When we last left our hero, he had re-soldered all the Via rivets on one of the 510 “PS”, core memory sense amplifier modules in the CDC6500, and the machine was working.
That lasted about a day, and the memory went away again. What was wrong this time? You guessed it, bit 56 in bank 36 was bad again. Third time is the charm: I am going to replace this module! I head off looking for a spare PS module. Where did we put all those spare parts we got with the machine? Oh wait, we didn’t get any spares with the machine. Bummer!
This is where I get to practice my “MAD Skillz”, and make some spare PS modules. What does a PS module look like? What does CDC give me?
On the left side we have an actual schematic of one of the 4 amplifiers on the module, YAY! Having been around this block before, I take apart the offending module and check to see if it matches. Wiring wise, yes it matches, but the values have been changed to protect the innocent.
After a while playing with the newest version of Eagle, I ended up with this:
The easy part is done, now the fun starts! The circuitry for the module is split between two printed circuit boards, one that connects to the odd pins on the connector, and has odd numbered transistors, and one for the even bits. Eagle really doesn’t understand this, so I have to fool it. First I put test points on each side of all components that go between the boards. I have to then add in the wire jumpers that also go between the boards, and I end up with another schematic:
I take this schematic and duplicate it into odd and even sides, then I write an Eagle script to delete all the between boards components, and all the test points that belong on the other board. Here is what the schematic for the odd board looks like, pretty terrible:
Now I have two schematics: PS_O and PS_E, and I do the PC layout thing. I have the original boards to use as an example, which I follow very closely so that timing and signal integrity will be as close as I can to the original module. Here are pictures of the odd and even layouts:
But wait, I’m not done yet! Remember Eagle doesn’t understand the whole module. I now have to verify that the two boards, together with all the components that go between, match that original schematic I started with.
I go over every line on the board pairs, and the schematic highlighting them as I go, until EVERYTHING is highlighted.
Done yet? Grumble, grumble: No! I have forgotten to identify which end of the diodes and polarized capacitors have the band on them! If I want them assembled properly, I guess I should do that before they go out to FAB!
Back to the PC mines…