Hi Jock
Had to dig deep in the memory banks for this one!!!
Mine was a Junghienrich in disguise,with the control panel at the rear right hand side looking forward. If yours is the same there are some small red and green leds on the printed circuit board ( you have to use a small flexi mirror cos they are behind it and difficult to see.) They flash up a code wich I don't have info on. Mine had a solid red on the drive which after sending the whole panel away was told it was uneconomical to repair( main transistors US )
Have fun
Regards
Kev