Making the repair on the core stack - part 3

REINSTALLING BACKPLANE + STACK INTO THE IBM 1130 FRAME

Once the core stack was screwed onto the backplane, the hard part was over. The backplane enters the frame from the back with the core stack side moving through the opening. The stiffener around the backplane sits against the frame. The various plastic holders and their hex style screws get attached to lock the backplane into its place on the machine. I put the two bottom holders partially on the frame, so that the backplane could rest of them while I attached top and side holders to refine the positioning. 

Two of the plastic holds cracked in my hand - a common issue with the loss of plasticizer in the plastic parts that were used in these machines. 

POWER CONNECTORS GO BACK ON THE BACKPLANE

The power rails reach the backplanes through push-on connectors whose wires run vertically along the sides of the frame. These provide the standard +6, +3, -3 and ground power connections. The memory circuitry also uses the +12V supply, which is connected to a single pin on a card slot for the voltage regulator card using a smaller push-on connector. 

TOP CABLES RECONNECTED

Three connectors plug into the T1, T3 and T4 slots near the top of the backplane, carrying the data, address and control signals between the CPU logic in other compartments and this memory compartment. I pushed them into place. 

SLT CARDS REINSERTED

The circuitry for the memory is implemented on SLT cards that fit in card slots on the backplane surrounding the area where the core stack is mounted. Rows 2 and 3 have room all the way across for cards. It is not fully populated because the core memory design for the 1130 is also used for the IBM 1800 system, which has additional locations outside the 8K which are addressed by an AUX bit. The 1130 does not use AUX memory locations and can dispense with the cards that support that function.

Rows 4, 5, 6 and 7 have the core memory stack in the middle, but columns A, B, M and N are available. Mostly these slots are used for the cards that support the sense/inhibit functions, each card handling two bits for a 4K portion of the 8K memory. The memory has 18 bits (a word in the 1130 is 16 bits, plus a parity bit each for bits 0-7 and 8-15) so we have eighteen cards for this function, sixteen of them in rows 4-7. 

POWER UP AND TEST OF MEMORY

I used the Storage Load and Storage Display functions from the CE switches to test the memory. A pattern is set in the Console Entry Switches on the front of the typewriter, then Storage Load will cycle through memory writing that pattern. Storage Display will cycle through memory reading every word. By varying the patterns I can be sure that the memory is being written and read correctly, without any parity errors. 

Alas, bit 7 in the lower 4K is still not correct. I popped the SLT cards for all the connections on the troublesome jumper blocks F4 and H4. All were good except for bit 7 low. The original fault was a break in continuity of one of the wires for a 2K loop on the core plane, while the other loop wire and the common connection to both was fine. Now, the common connection, which is tied to ground on the SLT card, is not connected.

LOOKING AT OPTIONS

The obvious but labor intensive approach is to remove all the jumper blocks, cards, power connections, take out the backplane and deinstall the core stack from it again. I could then repair the wire which evidently has pulled off of pin B13 of F4. This does come with the risk that other wires might be pulled when I put everything back together again.

I am considering ways to provide the common point connection without separating the core stack from the backplane. I am looking for ways to make this happen while preserving the noise prevention effect that the twisted triplet of wires provided for the bit 7 sense/inhibit signals.