Thursday, April 23, 2009

Adventures in Backlighting, Part 3: The K5m Lives!

So having figured out the cut-to-size EL sheet, it was time to put it to use. Out came the K5m to the workbench. Once I recalled how to open the jigsaw-puzzle case, I saw this:

The half on the left, laying flat, is the front panel. The right half is the rear half. A closer view of the front:

To cram the power supply into the K5m's case, they had to split it into two pieces. The transformers were crammed into a corner of the back part of the case, and the voltage regulators were put on a separate board, which is the tan-colored board on the left in the photo above. This board sits atop the LCD display board on standoffs, and it also contains the backlight inverter, which you can see below as the white box on the right. In this photo, the power cables have been disconnected from th board and it is in the process of being removed. To the right of the inverter, the small connector and (very short) cable that routes the inverter output to the display board is still connected. Note that if you have the keyboard version of the K5, this board is not here, but there is a ribbon cable that runs across the back of the display board. On the keyboard K5, you have to remove one of the panel boards to get the ribbon cable out of the way; see here. On the K5m, once you have the regulator board out of the way, the display board is free and clear.

Below is the back of the display board, exposed. Note the metal clips holding it in around the edge. To remove it, you must get some needle-nose pliers and very carefully bend those clips out of the way. They will only tolerate being bent so many times, and the pressure they provide to hold the board in is essential to the LCD working properly. Don't bend them any farther than necessary to get the board out. The gray ribbon cable at left is soldered in at this end, but can easily be unplugged from the CPU board at the other end. The dark green board below the display board is the panel board.

The top of the liberated display board, showing the LCD screen. The backlight is soldered in at the top end, in this picture. For the moment, we put this aside; we'll get back to it later.

The next step is to remove the dead inverter and retrofit the new one. Whatever brand of new inverter you buy, it's almost certain to not fit in the same holes as the old one. Here's the regulator board with the old inverter removed. The connector on the left is the inverter output going to the display board; the connector in the right is for the raw juice coming in from the power transformers.

Here's the regulator, which I mentioned in part 1. It's a Hantronix HVI-5E ordered from Mouser. It has three pins, in the same pattern as old inverter:
  • pin 1: +5V in
  • pin 2: common (return for both the +5 and the inverter output)
  • pin 3: inverter output, 90VAC @ 400 Hz

To my surprise, pins 2 and 3 of the new inverter actually fit in the same holes on the board. If I could just figure out what to do about pin 1, I'd be golden. I considered bending it over and running a jumper wire over to the hole, on the component side of the board. But then I thought of a cleaner solution: I could tell that the board was a single-layer board (hold up to a strong light if you have any doubts). Could I drill a hole for pin 1? Looking on the back of the board, there were no traces in the area that I had to drill. So I drilled a new hole. In the photo below, the old hole for pin 1 is below and slightly to the left of the drill bit:

Note: do this trick with a circuit board only if there are no traces nearby, and you are absolutely certain that it isn't a multi-layer board! Else you may cut an internal-layer trace, or accidentally short power to an internal ground plane, both equally nasty and non-fixable.

Success! Here's the inverter pins protruding through the back of the board. Note the lack of a pad around one of them; that's pin 1 in the new hole:

It was a trivial matter to install a jumper to connect the pin to the old hole location. Here it is, with the jumper installed and the pins clipped:

And the result. This turned out to be a good solution; the inverter is secure on the board, and the existing connector and cable to the display board are still functional.

Now, on to the actual backlight replacement. This turned out to be more trouble than I was expecting. First, the end of the existing backlight, which is a plastic piece that just slips in the slot between the LCD and the board underneath. It has these two tabs soldered in at the end, which are connected on the board to the cable that goes to the inverter:

Unsoldering the tabs is straightforward. Once that is done, the old backlight is just pulled out of the slot:

Here's what you see with the backlight removed. The grey are is the actual display area of the LCD. Note carefully the rubber strips at the left and right edges. Those rubber strips contain many tiny electrical connectors, which simply touch (they aren't soldered) many equally tiny surface-mount pads on the board. There is some kind of conductive glue holding the rubber strips to the board, and it isn't very strong. Also note that there are some exposed vias underneath the LCD.

Here's our new backlight, cut down to the proper size, and with two strips of conductive-glue copper foil attached to the tabs on the bottom, and crimped at the ends to form contacts. I took it to work and cut it with one of those guillotine-type paper cutters to get a clean edge.

Problem: the old backlight was insulated on the bottom. The new one has exposed conductive surface on the bottom, and we can't let that come into contact with those exposed vias. So I had what I thought was a brilliant idea: I'll just put the old backlight back in, unconnected, and put the new one in on top of it! That way, the old backlight will act as an insulator. It was a tight squeeze, but I crammed it in. Here's the result, with the copper foil tabs newly soldered to the pads on the board. I was so proud of myself...

... until I reassembled the display to the front panel and found that it the LCD no longer worked properly; it displayed black lines across the face, and had rectangular patches that didn't work. I took it back apart, and found that one side of the LCD contact strip had come unglued from the board. When I took it out, the whole backlight arrangement fell out the side! Disaster, I thought. At first, I thought it was unsalvagable, until I realized that since the contacts weren't soldered, maybe if I just put it back together without the stack of the old and new backlights forcing the LCD up, the pressure from the metal tabs that hold the display board in (and the display itself against the front panel glass) would hold it together.

At this point, it occurred to me what I should have done about the conductive backside of the new backlight: simply cover it with electrical tape. This I did, and once that was done, it easily fit back in the slot by itself (it's actually quite thinner than the old backlight). So, starting over from here again:

I was able to slip the new backlight back in, and since the LCD was coming up on one side, I was also able to get my finger in there with a piece of electrical tape to keep the copper foil from touching the board, while getting the tabs on the end of the backlight stuck back on to the ends of the foil tabs. After a couple of false starts, I got it back in there and got the whole thing reassembled, and all of the cables attached again. Here it is, nearly ready to close up; you can see the power transformers on the right:

Every time I open thing thing up, the metal case gets a little more warped and there are a couple of more case screws that won't go back in:

But it works! I can actually see the display again, and the light is a nice white color! Here it is in all of its additive-synthesis glory. (Weird shadows and colors were caused by reflections of ambient light in the room.) I noticed that the very top row of dots on the display is very faint, but I'm not going to mess with it; I really don't want to have to take that display board out again.

Now, while I had this beast open, there were a few other things that I could have done that are recommended for K5/K5m owners. If you go back to where the panel board is mentioned, you can remove that board and clean the button contacts (click on the link and scroll about 2/3 of the way down) to make the buttons work better. You can also upgrade the OS by installing a new ROM available from the Yahoo K5/K5m user group; the latest version for the K5m is 1.2, and for the K5 keyboard is 1.3. (It displays the OS version at boot time.) The ROM is socketed so it's an easy replacement. I had already done the button cleaning on mine in a previous servicing, but I do plan to buy the OS upgrade ROM and open it back up later to install that. And there is a mod to improve the output level (kfuenf again; scroll down to near the bottom); I had also done that previously.

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