HP 8568B Front panel unresponsive.

[Simon Spiers]

Some of you may know I purchased a non working RF section model 8568b a few months ago and thought i had managed to successfully repair it. I had been using for a few days when the same fault came back.
On power up the unit shows the check instrument light and the keyboard is unresponsive to commands despite some of the yellow led's being on to show input selection ect. I know the lower RF section will respond to commands without the IF section connected and also that there is not a stuck key. One of the tests to confirm this is to power the instrument op and hold any key except lcl and press instrument preset. ~This should flag the check instrument 1 light to confirm the keyboard is faulty. This appears to work correctly so I do not suspect the keyboard or its controller on the A12 board.
I have also swapped out both the A12 and A15 board (RF and keyboard interface and processor board) without any luck.

My question is, am i looking for a stuck data line?  If ?I remove the offending board, will this release the keyboard ?
The Processor board shows no error when connected to the IF section but does when standing alone. This is what it did when it was briefly working.
Any clues would be most appreciated.