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{\Large \bf  Repairing HP9800 Desktop Calculators -- Part 1\\
Tony Duell (ard@p850ug1.demon.co.uk)\\}
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{ \large 

\section{No Boardswapping!}
There is only one way to repair a calculator (or any other electronic or 
mechanical device for that matter). That is to understand how it works 
and what it should be doing, to make measurements to see what actually 
{\em is} doing, then to work out what faults are causing the observed 
behaviour to differ from the correct behaviour and then (and only then) 
to correct those faults. Essentially random replacement of large parts of 
the machine (for example complete PCBs) will get you nowhere fast!

With that in mind, I will start this series of articles by describing how 
the HP9810, HP9820 and HP9830 calculators work. I will not describe every 
last gate in the machine, but I will hopefully explain the major sections 
in a way that enables the functions of all the components to be 
understood. I will then describe how to dismantle and inspect the 
machine, trace faults in the logic and perform the mechanical repairs 
that are almost always necessary.

For the purposes of the description, I will divide the machine into the
following sections : The bit-serial, microcoded {\bf Central Processor}; 
The {\bf Memory System}, containing the ROM and RAM boards, the memory
address and data registers and the associated control circuitry;  The {\bf
Display} and its driver circuitry;  The {\bf Keyboard} and its encoder
circuitry;  The {\bf Printer}, or in the case of the 9830 the printer
interface;  The {\bf Storage Device} -- either the magnetic card reader of
the 9810 and 9820 or the cassette tape drive of the 9830;  and finally the
{\bf Power Supply}

\section{Sources of further information}

In order to follow the descriptions, it is necessary to have the schematic
diagrams for the machine, and data sheets on the ICs used. Therefore a 
TTL Data Book is very useful, but it needs to be a fairly old one to 
include some of the devices used.

Many of the ICs ins the actual machine will be marked with an HP `house
number' rather than the industry-standard code. A list of equivalents,
such as the ones published in HP `Bench Briefs' is almost essential when
you come to do actual logic repairs. It is important to note that devices
are removed from later lists (if no instrument supported by HP at that
time uses the device, it will be removed), so you may have to check
several such lists. 

The HP service manuals are what I somewhat pejoratively call `boardswapper
guides' but they do contain a little useful information and are worth
reading too. Finally, to fully understand the operation of the processor,
is is useful to have a source listing of the microcode. 

And so, let us begin the real articles with the : 

\noindent
{\Large \bf CPU Theory of Operation\\}

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