I reengineered the tool to add the extra delays I am seeing in packs that boot properly. Initially, the tool will write zero words equal to about 82 us of delay before it starts the 28 preamble words of zero. In between records, the tool will insert two extra words of zero.
The microcode uses constants for the delay words between records, but the actual execution takes one more loop than the value in each constant. Thus, if a constant of 4 is described, the microcode does this task five times. It was easy to overlook this when interpreting the microcode listing, but the logic analyzer makes it clear.
I don't think the initial delay is critical, as the Alto did read the header record of the sector correctly before stumbling on the label and data records. However, the extra two words between records is essential. The way an Alto moves between records ensures that it has already passed the sync word I wrote before it starts scanning for it, thus guaranteeing a misread.
IBM 1130 PANEL UPGRADE
I received my sample SCR and diode devices, allowing me to build a prototype to test driving indicator lamps with a 7.5 VAC supply and logic signals conforming to IBM Solid Logic Technology specs on both the signal and lamp test inputs. If this works well enough, I can design the circuit board to implement all the light positions in the 1130 pedestal box.
COMPUTER HISTORY MUSEUM MACHINE UPGRADES
I worked with Bill Newman who was applying an upgrade to four of the 026 keypunch machines, replacing the original selenium rectifiers with modern silicon diodes. We are doing this to avoid the noxious and slightly toxic fumes that Selenium rectifiers emit when they fail, as a safety measure.
We completed two of the four machines completely, meaning replacing the bridge rectifier as well as six separate selenium stacks used as diodes. Two other machines had their bridge rectifiers replaced, but we didn't have enough single diodes to replace the stacks with six diodes apiece. We will complete those on Wednesday.
I brought in a supply of 220 ohm 2W resistors to refill the empty drawer in our workroom and to complete the modified power sequencing SMS card for the 1401 systems. The original cards that sit in the machine have a pair of resistors that get extremely hot in operation, scorching the actual card underneath.
A 1401 power sequencing schematic shows an alternate set of components that run much cooler. I changed the card to match. This involved three new resistor values to drop the bias current, reducing the power dissipated from 3+ watts down to fractions of a watt, in conjunction with a changed transistor type.
We placed the modified card in the machine and verified that it successfully sequences power up and down. Next, I will build another such card since the 1401 uses a pair of them. The next card was all but complete, except for the IBM type 030 transistor, which I couldn't find in our stocks.