People often diss Doctor Who's mini playfield lift mechanism, mine's pushing 26 years after the initial rebuild and I've never had any issues with that part of it.
Just below the first stage however sits a row of 5 weird looking targets. They're optical, meaning each of them has a IR LED + phototransistor pair, which gets interrupted by the target when it gets whacked by a ball. While the design is very robust, every now and then one of the LEDs gives up the ghost.
The targets are in pretty hard to reach place, which requires pulling the mini playfield out of the game and disassembling the whole front part. When this happens, it gets a full service of regreasing the rails and the gearbox.
Also seen here is my go-to trick for diagnosing optos: just look at them through a digital camera. They always seem to emit at least some visible light even with the modern phones having an IR filter.
#pinball #repair #pinrepair #retrotech