Tuesday, March 21, 2017

1401 Computer and TV-3 Tube Tester progress


Today I dug up the proper manual and schematic for my unit, as I tired of waiting for the printed copy to arrive. The seller of the printed copy promised "fast and free" shipping with delivery tomorrow, but when they actually mailed the copy it went out media mail, the dirt cheap way to send paper items that will take it until late next week to arrive. Grrr. At least I have the online version I downloaded.

I printed the schematic and my guide to the eight push buttons, allowing me to begin tracing the power path from the 83 rectifier tube out to the plate pins of the tube test sockets. I quickly discovered that it was simple oxidation on the rotary dials and push buttons - exercising them a bit cleared up the problem and viola! - 150 plate, 130 screen, 40 bias and other voltages appeared as expected. 

I will apply some deoxit to the contacts of the switches while I wait for my new DMM to arrive tonight, which will substitute for the meter movement and let me further check out the functionality of the tube checker. All is looking great with the exception of my saved then destroyed meter movement. 

Once the DMM was received, I installed a series resistor of 2400 ohms, to substitute for the original meter resistance of 2365. The results were reassuring. even with some signs that contacts are not yet fully cleaned. Setting the line voltage to 117VAC gave a reading of about 73 uA, which is roughly half scale. 

Based on that, I extrapolated the readings I got for several tubes, linearly, and the answers for tubes I own and tested were reasonable. If I can get a replacement meter installed this tube tester will be ready to use. 


I invested some quiet time with the 1401 when nobody was around, just me and the powered down system, in order to trace out the wiring that must be reconnected to the Start Reset button. First I had to decode the locations on a small backplane into which all the front panel components are wired, then trace the wires and match them to the logic diagrams, finally resolve any faults.

IBM's scheme for these computers is called SMS, which defines the cards, backplanes and other aspects of the machine's construction. SMS cards have 16 traces on the edge that are inserted into a card socket on the backplanes. The pins connected to the traces are labeled A thru R (skipping the letters I and O is an IBM 'thing' apparently because of the potential for confusion with the digits 1 and 0.)

Cards populated with transistors, resistors, diodes and so forth are plugged into the card sockets or slots on a backplane. The typical backplane has 26 slots, numbered 1 to 26 and each slot has pins A to R. The backplane connecting the front panel, however, had only 24 slots. It has the slots facing downward below the backplane, with the cards plugged upward from below.

First step was to determine the orientation. Pin A was the furthest in and pin R is closed to the front panel. Slot 1 is furthest to the left, when looking in from the front panel, while 24 is to the right. There is a rats nest of snarled wires wrapped around the 384 pins sticking up on the backplane, making it hard to count rows and pins, or to get to a specific pin with a probe. 

The logic diagrams (ALDs) will list the spots where signals go on or off a particular backplane, gate or module. In the case of signals connected to the front panel, they went through this backplane 01A2 Axxy where xx is the slot number and y is the pin number. 

I found the various locations, hooked on a circuit tester, and then found the wire that corresponds to it hanging loose next to the Start Reset button. In this way, I identified all 8 wires that run to the switch and knew which signal they contained. That also means I know which terminal on the switch they are to be soldered to. 

One problem we had identified earlier was that none of the eight wires had +6V present, yet one of them must for the switch to work properly. On the logic diagram, this is the +6V CK3 line, which means circuit 3 from the +6V power supply. The backplane also had +6V CK1, CK2 and CK4 wired to it. These are all electrically identical, other than using different wires to bring it up into the card slot. 

I found that the pin which brought +6V CK3 was completely open, not connected at all to the other +6V circuits. They were all mutually connected and would measure properly when the machine is powered up. The pin for this power is 01A2 A24M and that means a paddle card plugged into slot 24 (a stub of a circuit card with discrete wire cables connected to the 16 traces) had one wire which was CK3. It either melted or a fuse element somewhere in the line melted open.

Fortunately, there were several other pins that had +6V from other circuits. I moved the wire that runs down from the Start Reset button over from A24M to A22R, which is +6V CK4, and we now have good +6V running to the switch.

Tomorrow (Wednesday) morning we will solder up all the wires, reinstall the switch and verify everything is good. It is likely that the melted line for CK3 occurred when we had a major short between power supply lines at the switch. The short blew 20A circuit breakers on the power supplies, certainly enough to vaporize a bit of wire.

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