While I wait for my replacement bulbs to be delivered, I can get row L5 properly inserted in the honeycomb and then deal with the intermittent bulb contact in row L4. The lights, when on hand, will allow me to finish up row L6.
I did manage, after quite a bit of tweaking, to wedge L5 in adequately and all the bulbs lighting properly. Still having problems dealing with the erratic bulb on L4 - I am trying to manipulate the bulb hoping that I can get it to light reliably without having to pull row L4 out and start over. It will all come down, as it always does with the light panel, with trying to get the last couple of lights working and the last board inserted properly.
I received my bulbs and proceeded to build 16 lamp holders with these 715 mini bulbs. They are electrically identical to the full sized bulbs and produce the same light output, so my only issue is fitting them to the much larger lamp holder.
I use shrink wrap tubing on the wire leads of the bulb, from the glass down to the point where the wire lead will disappear into the pinhole of the lamp holder. This protects against the leads touching in a short circuit and also stands the bulb up further out of the holder to better match the filament locations of the original equipment bulbs.
I did a test on bit position 2, the one that I had found to be defective yesterday morning but that appeared okay when retested later in the day. I decided to hook up one bulb I have that works okay and give a test - it didn't light.
I then put on my new bulbs on row L6 - the last row to be inserted and therefore the one with the worst minimal access and maneuverability. Bit 2 is working, so I am not sure why I keep getting conflicting tests, but I tried to insert the board as it was.
My mini lights stick out and are easily bent up or down so they won't fit in the honeycomb cells. Very tedious. All insertions begin with wrestling the board to the right as far as possible, since the power and lamp test wires are connected at that end and tend to spring out, pushing the board too far left.
It requires pushing rightward while pivoting the board around and getting sixteen lamp holders to enter their intended positions. Unfortunately, while pivoting the board, the gate lead snapped off the bit 0 SCR module. Now, I have to remove and replace that SCR before I can continue. Arrrgh.
I hoped I could dig out some of the encapsulation and save this SCR by soldering a new pin to the stub. Unfortunately, the bakelite covering crumbles easily and does not bend at all like plastic (or is a plastic whose plasticizer has departed this mortal veil many years ago). I managed to break off a lead inside while trying to clear space around the pin.
I put one of my donor SCR modules in the bit 0 location, and managed to get the board inserted almost firmly. It is wedged with very slight friction, mostly from one side, but I can't seem to wrestle it to a firmer fit. Still, it is in place and all the lights on row L6 light.
The final problem is the errant light on row L4, bit position 8 which I have to finesse until it works again, or pull the row and deal with the light issue more directly. I noticed that bit 9 is 'hot', always on and when I swap the signal connectors the flaw stays with lamp 9. Time to pull the board out and work on it.
Using my analyzer, I see that both the bit 8 and bit 9 SCRs are bad - 9 is always on and 8 is inoperative. I had only removed a total of 5 donor SCRs and have already used 4, so I ALSO have to remove row R5 and take off more SCRs from the CE light positions.
Two SCRs replaced, all bulbs lighting properly, so I insert L4 into the honeycomb. A light on position 10 is dark now. Remove board, find that the brittle leads at the glass envelope broke on the lamp. I tore it out and put in one of my mini bulbs, tested that all 16 lit, then reinserted. The bulb at position 8 is dark now. Removed board, tested lamp, which had gone open circuit.
New mini bulb in position 8 lamp holder, testing that all 16 lights illuminate during lamp test. Bend and pivot the holder into place, but before wedging it tightly, test again. Now bulb for position 5 is dark. Aaaaaaaaarrrrrrrrrrrrrrrrrrrrgggggggggggggggghhhhhhhhhhh. I hate this panel.
I may have to replace every one of the hundred or so original bulbs if they keep snapping during insertion into the honeycomb. At a minimum, if I don't disturb any other board, I might face more lamp failures on board L4.
I have 14 remaining original style lamps on board L4, one of which is bad and needs replacing. The question is whether I tear out all the other bulbs and go totally with mini lamps, or hope that the string of bulb failures will stop.
When I swung the board out yet again, it was clear that the bulb for bit 5 had self destructed. The bulb was sitting in the honeycomb while the lamp holder was still attached to the board. I replaced this with another mini bulb, tested all were lit, and managed to insert it with all still working properly
I also see that the sheet metal of the rear door comes perilously close to the signal pins on the back of the boards, something I don't like. I put an insulating layer on the offending area before I closed things up.
With the rear of the panel gently closed, I went around front and did another lamp test. Damn!! One bulb on L5 (Accumulator) and two on L6 (Extension) are off - tapping the panel slightly made a couple flicker on, but basically I would have to open the panel up and iterate endlessly with bad bulbs and contacts for L5 and L6. Meanwhile, every time I touch one thing, there is a risk another bulb will stop working.
I decided I can live with the dark positions, because I have already put in almost forty hours of frustrating work and still can't get all 156 bulbs working at the same time. The next time I touch this, I think it will be to strip out the current boards and SCRs to replace it all with some alternate system I build. This makes the machine impossibly fragile and undependable, as it sits.
I truly hate working on the light display and hope that I won't have to do anything to it for a very long time. The way the panel is built in my system is described in the ALDs and other docs as "early 1130s" style. It has the SCR modules on the circuit boards attached to the rear of the lamp holders.
After those early machines, IBM changed to put 9 boards on the rear cover, each handing up to 20 lamps, with an unknown method of cabling to the lamp holders and for holding them in the honeycomb. I imagine it to be much easier to change bulbs and nearly impossible to blow SCRs with the new method.
To make a mid stream change of this scope to the machine probably meant there had been a flood of complaints from the CE organization about how difficult and time consuming it was to fix machines with the prior design. May also have been a very high rate of parts usage from blown SCRs on these boards.
I can probably build new SCR circuits so that I can either restore the full 8 CE light positions or replace others if more fail over time. I just need an SCR that can comfortably handle the .114A 5V load of the lamp, take the 7.5VAC input power and doesn't need any heat sink. I can wire that, plus a 6.8K resistor, onto the board in place of the encapsulated module used by IBM.
The order from Digikey is on its way - 10 sets of SCR and resistor - which I can test out on a CE light position first and then use to backfill all the modules I previously removed. It will mean opening up this bedeviled panel again, but only to touch row R5 which is relatively easy to manipulate. I won't touch any other rows for the near term.