The Servogor 281 is a plotter from 1979, which I aquired with no manual. It would, upon boot, attempt to retrieve Pen 1, but would do a pretty poor job, and would end up ramming itself against part of the wall.
To re-calibrate the Y-axis (for those playing at home), you need to do the following:
Remove the panel underneath the control panel. It is removed with 2 screws, on the bottom of the machine.
This will reveal the CPU board and Communication Board.
On the CPU board (the one with 2 LEDs), There are 4 tiny variable resistors, on the edge of the board, so you do not need to remove the board to access them. These are labelled X0, XE, Y0, YE. I assumed that *0 represents the origin calibration, and *E the max value calibration. The YE is the resistor which is required to be altered for pen alignment.
Turn the plotter on, place it in local mode, and press the scale button. This should place the plotting arm more towards the middle of the surface. Then by twisting the resistor knob slowly, you should see the pen arm slide along the y axis. It takes some trial and error, with a press of the reset button (on the CPU circuit board) to test the alignment, but eventually it should behave itself and collect the pen.
On a side note, the Resistor knobs had a small dab of paint on them, indicating some position of the knob, but I am not sure if this was factory added, or by whoever serviced the machine last (if ever). In any case, the resistor knobs do so many revolutions that using it to calibrate is not helpful, but may indicate which knobs have been tampered with, and which are still in the original positions.
A bargain find, shipped from overseas, but does exactly what I needed.
1080p 4-Port HDMI Input to HDMI + Optical/Coaxial Audio Output Converter (100~240V AC) to be more specific is what I bought. Look for it on google if you are trying to find one.
It allows the audio to be split from the HDMI to input into an older non-HDMI compatible amplifier via a optical / coaxial digital audio cable. (It also happens to have a switchbox, so I can extract this from 4 of my HDMI sources easily, if I had that many).
It is pretty simple, and needed a power socket adaptor, but alot cheaper than buying a new amplifier, thats for sure!
It occasionally required a hard reboot. Not sure why.