Saturday, November 17, 2018

Some repair work or preparation far from the AGC


Building driver for Core Rope Simulator modules

Ken joined us today to work on a few remaining issues with the Core Rope Simulator modules before building his driver box that will restore these to operation feeding ROM contents to the AGC. Two issues pertained to the external connector and the other two focused on internal connectivity.

The modules have a round Amphenol 39 pin connector which would be connected by cable to an external box that held a tape drive and circuitry. The purpose was to serve up a ROM image to the AGC in lieu of having the six physical core rope modules in the computer.

Ken had found a mating Amphenol connector on ebay but it wouldn't fit into the module's connector because of a mismatch with the keys. Keys are raised edges around the barrel of the connector that need a corresponding slot milled in the other side. The key guarantees that the proper cable is attached and ensure that the pins are oriented correctly.

We had milled some of the keys off but one mismatched key remained. Marc took it off with his vertical mill and we then tried to put the new connector into the one on the module. It barely made contact. We had discovered that the connector bought on eBay had recessed female pins but the original connector in the module expected the pins to be flush with the face.

We then had to mill off the face of the connector until the recessed holes, formed with hard rubber, were removed and the female pins were flush. The connector was ready to insert but we still had some severe oxidation on the male pins.

One of the modules had a screw on cover on its connector, but the other connector was exposed to the elements for decades. The pins on the second connector were so oxidized that they were electrical insulators.  We used Deoxit with an electrical toothbrush until we had removed enough of the oxide layer that the connectors could be mated with good connectivity.

Moving on to the internal issues, we first examined some wires welded to the top of the cordwood assembly in the module. To the eye they appeared to have solder blobs potentially shorting on adjacent pins. Under the microscope, however we discovered both good and bad news. The blobs were some form of sealant placed on the connection after it was welded, not solder blobs. However, we did see some corrosion of the component leads and rust. We don't think they affected connectivity.

The other problem was with the DipStick modules. These are a prototyping or production method of connecting DIP chips with wire wrap. They are a tray that supports five 14 or 16 pin DIP chips, with the chip sitting on a triangular top part and that part inserted into a triangular valley in the base.

The legs of the chip are sandwiched between the gold contacts of the top and the gold contacts of the valley in the base. A metal bar would snap across the chips on the top, holding them in place. The metal bar deteriorated in all of the DipStick modules and fell into pieces, thus the chips are no longer supported.

We tested connectivity using the +5V and ground pins which are an easy set of pins to check, but found that only about half the chips had a functioning connection. After some investigation, Marc found that the issue was tied to the shape of the DIP chip pins.

The top with its triangular shape has no springiness to its contacts. The chip legs must be making good contact as the contact won't bend to span a gap. DIP chips come with their legs splayed outward slightly compared to the angle needed to insert a chip in a socket, as any hobbyist will remember. The user generally has to bend the legs slightly inward in order to insert them in a socket.

Perhaps the broken metal bar would have forced the chip down against the top and pressed the legs into contact, but with the bar missing the chips could sit on the top without having the legs touch the inside contacts. The top has the contacts on the inside, under the body of the chip.

Our solution was to bend the chips to make the legs slightly closer together, so that they will slide on the top and its inside contacts with no gaps. Once the chips were properly prepared before reinsertion on the top, we had restored connectivity.

We are hunting for a second Amphenol connector to connect to the other module. At the same time, Ken is designing a circuit board to mount suitable connectors and a Beaglebone  to serve as the driver of these core rope modules. He will wire up cables to the two Amphenol connectors and plug the other ends into his circuit board.

Erasable Memory Module repair, advance work

Mike has built a 3D model of the inside of the erasable memory module (RAM) which helps us visualize exactly where we need the high resolution X-ray images and to plan out a repair process.
At this point we are going to mill a hole through the epoxy outer potting, open up the fiberglass board near where the broken wire is routed, and then laser weld a repair.

If the wire broke off right at the inner potting (RTV-11 surrounding the RCA built core memory stack), we can then dissolve a bit of that soft potting with relatively safe solvents to expose enough of the #38 magnet wire to accomplish the weld.

All this is based on our expectation for where the wire is broken - based on all the examined failures during the entire AGC program. When we do the CAT scan in mid December we will have exact information about the location of the break. This will inform our plan of attack for a repair.

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