How I Replaced An ADM-3A CRT-And Lived


Safety Warnings

Firstly, let me say that I am NOT a "safety nut". However, this operation deserves great respect in terms of safety.

For liability reasons, I will state that this is not a set of instructions on how to perform this task, or a recommendation to undertake it. This is simply a report on how I performed this task! I replaced my ADM-3A's CRT because it had been burned-in by characters at high brightness sitting on the screen for long periods of time. Also, the tube was "soft" in that it could no longer deliver much brightness without blooming. Although these days, few people replace CRT's because new monitors are so inexpensive, this unit has, I feel, some historical value. For about $50, I replaced my CRT and dramatically improved the unit so that the video looked just like it did 20 years ago - which is pretty lousy compared to even today's worst VGA, but in 1979, this was pretty good!

1. Mechanical

The CRT is glass (not safety glass, just glass) and contains a high vacuum and can IMPLODE if struck or abused. One can reduce, but not eliminate, the danger of flying glass by breaking the seal at the end of the tube (see step 5 below). So, while performing ANY of these tasks:

a.Wear a long-sleeve heavy cloth coat.
b. Wear OSHA-approved safety glasses.
c. Use heavy leather gloves.


2. Electrical

The CRT can retain a high-voltage charge. If you are in reasonably good physical condition, it isn't enough to kill or injure you in itself. The real danger lies in the reflex response after the shock, where you might hit the CRT, equipment, or yourself.

The easiest and most reliable way to deal with this problem IN THIS CASE is to not power the CRT for a week, to permit the charge to drain off. Note that larger CRT's on other equipment may retain a charge for (possibly much) longer times. So, son't power the CRT from the time you determine you need one to the time you get the replacement by mail order, and you should be OK!

Procedure

0. Read above concerning the week wait for the CRT to discharge. The unit should have been in an unplugged state and the power switch ON for this period of time. (Yes, I know that it can be manually discharged, but I am willing to wait.)

1. Open the clamshell and fold it back after removing the screws at the bottom of the case under the keyboard.

2. Noting the locations on the monitor board "under" the CRT, (draw a picture with orientation), disconnect the wiring harnesses to the CRT yoke, noting their positions on the monitor PCB.

3. Using a marker, note the orientation of the yoke (the copper wire knot on the back of the CRT tube) relative to the CRT (mark the plastic yoke frame on the side opposite the anode with the marker).

3. Very carefully, remove the small PCB from the back of the CRT neck. Wobble this part off only very minimally as you remove it.

4. Locate the vacuum seal at the rear of the CRT neck. Cover the main ADM PCB with a blanket, and also as much of the CRT, front and back, with a blanket, leaving only the neck of the CRT exposed.

5. Using a pair of needlenose pliers, break off the vacuum seal. You will hear a woosh as air enters the CRT. Although the danger of an implosion is now eliminated, the glass is still glass, and therefore fragile, particularly at the rear and neck of the CRT where it is thinner.

6. Remove one bracket/screw from the "top" of the CRT (as you see it).

7. Remove the grounding spring across the back of the CRT.

8. Remove the grounding screw/wire.

9. Remove the high-voltage wire from the wire retainer clip.

10. Remove the other screw while supporting the CRT so that the neck will not drop.

11. Lift the CRT/bracket assembly so as to expose the high voltage lead.

12. Remove the high voltage lead from the CRT. Pull back on the rubber shield and note the "clip" design of the connector. Bend the clips to disengage its prongs from the CRT.

13. Place the CRT face down on a pillow or blanket and loosen the screw around the yoke associated with the "C" clamp; you may also have to carefully loosen the screw for the other clamp (not entirely visible) further down the neck, but do this only as a last resort, and then not by more than a turn or two. Loosen the top clamp until you can definitely see that it is loose. A GENIUS at the factory, who no doubt very quickly learned otherwise, had somehow overtightened and stripped the Phillips drive part of my clamp's supplied screw, and there was no room to use its hex head; I had to delicately back the screw off from the other end, which, I assure you, was quite exciting.

NOTE: Remember, "righty, tighty: lefty, loosey" - or, you could crack the neck of the tube.

14. Wobble the yoke up and off the neck of the CRT while avoiding touching or bending the delicate wiring. Do not disturb the positions of any pieces of metal (magnets, possibly) on the prongs surrounding the yoke (these"tune" it). I found it easier to remove the tape above the yoke towards the back of the CRT.

15. Remove the brackets holding the CRT. Mine were riveted, and I had to drill out the rivets, while retaining the front metal "rollers" and the rear washers. Drilling out from the rear worked best. Do not drill out the "shoulder" inside the roller. While drilling, remember that there is a lot of non-safety glass inches away from your tools. Note the position AND ORIENTATION of the brackets on a piece of paper (I inadvertently put the grounding point on the wrong side).

16. Using 3/4" long screws in place of the rivets, place the CRT brackets, rollers, and washers on the new CRT in the same orientation as they were removed from the old one with the new CRT resting on a pillow or blanket. I used Lock-Tite to make certain that the nuts would not loosen, and this is very highly advised. If you have no intention of ever performing this operation again(!), superglue on the threads would also work. Try to keep the brackets "even" as there is some slop in the mounting holes.

17. With the new CRT resting on a pillow or blanket, place the yoke on the (NEW AND VERY DANGEROUS WITH HIGH VACUUM) CRT in precisely the same orientation as it was on the old CRT. Push it down towards the front of the CRT as far as it will go without forcing it.

18. Making certain of orientation, tighten the C clamp. Tighten the clamp(s, see above) ONLY ENOUGH to prevent the yoke from moving easily and absolutely no more. Bear in mind that you are clamping onto GLASS.

19. Install the high-voltage lead and cup connector into the "hole" in the CRT.

20. Replace the CRT into the housing, replacing the two clips and screws.

21. Replace the high-voltage wire into the wire retainer.

22. Replace the PCB at the back of the neck of the CRT (carefully).

23. Put the two deflection yoke connectors back into the main monitor PCB.

24. Replace the grounding wire to the CRT frame.

25. Make certain that wiring is dressed towards the CRT and will not get pinched when the clamshell is closed.

26. Replace the grounding spring accross the back of the CRT. Obviously, you must do this so that the spring is secure and will not fall on the ADM-3 PCB. If you deformed the spring ends when removing the spring, repair this.

27. Close the clamshell.

28. Plug in and turn on the unit and in half-duplex mode or via a jumper type characters. If you've done everything correctly, you should get an image and see characters. It is best at this point to fille the entire screen, 24X80, with W's and check width, parallelism, etc.

29. Adjust as required. It is possible that you may have to adjust focus, or possibly loosen the yoke slightly to orient the horizontal position to match the CRT by turning it. Horizontal and vertical height may need to be changed if the yoke has been moved back or forth relative to the front of the CRT. I would perform these adjustments with great care using long-length fiberglass or plastic tools. I needed to back down the INT BRIGHTNESS control (I suspect that someone had turned it up quite a bit to compensate for the aged tube).

30. Replace and tighten the bottom screws on the ADM-3A housing (but do not overtighten, these are inserts into Cycolac plastic).

31. The procedure is now completed. The ADM-3A should now be ready for many more years of service! Bear in mind that the ADM-3A doesn't have a "screen saver" to blank with, and therefore displaying the same characters constantly is not recommended. In "vintage demonstration" mode, this should not be a consideration.