Putting the correct 74LS08PC chips onto the board

FIXING THE CLOGGED HEADS OF MY HAKKO FR300 DESOLDERING GUN

My desoldering gun sucks the solder through a fine opening in the tip, but it can easily get clogged. The tips I use, 0.8mm and 1mm openings, come with ramrods to help clean them but once a serious clog develops, the rod will no longer work.

The problem tends to happen with a bit of debris such as a chip lead is sucked into the tip. When solder forms around it and hardens, we no longer have sufficient suction to clear the solder out of the PCB holes.

I ordered two new tips but also bought a hand drill set with small drill bits that I can use to clear out the two original tips. When these arrived, I was able to drill out the 1.0mm tip but did install the new 1mm tip to use removing the wrong chips.

REPLACEMENT OF THE FIVE MISIDENTIFIED CHIPS

Removing the old chips was the time consuming part. Putting new chips into the board and soldering was quick. I was fortunate to have dozens of 74LS08PC (quad AND gate) chips on hand. Due to the two chip removals, I had some pads that had lifted from the board - typically where the pin has the trace on the top of the board but uses a pad on the bottom as an anchor. I decided to attempt to touch up the top side of the pins in question.

BEEPING TO VALIDATE CONNECTIVITY

Because of the potential for broken connectivity due to the lifted pads and other issues in the rework locations, I beeped all the connections from the 90 pins affected, ensuring that I still had connectivity after my second serial repair in this spot.

I needed 2 extra short jumpers in addition to the one that was previously added to U6J. I have full connectivity so the replacement back to the proper chips is complete.

TESTING RESUMES

My initial test was to see that the machine powers up in the REST state and with the BACKWRAP signal off (high). A push of the Load/Rewind button should advance the load state machine to WAIT state and force BACKWRAP to ground. If these worked, it suggested that I had a board in decent condition.

Indeed, BACKWRAP is now high, which means inactive, when the board powers up or is reset. When I 'pushed' the Load/Rewind button, I saw the state machine advance from REST to WAIT, which should have lowered BACKWRAP, but it didn't.

I looked at the logic involved and found that the WAIT state is only one of the three conditions that must be correct in order to take BACKWRAP to ground. The other two are CART-PRESENT, since when the operator uses a tape with an autothreading band the state machine does not back wrap. The third is TAPE-PRESENT, a signal that is active if either of the phototransistors that look at Beginning of Tape (BOT) and End of Tape (EOT) is switched on.

Since I have no connection to the phototransistor board, the default state is the same as having both light sources blocked. The drive interprets that as an attempt to do a Load/Rewind after power-up with tape already threaded through the drive. In that case, we don't want to try threading.

I wired a pullup resistor to the inputs for BOT and EOT, which eliminated the false detection of TAPE-PRESENT. This left the logic primed so that when the state machine does to WAIT, the BACKWRAP should drop to ground level. That is indeed what happens!

It is time for me to step back and map out a comprehensive test plan, first verifying the default power-up states of all the output signals. Next I can test the major functions of the control logic:

• Powering up while tape is 'in the tape path', then readying the drive
• Driving forward both via input pin and by the test buttons on the board
• Repeating with reverse, both by test button and input pin
• Verifying operation of rewind command from input pin
• Trying Unload button and watching the behavior
• Trying Load/Rewind on ready drive, watching behavior
• Stepping through the load sequence for non-autothread cartridge
• Stepping through the load sequence for autothread cartridge
• Stepping through the load sequence for mini-reel

In the load sequence tests, I want to test both successful attempts and time-outs to insure that the state machine is working properly. This will involve simulating changes in input pins at specified times.