I explained two posts ago that the problem with this tape machine was IC 3, an SN75462 that was put on backwards and therefore fried. That IC was responsible for interpreting logic from the transport and operating the tape sensors’ locking solenoids. Here’s the data sheet for the SN75462, and I can’t say I understand it all, but the gist of it is that it has two NAND (or AND) gates followed by an NPN transistor whose collector is taken to a pin on the actual chip, so it’s not necessarily connected to Vcc powering the chip. This is called an open collector, and here’s a very good explanation of what it is, and what are some applications. Here’s a little drawing of what this looks like in this particular case (can also be seen in the data sheet):
Here’s the portion of the schematic that shows IC3 in the circuit, the anode of the zener (D59 in this case, but all of them do) goes to ground.
Now, when I was measuring the voltage at pins 3 or 5 of IC3 (after it’s been replaced with a functional one), what I saw was that it was “delivering” +24V when it was supposed to deliver +5V (Vcc), and that was very confusing because how could it put out such a high voltage when Vcc is 5V? Not only that, the +24V was identical to what I was seeing on the other side of the relay (that should have tipped me off..)
The answer to this is the open collector but also a KVL of the circuit. I was thrown off before because someone mentioned off-hand that the reverse diode pulls up the voltage, and that sent me on the wrong path for a while. I finally understood what’s going on when I drew the NAND gate + the transistor + diode + relay together:
Here’s how I understand it. When the transistor is ON, the collector is pulled to ground through the transistor, so current flows through the relay. When the transistor is OFF, no current flows through it so the collector is free to being pulled up. What does it mean, though? KVL shows it. There’s +24V on one end of the relay, but no current flows through it (the transistor is OFF and the zener is reverse biased), so there’s +24V on its other end. That other end is the collector, and that’s why I was seeing the exact same voltage on both ends of the relay.
Now, one might ask why use the zener at all? Pull it out of the circuit and you get the same behavior. However, when you open the switch connected to a relay, the relay shoots out a spike of high voltage. Without another path to ground, that voltage will fall on the transistor and burn it. Instead, the zener starts conducting as soon as the voltage across it is higher than 30V. A regular diode reverse biased won’t work because it won’t recover from being pushed to its breakdown region. And of course, a forward biased diode will always conduct so current will always flow through the relay.