The aim was to work out how the safety interlock worked – which uses the two relays on a board with digital logic and a big area with no components (photo).
This took a while as there is a wiring loom involved that disappears into the heart of the machine, and wire colours change as they pass through connectors.
A pleasant surprise was that, even after 40 years, the loom connectors are still available from RS Components under their original RS part numbers – they are ITT Cannon Trident types.
Another discovery was a reason for the speed control pot on the lathe to be loose, and its wires twisted around many times.
It looks the wires to the pot were clicked into an intermediate connector in the wrong order in the factory, causing the control to work backwards, leading to obvious operator confusion.
BTW, a crimp-on tubular ferrule (grey end) made a serviceable extraction tool for the pins in these Trident connectors.
This is definitely a ‘Mark 1’ machine – it is a Denford Starturn, designed to work with a BBC Micro – as it looks like the manufacturer intended to have the stepper drivers on this board (which would have been a neat set-up), and then moved to external drivers instead, bodging in 75LS38 quad open-collector NAND gate to interface with them – it is the chip sitting on its own, with a hidden mess of thin wire underneath.
These ICs are only rated for 7V on the outputs, and are being asked to pull down a 4k7 resistor from 12V, which is the second time in this machine that semiconductor voltage rating have been abused – the other was too much negative voltage on the SCR gates in the spindle motor drive board. (This pcb also has mains voltage tracks running close to logic voltages – hmmm)
Moving to external drives also explains having two monster transformers in the machine – one now just providing 12V for the two relays and 5V for the logic, but probably intended to power the on-board stepper drivers at one time. The other, a big torroid, powering the external drivers.
The next model: ‘Starturn 4’ had a separate relay board and a separate +5V and +12V power board.
Anyway, the interlock relays were finally understood and it is all very sensible, with the two relays implementing a no-volt release, emergency stop, over-travel stop, and push-button on-off control for the spindle motor.
It is very tempting to use this interlock as it is, especially as the board creates handy 5V and 12V rails, but the logic is not going to be used so it wastes a lot of space, and the having mains voltage so close to that 5V is a worry.
