In my articles on repairing the HP9800 series of desktop calculators, I 
covered the printer interface in the HP9830, but I did not cover the 
HP9866A printer that is normally connected to this interface. In many 
ways this was reasonable since the HP9866A is a separate instrument, it 
is not part of the HP9800 series of machines. On the other hand, the 
HP9866A is required for many uses of the HP9830, and thus it is useful 
to be able to repair such a device.

In this series of articles, I will remedy the omission and cover
troubleshooting and repair of this printer. I shall assume that the reader
has already read the HP9800 series of articles and thus, since much of the
general repair tips, necessary tools and test gear and safety information
are the same for the HP9866 as for the HP9800 machines, this information  
will not be repeated here.

As in the case of the HP9800 calculators, the official HP service manual 
is mostly a `boardswapper guide', although it does contain schematics of 
the {\bf Logic PSU} and {\bf Motor Driver} PCBs.  A full schematic is 
however available and is almost essential for repairing this printer. The 
source listing of the control state machine is also available, but this 
information will be given in a future article in this series. 

