Wednesday, August 17, 2016

SCRs replaced on light panel, nearing end of bulb insertion; checking out repaired drum for plotter

My day to meet the rest of the 1401 restoration team at CHM, but can get some work done before and afterwards.


Well, the replacement bulbs I ordered are not going to arrive today as promised on the selling website. They are coming via USPS and should be delivered by the end of tomorrow. I can't close up and finish this until I have the lights in place.

This morning, it was time to verify the bad SCRs and carefully desolder them from the PCB boards. Four of the left rows have a bad SCR - one apiece - which need to be replaced. Using my semiconductor analyzer device, I can check each suspect SCR (actually an SCR plus a resistor in a molded case). This allowed me to validate the SCRs being removed.

I had removed five SCRs from the "CE Signal" section of row 5 right, in order to repair bad SCRs on the lamps for signals that are normally used. The CE lights are ad-hoc displays that a CE will jumper to some condition to help in debugging a problem. I can use my oscilloscope and other tools instead of such lights.

With the donor SCRs removed, I populated the remaining lamps on row 5 right and inserted it into the machine; now the right side is complete. It is a pleasure to see 2/3 of the boards in place and every light in those rows lit during a lamp test.

I made use of three of my new mini lamps and indeed they look exactly like all the other incandescent lamps when viewed through the panel. I decided I will use shrink wrap as an insulator around one of the two legs of each mini bulb I make up, as there is a risk that if the bulb gets rotated it will bring the two leads into contact with each other and short out another SCR.

The failed SCRs were validated using my analyzer and are bit 10 on left row 2 (SAR), bit 5 on left row 3 (SBR), bit 5 on left row 5 (Accumulator) and bit 2 on left row 6 (Extension). I began my morning by working on L2. Somehow by the afternoon, bit 2 of row 6 was working fine, no issue with the SCR.

The SCR for bit 10 is in the most accessible position of the three failure points (10, 5 and 2) on these boards. Using solder wick and careful work, I released the failed SCR and cleaned up the board. I then took my salvaged SCR that I took out of the CE light positions, dressed its leads and inserted it in the board. Once soldered up, I could put on the bulbs, verify that all lamps lit under test and then put that of lights into place in the honeycomb assembly.
Board with defective SCR module removed

Underside of board with bad SCR module removed
Three more to do - L3 (SBR), L5 (Acc) and L6 (Ext) - with work resuming after a break on board left 3 to replace the SCR in bit position 5. These boards are wired in place and require quite a bit of torque to twist them enough to put the solder face of the board somewhat upward where I can reach it.

I need to hold the board twisted, apply the soldering iron tip, hold the wick in place, and manipulate the SCR to get it out, which takes four of the two hands I own. Persistence, creativity and luck combine to overcome this hand shortage and I completed L3 and inserted it with all bulbs working.

As so often happens with this poorly designed hot mess of a light panel, a bulb stopped working on a previously inserted board, L4. I have to investigate it, correct the problem and then get back to work on completing L5 and L6. I ran out of morning work time at this point and had to head over to CHM.

While I was out on the way to CHM, I stopped and bought some heat shrink tubing, which I will use with the mini (715) bulbs to ensure that the two legs can't twist into contact with each other. I made up one lamp using this method, which also allows me to stand the bulb higher off the socket and closer to where the filament sits in the original equipment bulbs.

I came back in late afternoon and commenced work, at least as far as I could get with the bulbs on hand. I replaced the bad SCR on board L5 at bit position 5, inserted my remaining bulbs and found that 4 and 14 were dark under lamp test. I also pushed the ground pin into the unlit bulb on L4, restoring it to proper operation.

I checked the bulbs in those positions and found them unusable. To be sure that everything else was good, I moved bulbs from 6 and 15 over to 4 and 14, ensuring that every bulb glowed. I can't insert row L5 into the honeycomb until I get my new replacement bulbs and place them on the two open spots, but otherwise it is ready to insert.

Row L6 tests fine for all 16 SCR positions, the problem I thought I had spotted on bit 2 is either cleared up on its own or was a measurement error. Therefore, it is my expectation that once I have the replacement bulbs tomorrow, I can put together than 16 lamps I need for this row and the 2 needed to wrap up row L5.

In early evening, I decided to make two more working lamps from the stock I had. There were a few bulbs which worked but whose wire leads were too short to reliably hook over the ends of the lamp holders. Other bulbs were dead inside but had good long leads.

The solution was to cut off the long leads from bad bulbs and solder them to the too-short leads on good bulbs, allowing me to mount the bulbs in holders successfully. I added them to the L5 board and have a full set of 16 working lamps for this row (Accumulator). I am still finessing the insertion of the board into the honeycomb, not yet done with L5.

The darned row L4 bulbs are flickering off again - the original unlit bulb came back along with a nearby mate. The least amount of flexing of the board and connectivity to the bulb is lost or regained. I will have to pull L4 out and reseat the bulbs more carefully, since this is unacceptable.

Now just in wait mode until I have 16 bulbs to put in holders and insert into the final L6 board. USPS, please be unusually timely and dependable. Just a few more tedious hours and frustrating setbacks until the entire light display is together and all lamps light up.

Only lower two boards on right of picture left to do, light panel coming together

I reassembled the drum with its internal braces, side mounts and brackets. It is still a bit too much out of true in a couple of spots, but those should be recoverable. The remaining issue will be a few rough spots on the drum surface which might be able to be sanded out (thinning the metal in that spot).

Drum reassembled, first view

Second view, drum rotated about 90 degrees

Third view, rotated another 90 degrees

Final view, about 270 degrees from initial view

The risk of sanding it down is a punch thru. At worst, however, it would have to be bridged with automobile dent repair materials. I am feeling cautiously optimistic about the potential to restore this to operation.

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