heidenhain vrz731b fault

Hi, new to the forum, hope somebody can point me in the right direction?
Just bought an old heidenhain vrz731b 2 axis readout, bought as a non worker but found plenty of burned out components on power board, sorted them out and got unit to show all numbers on display, very happy after long day of testing.
Took unit to my friends workshop to try on his scales (working on his identical set up) but not counting? The readout has a lit led on the right of each axis readout and a minus sigh on left of neadout, presumed this was because scales were not fitted but his does not have these lit? Downloaded manual but these get no mention? Presume they are a fault light??
brought unit home to have another play and the scales lit up without the extra lights but after 5 minutes they came on again!
Anybody any ideas?
Cheers,
Philip