IRRATIONAL COMBINATIONS
Each logic gate type has defined behavior. The simple combinatorial gates in particular have easily checked conditions. The inverter must have opposite states on the input and output. AND gates must obey the logic equation, only having a positive output if all inputs are positive. Of the 15 types of chips on the board, eleven fit this category where some combinations of states represent a failed chip.
The NE555 timer chips have defined behaviors but no irrational combinations to check. The LM311 op amps do not have illogical combinations but I can check their behavior by varying the input signals thus they can be separately checked. The
CHECKING ALMOST FIFTY CHIPS FOR IRRATIONAL COMBINATIONS
I printed the data sheets of all the possible irrational combinations so that I could perform the checks on the logic board. Previously I had pored over the five pages of schematics and built a list of the chip types at each of the 94 locations. Armed with this, I powered up the board and began stepping through all the chip tests.
I found quite a few bad chips. This board had previously had a missing inductor and a broken resistor, then I discovered a bad inverter chip which is why I thought I would do a scan for irrational combinations prior to replacing the inverter.
The final tally is five 74SL00 and one 74LS14 chips.
MY SUSPICIONS ABOUT DRIVE B
With this many chips bad, I suspect this is evidence of insurance fraud. The reason for my suspicion is the signs that this board failed in service on some other drive. With the high number of bad components, it would have been either sent for rework or scrapped.
However, since a shipper damaged the drive with a forklift, the entire drive would be written off by the insurance company and the unit scrapped. By rights, the intact boards inside belonged to the insurance company.
If an unscrupulous organization took the opportunity to swap out all the good boards, repopulating the card cage with bad cards, they would avoid the charges for the bad boards and obtain good spares from the drive that is now property of the insurance company.
Other explanations are possible so I can't be totally certain. When the drive fell backwards and bent its frame, perhaps the shock broke leads inside the IC packages. It is a very high number of chips to have damaged like this, but it is possible.
The five bad 74LS00 are all marked El Salvador, but the 74LS14 is not. Some of the good 74LS00 are also from El Salvador and that is the only source for all the 74LS00 chips. There may be a bond on the chip, as it was made in that factory, that is susceptible to shock, leading to a high failure rate. The fact that only 6 of the 84 chips failed and that five of them were this type, may support that theory.
As a counter theory, the board has pen ink marks on some pins of the chips, which likely occurred as a technician traced a path while debugging the board. Once they discovered that six chips were bad at a minimum, they likely tossed it in the scrap pile where it waited until a drive was damaged in transit.
PLANS FOR THE BOARD AND FOR DRIVE B
The major hassle in reworking these six chips is that I have to clear solder from 84 pins to remove them, then insert the replacement chip pins in the partially clogged holes. The cost of the chips is insignificant, it is the time and effort that looms large.
There is no guarantee that the chips which passed the irrational combination tests are working properly, only that they weren't provably bad like these six. Some other bonds may have failed or portions of the chip dies might be bad but not show up with the particular combination of inputs at idle.
Since I am doing this restoration for the hobby fun, having no purpose for the drives once they are running, I have to think a while about whether to proceed.
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