Tuesday, April 23, 2019

Physical construction work for IBM 1130 console display panel alternative lighting

Mounting the power supply components

I have four components to the power supply for the panel, three of which can be mounted with a screw onto the posts conveniently provided on the swing-down rear door of the panel enclosure. None of the holes in the components line up well with the posts. The fourth component is a fairly large filter capacitor, a cylinder shape without mounting points of its own.

The classic method to mount the filter capacitor would be a clamp that circles the capacitor and provides screw holes. The cylinder would project upwards from the door, the axis of the cylinder at right angles to the door, way too tall to fit inside the relatively shallow enclosure. Instead I want a mount that places the axis of the cylinder parallel to the rear door, so that the capacitor projects outward only by a bit more than its diameter.

The challenge will be fitting everything between the board and the swing door. There is a bit less than 2" clearance from the back of the PCB to the swing door, but the posts and a raised lower ridge on the door eat into the space by up to 5/8".

Assuming that I can fit the capacitor between posts, a clamp can be attached to one of the posts to secure it in place. These are 1 3/4" diameter capacitors, which can be mounted with a bracket that I just ordered from Digikey. These should just fit. In a few days I will have the hardware here and can mount the capacitors onto their final position on the swing door.

Filter capacitors used for power supply
The two buck converters needed adapter plates since neither of them have mounting holes that directly match the posts on the rear swinging door of the 1130 enclosure. I am having the adapters laser cut at Ponoko out of 1/16" plastic.

Interfacing with the computer wiring system involves connection to the 7.5VAC main lighting power, ground and the 155 discrete signals for 154 lights and the lamp test control. There is a terminal strip on the left side of the enclosure (viewed from the front) which provides the 7.5V on the top two lugs, common ground on the middle two lugs, and lamp test signal on the bottom two lugs.

The way that IBM designed the 1130 computer, they have a 7.5V AC transformer wired with one side tied to ground, so that the other side swings to both positive and negative excursions during every cycle. This does present problems with my initial power supply design, which was going to use a full wave bridge rectifier to give me DC for the buck converter boards. However, if one side of the input is tied to the negative output, the full wave design won't work properly.

AC feed tying one side of transformer to ground
Supply connections inside light panel enclosure
Instead, I must use it as a single diode of the component to rectify, producing only positive half cycles. This requires a filter capacitor to smooth out the more substantial ripple I will be experiencing.  I selected 9000 uf caps for the output of the rectifier diode and again for the output of the 5V buck converter.

To attach properly to the barrier strip inside the 1130 enclosure that provides the 7.5VAC, ground and lamp test signals, I have to create wires with lug ends to fit on the strip. My wire has to be bigger than 20 gauge (or two somewhat smaller wires, each attached to a barrier strip terminal) to properly carry the max current. I chose 18 gauge stranded wire.

The lamp test wire can be much thinner, since I am simply detecting the voltage level with a PCA9505 multiplexer chip. I do need to put a push-on connector for the lamp test to fit it over the pin on my PCB for this connection.

The power connections are flat solder plates thus only one end of the ground wire needs a lug end. The other solder plates run to the buck converter boards supplying 3.3 and 5 VDC. A lug plate is needed on the wire to provide the AC to the rectifier diode. The two filter capacitors need lugs on one end of the wires connecting them to the rectifier and the buck board, otherwise connections are soldered.

Since I must wait for the adapter plates to mount the two buck converter boards, I figured I could complete everything else and wait for the final physical assembly when they arrive next week. I installed the rectifier and two capacitors along the swinging rear door, but left the two converter boards dangling so they can be screwed onto the plates later.

Adding insulation and support material to the rear of the panel

The honeycomb behind the front panel is actually a number of blocks that are glued together and then anchored to the face plate by two threaded studs rising along the mid-line of the face plate. A metal plate extends across the mid-line and is held in place with two screws.

The PCB has solder mask and no components along the line where it will sit atop the metal plate and screws, but with time the screw heads may dig into the solder mask and eventually short. Further, since the PCB is planar but the screw heads make elevated points on the rear of the honeycomb, the PCB wobbles and won't sit flat.

My solution is to overlay the metal plate and screws with a long narrow foam strip. Small pieces of the foam will be applied at various points on the rear of the honeycomb to cumulatively provide a planar support onto which the PCB can rest.

Padding/insulation between honeycomb face and PCB face
Minor repairs to the IBM panel before installation

The IBM panel is a plastic front with honeycomb segments glued to its rear. Two plastic blocks are glued to the sides of the honeycomb outside edges. These are screwed onto vertical metal brackets to hold the light panel in place inside the enclosure. Both of the plastic blocks have broken free from the honeycombs due to deterioration of the glue used for manufacturing at IBM.

Edges of honeycomb where plastic blocks detached due to adhesive failure
 The bracket is attached to the top and bottom of the enclosure and has two screw holes facing sideways into the plastic blocks. However, getting a screwdriver in the enclosure to install or remove those two screws is quite difficult.
Plastic block attached to its bracket
The only effective method is removing the two end switch panels to get access. These metal plates have threaded studs on the rear that fit into an enclosure hole and are tightened with a nut you can reach from the rear. I have to remove the 4 nuts and swing the two plates out of the way, allowing me to put a stubby screwdriver in from the front and access the screw holes for the plastic blocks.

Re-gluing the plastic blocks to the honeycomb sides using epoxy
After removing both plastic blocks, I used epoxy to glue them onto the honeycomb sides. Once this was firmly set, and the standoff insulation applied to the honeycomb rear faces, it was time to reinstall the honeycomb.

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