Replacing defective chips on the drive B Control Logic board

FINISHING SOME CHIP TESTING

I did look at the outputs of the 96L02 since it has both Q and Qnot, which must be different. The LM311 op amps I tested by swinging the input voltage and watching the output. The open collector 7416s drove off board circuits and had no pullup resistors, so I added it to the probe and performed the irrational combination test. The only chips for which I had no test at all were the NE555 timers and the ULS2003 which drove indicator lamps and thus was open collector too.

DEFERRING THE DECISION

I began to remove the bad chips, letting the effort involved determine whether I will continue to take all of them off and replace them. The method I used makes use of a vacuum desoldering station, a soldering station and clipping away the chip body.

I snipped off all 14 leads up at the chip body, removing the IC itself. This left me with short metal pins that were soldered into the board. I attached forceps to each pin, dangling on the underside of the board, while I applied heat on the top side of the board. The goal is to have the pin pulled out cleanly.

Once all the pins were removed I couild use the Hakko vacuum desolderer with a small tip size to suck the solder out of the holes. That gave me six chip sites with all the holes clean and ready for the new chip. My decision was made - time to replace the chips with new ones and continue testing.

VERIFYING THE SUBSTITUTE CHIPS ONCE SOLDERED TO THE BOARD

I have a supply of all chips on hand, so I began merrily installing and soldering the 84 pins to the board. Before I power up a board where I have replaced chips, I do some continuity testing to ensure that the pins of the chips are hooked into the circuit per the schematic. This is particularly important with the visible desoldering heat damage at one location.

New chip in place - some heat damage to board on lower right

Resoldered pins on the bottom of the board

For the 74LS14 chip, I had to check the six inverters, testing that the input came from the other components on the board and that the output went to its intended destination component. I found a problem with one of the inverters - the input pin was not connected to the resistors and capacitor as it should be. The solution is a small wire from pin 13 of the chip to the resistor.

The other five chips, each a 74LS00N, have three three-input NAND gates apiece. This means I needed to verify connectivity of 60 pins, 12 per chip. Fortunately, all checked out fine, so only one rework wire needed.

The final complication to sort out was existing rework on the board, consisting of several small blue wires that connect to various chip pins and other components. I wasn't sure what this was doing, so I recorded all the connections, located the chips on the schematic, and drew out the purpose of the rework.

Part of the rework I found on the board

Once I began tracing it out, I found that these were replicating connections shown in the schematic. That suggests that these were added to compensate for broken traces that occurred at some prior time. Another sign that this was an old and perhaps scrapped board stuffed into the cage after the shipping damage.

INDUCTOR REPLACEMENT NEEDS REPLACEMENT

The inductor I bought from Anchor, a 100uh axial, has almost 5 ohm resistance. Since this is a choke on the 5V input to the board it is going to drop a fair amount of voltage - for example if the board draws only 100ma then the inductor will cut the 5V supply to 4.5V at the chips.

Digikey had a replacement that can handle up to 1.3A with just 0.2 ohms resistance - a much better component for the board. It should arrive in a few days and I will swap it for the inadequate one.