Tuesday, August 16, 2016

Finished cleaning and testing lamp assemblies, began reassembly of light panel; plotter drum much improved


I will have to wipe the SCR PCB pins with vinegar/salt and then again with sodium bicarb, since there is so much corrosion on those pins too. Some of the lamp holders were extremely difficult to break free, because the pin and lamp holder pinhole had almost welded together with oxides.

I ended up with 18 broken, dead or questionable lamps out of the 156 in the pedestal. A bit over a 10% failure rate, although it is hard to know what part was already dark and what broke during the handling. With 80 replacement bulbs (the mini kind) on order, I should be able to close up the light panel with fully working bulbs, with plenty of spare bulbs for future bulb failures. Thirty of the bulbs are fast-tracked to arrive Wednesday, because I don't want to wait too long before firing up the system again.

I rehabilitated four of the bad lamps, leaving me 14 that need replacements. I had four of the mini 715 bulbs on hand, which I installed in holders - now just ten lamps that need the new bulbs coming on Wednesday. Eight lamps are CE indicators, meaning they are only used if I hook them to some signal I am monitoring, otherwise they remain dark, but that still leaves two of the system states I wouldn't see unless the lamps are fixed.

The honeycomb sections are very firmly in place and there is a slight amount of friction on the sides as it slides into the metal enclosure, rubbing along the vertical plastic bars that should hold everything in place. I suspected that the friction would grow too much when I had the plastic glue on the side plates, so I did some sanding down of the side surfaces of the vertical plastic bars. That should make the fit easier but also improve contact area for gluing.

Not sure how I will get some side pressure introduced to firm up the plastic bonding, but the alternative of removing the vertical plastic bars seems too prone to problems. Maybe I will loosen one side so that it can slide left-right, use it to get sufficient bonding pressure on the panel and tighten it up after everything is set tomorrow.

I ended up getting a good left and right side adhesion holding the light panel in place without having to loosen either of the vertical. Now I can carefully insert lamps into the honeycomb.

I didn't want to power up until I have all the bulbs inserted firmly on the SCR PCBs and all PCBs insulated from each other and the metal enclosure. Sheets of paper insulate the PCBs that are not yet installed. I do a lamp test to verify that all 156 bulbs are working properly before I begin seating each PCB painstakingly into the honeycomb. After insertion, I check to find if any no longer light.

I am very very slowly inserting PCBs and their lamps into the honeycomb. This is extremely tough - there are up to 16 lamps in a row, where all 16 must fit through the small bulb diameter hole and all 16 holders must be aligned close enough to push into the honeycomb.
Partway through the hours of work replacing lights in pedestal
It was easy with the bottom right row that has only two bulbs, but the expected horrific task for the upper left (IAR) row. When I got the sockets wedged into place, the bulb for bit 1 had packed it in (or won't light, anyway). Have to pull the darned PCB out and start over. It will just be that much harder once I am down to the last row on each side, when all my maneuvering room will be gone.

By lunchtime, I had the top right and top left rows, the bottom right row and had inserted the right row just below the top but one of the lamps is not lighting. Have to pull it out and work on that lamp (the E2 lamp). For convenience, I am going to label the rows 1 to 6, with 1 on the top and 6 on the bottom, then refer to right or left as seen from the front of the machine.

After lunch, I fixed the E2 light in row R2 and moved on to populating the 16 lamps in row L2 (SAR). I discovered that the SCR for bit 10 is not working - even if I swap the signal lines between it and an adjacent SCR, the lamp doesn't light. I will need to replace it.

I have eight lamps on the panel that are intended for CE diagnostic displays - if wired up by the CE, they will light when the connected signal is '1'. They have no 'normal' use and therefore are the best place to borrow working SCRs.

SCRs on the narrow PCB, pins at top go into lamp holders
Bottom of SCR PCBs
I populated and inserted rows R3 and R4, which work perfectly. R6 is already in place and I won't try to install R5 since it is the donor board with SCRs that can be relocated to more important positions such as L2 bit 10.
Right side, all rows but 5 in place
I placed lamps in L3 (SBR) and tested them to be sure that the lamps and SCRs are all functional. On L3, bit 5 SCR is dead. Time to move on to L4, L5 and L6 to round up all the bad SCR positions before I begin repairs.

L4 (AFR) worked great and is inserted into the honeycomb cells. L3 is malfunctioning for bit 5.  L5 has bad SCR for bit 5.   L6 has bit 2 SCR bad.

You might wonder why all these SCRs have failed. The answer is a design that has zero tolerance for error. This is compounded by a total lack of warnings in maintenance manuals, schematics or other documentation for the 1130.

The bulbs used in the light display have bare wire leads. These are threaded through the pinholes in the bulb holder and wedged into a narrow slit on the outside edges of the bulb holder. If the remnant of the bulb wire is more than a few millimeters long, it can project sideways towards the adjacent bulbs. If two such wisps meet, it shorts the SCR directly to ground.

Undoubtedly, in my earlier attempts to replace bulbs, particularly when I hadn't gotten the row wedged back into the honeycomb cells, I had such wisp to wisp contacts. If one existed at the time that a signal tried to light the lamp, the SCR would be driving into a dead short. Very hard to trim short to the degree needed to avoid this risk.

I have a total of 4 SCRs which are defective on several left side PCB positions, removed them, and unhooked donor SCRs from the CE lamp positions 8 thru 3 (to have one extra on hand). At this point I ran out of energy - while it was only the early evening the lingering effects of my cold stopped me for the night.


Stan Paddock, a fellow member of the CHM's 1401 restoration team, found a way to undent the drum for the plotter, sufficiently to let me reassemble  it and return it to service. It is still not perfect, but much more usable and close. I will reassemble the drum on its axle and then work on evening out the surface.

I anticipate that I can sand down high spots, fill with paint and repeat until it is smooth and even enough for use as a plotter. Not sure how long it is going to take.

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