HP 54501A Timebase cal error

[MarkC]

I recently found an old HP 54501A that had been left in a cupboard for 14 years.  Unsurprisingly it did not start up and after some reading I realised the NVRAM was dead, changed the DS1230Y for a DS1235Y and it cals without error.

Unfortunately, there is an error message of "failed interpolator cal" that will not clear and when you run self test, acq, timebase it fails with an error of 0000 0011 0000 1000

Any ideas would be appreciated.  Thanks.