NeXT Megapixel Display N4000A Repairs

 As you may have seen previously on my blog, I'm fortunate to be the custodian of a NeXTStation setup that I purchased at last year's Vintage Computer Festival - Midwest.

Since blogging about that machine, I've recapped the mainboard, added an External SCSI CD-ROM so I can load software the old fashioned way, and upgraded the Hard Drive to a 2GB Quantum Fireball (Fingers crossed, it's still working!).  One outstanding issue with the machine has been that the audio on the monitor has not worked since I owned the machine.

According to this article, on the nextcomputers.org forum, it was likely that a circuit board in the monitor need to be recapped.  I finally got the time to tear into the monitor and have a look.  What I found surprised me, and so I thought I'd put this out here for the community so you can potentially avoid these pitfalls.

First let me start with the obligatory warning about working inside of CRTs.  I recommend you not do what I've done. If you do decide to go ahead, then watch a video or three about how to properly discharge a CRT before you work on it.  OK now that that's over with...

First, it was pretty easy to tear down the monitor, but I did make a mistake along the way removing the stand from the bottom of the monitor.   You don't need to do that, and if you do release the tension on the monitor stand's hinge, it's difficult to get that back in adjustment, so I'd recommend that you not remove the stand or mess with the hinge.

The back comes off with four Allen screws...

This exposes the old school RF shielding.  I'd recommend putting this back on when you are done (I'm looking at you Adrian Black).  The shielding is held on by four screws on one side and five screws on the other, they are all the same length.   When they are off, the housing comes back just a bit to release from some tabs, then can go straight up off the monitor...

This exposes the entire CRT and both boards.  Now would be the time to discharge the CRT if you were following along, which of course you are not because this is dangerous...

The main circuit board cover the back of the monitor, but the sound board is on the bottom of the enclosure.  It cannot be removed without taking the electronics off the chassis, but this is pretty easy to do.  

On the bottom of the monitor, there are two large screws that hold the electronics portion of the chassis onto the sub-frame that holds the tube and the stand.  Removed these two screws...

Disconnect all the cables from the tube, including the high voltage, the yoke, and the tube socket at the end of the tube.  Once they are disconnected, the rear of the chassis with the electronics on it can be pulled straight back.  It may be easier to pull it back just an inch to get your hand in to remove the socket from the end of the CRT.  Once all the wires are de-routed the electronics are more accessible...

The large circuit board must come off the back first before the connector board with the sound circuit can be removed.  There are two connectors going from the large board on the back to the small board on the bottom, remove those.  There are five screws holding it in.  Remove the screws and set the large board aside.  Note:  I did a careful inspection and some cleaning on this board, but I did not see any of the small electrolytic capacitors on this board, so I didn't do any recapping on this board at this time.

The small circuit board is held in by two screws inside, and two nuts on the outside of the enclosure where the cable attaches, remove those...

The circuit board has a Dozen (12) Electrolytic Capacitors. They are 47uF, 16 V rated caps...

Remove and replace the caps.  In this case the monitor was from 1991 and so it was suffering from capacitor leakage and needed some minor rework on a couple of the traces.  I'm glad I caught this in time.

But, there was one more twist in the story.  When I tested the monitor, the audio was not restored to function.  After measuring resistance on the speaker, it was open circuit.  I was able to source a replacement speaker from Rob at Black Hole, Inc, and the monitor and speaker are now functioning normally.

The final hardware upgrade I'm planning for this machine is to replace the Spinning HDD with a Solid State device, but in the meantime I've got alot of learning to do about how this machine works and what its capabilities are.