Makes sure you see the microphone grill on the front and it should be compatible. Unfortunately no schematics exist. There is 12V and -12V going into the monitor. One of these rails likely shorted, so I would start by checking/tracing everything immediately connected to those.
I have some relevant information on pinouts here:
http://asterontech.com/Asterontech/next_adb_conversion.html
Quote from: barcher174 on January 23, 2025, 03:48:44 PMThe tubes are exactly the same in the n4000a, with the only difference being the sound board and possibly the monitor mount. Makes sure you see the microphone grill on the front and it should be compatible. Unfortunately no schematics exist. There is 12V and -12V going into the monitor. One of these rails likely shorted, so I would start by checking/tracing everything immediately connected to those.
I have some relevant information on pinouts here:
http://asterontech.com/Asterontech/next_adb_conversion.html
did some more testing. Im beginning to think its the audio board in the monitor. I verified the video output is working, i get good video and h and v sync off the nextstation.
oscilliscope on the input and output side of the horizontal driver on the CRT - yields NOTHING. like the CRT is getting no signal at all.
that link above is a big help - i just pulled the audio board and getting the pinouts that go to the monitor is gonna help me diagnose more.
god i hope its not that board because that obviously wouldnt transfer from the A to my B!
update - THANK YOU for that link.
Just verified 100% that the sound board is working fine. video, HSYNC and VSYNC all match from the computer connector to the output to the CRT board. Onward!
@barcher174 So i did some further testing - on the 19 pin connection, when the blue/grey cable from the sound board to the CRT board is connected internally - there is a short from the + and - 12V to ground readable on the other end of the video cable.
When I disconnect the sound board from the CRT board - the short goes away.
With the sound board disconnected from the CRT - i get a good 12v and -12v and all good signals out of the board.
Am i right in thinking theres a short to ground somewhere on the CRT's board?
Quote from: barcher174 on January 23, 2025, 03:48:44 PMThe tubes are exactly the same in the n4000a
I don't think this is so? The ADB mono display is the N4000B if I'm not mistaken. The B model doesn't dim because it has a different tube: see section 5.41 of the NeXT FAQ (
https://www.levenez.com/NeXTSTEP/faq.html).
I think this message ("The N4000B, not the N4000A, is the one that doesn't dim as easily, check the FAQ") is the thing I post about the most on this forum by a considerable margin. I see that Wikipedia is wrong about this.
Quote from: stepleton on January 23, 2025, 05:41:14 PMI don't think this is so? The ADB mono display is the N4000B if I'm not mistaken. The B model doesn't dim because it has a different tube: see section 5.41 of the NeXT FAQ (https://www.levenez.com/NeXTSTEP/faq.html).
I think this message ("The N4000B, not the N4000A, is the one that doesn't dim as easily, check the FAQ") is the thing I post about the most on this forum by a considerable margin. I see that Wikipedia is wrong about this.
The n4000 contains the original tube with dimming problems. (And also the feet liquify)
The n4000a is the non-adb model with the new tube
The n4000b is the ADB model with the new tube
Both the a and b are identifiable by the addition of the mic grill on the front.
Quote from: jdanna on January 23, 2025, 04:44:02 PM@barcher174 So i did some further testing - on the 19 pin connection, when the blue/grey cable from the sound board to the CRT board is connected internally - there is a short from the + and - 12V to ground readable on the other end of the video cable.
When I disconnect the sound board from the CRT board - the short goes away.
With the sound board disconnected from the CRT - i get a good 12v and -12v and all good signals out of the board.
Am i right in thinking theres a short to ground somewhere on the CRT's board?
Yup, that sure sounds like it. The soundboard just passes the power and video through, so I wouldn't have expected the problem to be there. That should help isolate significantly. I would assume there's less to trace on the -12 side (but I'm just guessing).
Just in case, here is my n4000 recap guide that has the adjustment pot info if you get it going again:
http://www.asterontech.com/Asterontech/NeXT_MegaPixel_Restore.html
Ugh scratch the short idea. I was checking the wrong pins, i had my orientation of the DB19 connector backwards in my head.
So 12v + and - are all good to the CRT board. But somehow there's no signal getting to the H or V drivers. I checked on both sides of the transistor, and at the test point, and they get nothing.
Quote from: barcher174 on January 23, 2025, 06:18:54 PMThe n4000 contains the original tube with dimming problems. (And also the feet liquify)
The n4000a is the non-adb model with the new tube
The n4000b is the ADB model with the new tube
Well, the FAQ disagrees about this, saying:
QuoteThe other type of CRT cathode is the I-cathode or dispenser type. This type of cathode is porous and continually brings new activation material to the surface. Its lifetime is 40,000 hours or more. The last of the NeXT monochrome monitors (N4000B) used this type of CRT and they don't go dim. There aren't many of that type around because NeXT quit the hardware business after producing only a few thousand. If you can get an N4000B monitor, you won't ever have to worry about a dim monitor.
This should be easy to check.
@jdanna , would you mind taking photos of any identifying labels or markings on your CRT? I can open up my N4000A and then perhaps we can compare to see if we really do have the same tube.
Quote from: stepleton on January 24, 2025, 12:28:09 PMWell, the FAQ disagrees about this, saying:
This should be easy to check. @jdanna , would you mind taking photos of any identifying labels or markings on your CRT? I can open up my N4000A and then perhaps we can compare to see if we really do have the same tube.
well i have an A unit coming from ebay, and a B unit here - once i ahve both ill compare!
@barcher174 sorry to keep tagging you - but ive been tracing down the circuits as best i can without a diagram. I found a zener diode labeled ZD601 that seems to test as completely open and failed.
Any idea what voltage that should be replaced with? its so small I cant tell if theres any writing on it.
edit- nevermind. it tests fine once removed from the circuit.
ugh back to the beginning
Hopefully with the new megapixel coming you can do some board swapping to save some sanity.
Honestly I'm no expert on CRTs. It seems like maybe this would be a good place to start:
https://www.alldatasheet.com/datasheet-pdf/view/17612/PHILIPS/CR2424.htmlI assume there would be a 24V supply voltage. -12 --> 12V