It is now time to take the machine apart. I shall begin by describing 
how to dismantle the machine into its major modules (since the HP9810 
and HP90280 have very similar construction, they will be described 
together) and then how to dismantle the keyboard and rear panel (these 
sections are very similar in all the machines). A final section explains 
how to dismantle the optional ROM modules.

\section{Dismantling the HP9810 and HP9820}
Start by removing any ROM modules and opening the printer cover.  Remove
the 4 countersunk screws under it. These retain the top plate of the
machine which can now be bowed up by pulling up on the printer cover and 
sprung out of the side plates. Put the top cover aside. 

Next remove the keyboard which is anchored by 4 screws under the machine. 
Stand the machine on its back and remove the 2 screws right at the front 
edge and then the 2 further back. The former tap into metal bushes in the 
relatively weak plastic front strip, the latter into metal posts on the 
keyboard chassis. By removing them in this order, the plastic parts are 
not subjected to undue stress. Put the machine the right way up again and 
lift the keyboard assembly to free it from the printer paper guide. 
Unplug the keyboard cable from the {\bf Backplane} under the front of 
the printer, then unplug the in-line connector leading to the mains switch 
and put the keyboard assembly aside for the moment.

Before removing anything further, look at the layout of the machine. On 
the left is the memory box containing the memory and memory control PCBs. 
Moving to the right, we see the 4 CPU PCBs. The peripheral section is to 
the right of that, containing the printer and {\bf Card Reader Interface} 
PCB, with the {\bf I/O Backplane} behind them. Finally, on the right are 
the 3 power supply PCBs under a metal heatsink cover. The {\bf Display} 
PCB is at the front on the left side of the machine, the card reader 
mechanism is in front of the power supply.

As you dismantle the machine, look for `obvious' problems such as burnt, 
broken or missing components. It is quite likely that you will find a 
crumbly powder inside the machine, which is the remains of the original 
card reader drive roller. You may also find a sticky black substance on 
the {\bf Backplane} at the front of the printer, which is the remains of 
the platen roller. Repairing these mechanical problems will be covered 
later. 

You can dismantle each section independently of the others, but let's 
start at the left and work rightwards. On top of the memory box are 6 
screws. Remove the 4 along the front and rear edges, leaving the 2 at the 
left side. Using the ejector handles, free the {\bf Memory Backplane} 
from its edge connectors and lift out the memory box. Remove the 
remaining 2 screws on top and the corresponding pair on the bottom and 
remove the side cover. Take the memory PCBs out of the memory box. You 
may wish to put these PCBs back in the memory box after inspecting 
them, if not, then store the ROM and RAM PCBs in anti-static bags.

Back on the main chassis, remove the 4 CPU PCBs, the {\bf Display} PCB 
and the {\bf I/O backplane}. Unplug the edge connector at the front of the 
{\bf Card Reader Interface} PCB and unplug this PCB from the {\bf 
Backplane}. Free the stud from the power supply casing and lift the PCB 
out. Unplug the head connector (a little PCB with 5 pins that plugs into 
the {\bf Card Reader Interface} PCB) to completely free the {Card Reader 
Interface} PCB. Remove the 4 fixing screws and take out the complete 
printer unit. 

The heatsink on top of the power supply area has 7 screws on it. First 
remove the 4 pan-head ones (these screw into heatsink brackets on the 
PCBs). Then remove the 3 countersunk screws at the right side and take 
off the cover. Remove the 3 PSU PCBs; the {\bf +/-12V PSU} PCB must be 
removed before the {\bf +16V/+20V/+24V PSU} PCB.

Remove the 4 fixing screws and lift out the card reader mechanism.

The next section to remove is the complete rear panel. Unscrew the 4 screws 
holding the chassis-mounted edge connectors inside the power supply area 
of the machine; these carry the transformer secondary winding connections. 
Loosen the 11/32'' nut at each side that holds the rear panel to the side 
panel, then remove the 3 countersunk screws on the underside of the 
machine at the rear edge. Pull the panel off rearwards, feeding the switch 
cable through the cardcage metalwork.

You may not have to dismantle the machine any further, but if you wish 
to remove the {\bf Backplane} for cleaning or repair, continue as 
follows. Unplug the {\bf ROM Module Backplane} cable from the left edge 
of the {\bf Backplane}. Remove the 2 screws holding the ROM module hosing 
to the cardcage and the 2 nuts holding it to the left side panel. Take 
out the ROM module housing and backplane, if you wish to remove the 
backplane from the housing, it is held on by 5 screws.

Next undo the 2 screws holding the 2 sections of the cardcage together. 
Loosen the nuts  holding the left section to th left side panel and slide 
the T-bolts along the slots. Remove the screws holding this section to 
the base of the machine and remove it. Remove the right hand section in 
the same way. Finally, remove the remaining {\bf Backplane} retaining 
screws and lift it out. 

After cleaning these parts, and doing any repairs that are 
needed here, refit the {\bf Backplane}, card cage metalwork and {\bf ROM 
Module Backplane}, but leave the other parts of the machine dismantled 
for the moment.


\section{Dismantling the HP9830}
Begin by removing the top cover which is held on by obvious screws, 
Under it you will see pair of hold-down bars over the top of the PCBs. 
Undo the single screw  and unhook the upper hold-down bar from the PSU 
heatsink. Then lift out the other bar.

The keyboard assembly should be removed next. Stand the machine on its 
back and remove the 4 screws on the underside of the machine near the 
front. This is one of the few times where it is better not to remove the 
screws in a diagonal pattern, instead remove the 2 screws on the front 
edge first (these screw into metal bushes in the relatively fragile 
plastic front of the machine) and then remove the 3 further back (which 
screw into metal pillars on the keyboard chassis). With the machine the 
right way up, lift the keyboard up to clear the eject lever of the 
cassette drive and then unplug the {\bf Keyboard Cable} from the 
backplane (just in front of the {\bf Display} PCB. Unplug the in-line 
connector on the mains switch cable and put the complete keyboard aside.

Look at the layout of the machine. The {\bf I/O backplane} is right at the
back, with a cable connected to the top edge leading to to the rear panel
connector, Of the main PCBs, the {\bf PSU} PCB is the rearmost one, with a
heatsink bracket screwed to the cardcage metalwork. In front of this are
a number of PCBs taking up the full width of the machine, these contain
the memory and memory control circuitry. The next area consists the {\bf
Ext ROM Selector} and {\bf Internal Ext ROM} PCBs on the left and the CPU
PCBs to the right. In front of the cardcage metalwork, the {\bf ROM
backplane} is on the left, in the centre are the tape control and {\bf
Display Driver} PCBs, with the tape drive itself to the right. Finally at
the very front is the {\bf Display} PCB. 

Remove all the PCBs from the {\bf Main Backplane}. Most of them come out
without difficulty, but 3 require comment. The printer cable must be
unplugged from the top of the {\bf I/O Backplane} before it is pulled out;
there's a fixing screw in the middle of the {\bf Display} PCB and the {\bf
PSU} PCB heatsink is fixed to the chassis by 2 screws. As you remove each 
PCB, inspect it for burnt, damaged or missing components, and put the 
ROM and RAM PCBs in anti-static bags.

Remove the {\bf ROM backplane}, the tape drive and the tape eject lever 
assembly, In all cases the fixing screws are obvious,

The next part to remove is the rear panel. Start by removing the nuts
holding the mains switch cable clips in place. Slide the T-bolts out of
the slot.  Slacken the 2 nuts that hold the rear panel to the sides of the
machine.  On the underside of the machine, remove the 3 screws at the rear
edge, then with the machine the right way up pull the panel rearwards and
unplug the secondary winding edge connector from the rear of the backplane.
Remove the rear panel entirely, feeding the switch cable under the card
guide metalwork. 

It is not normally necessary to remove the {\bf Main Backplane} or card
guides, but if you wish to do this, either to fully clean the machine or
to replace edge connectors on the backplane, continue as follows. Remove
the ROM door assembly and then the side card guides which are each
retained by 2 screws from the rear of the machine. There is an air
deflector plate at the left side of the machine which will come off when
you do this. Loosen all 8 nuts that hold the card guide metalwork to the
sides of the machine and slide the T-bolts along the slots, clear of the
flanges on the card guides. Spring the casing apart slightly and remove
the front card guide assembly. Then remove the 2 guides for the I/O
modules at the rear of the machine (5 screws each, a long screwdriver is
needed for the ones holding the guides to the base of the machine). There
are now no fasteners retaining the rear metalwork, spring the case apart
again and remove it.  Finally remove all the screws holding the {\bf Main
Backplane} in position and remove it. 

It is possible to remove the {\bf Main backplane} without removing the 
guides by simply removing the fixing screws and sliding it out towards 
the front of the machine. However, a machine in need of backplane repairs is 
likely to need fully cleaning too, so it's probably better to remove the 
guides

After cleaning the machine or repairing the {\bf Main Backplane}. refit 
the latter and the card guide metalwork to the casing -- and don't forget 
to put the T-bolts in the side slots before fitting the guides. Leave 
the rest of the machine apart for the moment.

\section{Keyboard dismantling}
There are 2 good reasons for dismantling the keyboard. Firstly to clean 
out the years of accumulated dust, coffee, and the like. And secondly to 
remove the mains switch so that the mains can be kept well away from the 
rest of the machine while you are tracing and repairing faults.

The first part to remove is the {\bf Keyboard Encoder} PCB. Put the 
keyboard upside-down on your bench with the display window section of the 
bezel overhanging the edge. In an HP9810 or HP9820 machine, unplug the 
keyboard cable from the end of the encoder PCB (The HP9820's cable will 
be completely free now, the HP9810 one is attached to the LEDs on the 
keyboard) and carefully ease off the {\bf Connector} PCB between the {\bf 
Keyboard Encoder} PCB and the keyboard itself. On an HP9830, there is a 
single PCB which connects the {\bf Keyboard Encoder} PCB to the {\bf 
Keyboard} and also carries the cable to the {\bf Main Backplane}; ease it 
off. Finally remove the screws and lift off the {\bf Encoder} PCB.

The plastic keyboard bezel is held in place by 6 screws. Four are easy 
to see at the sides of the keyboard, the other 2 are reached through 
holes in the {\bf Keyboard} PCB. Loosen the 4 at the sides so they have 
about 1/4" of play, then remove the last 2 screws. The one at the rear can 
be extracted using tweezers or long-nose pliers working between the 
keyswitches, the one at the front has to be shaken out. Remove the 
remaining screws and the bezel. The bezel is quite brittle now, and should 
be stored with care.

Back on the keyboard chassis, remove the front casing strip (6 screws) -- 
on an HP9830, desolder the power-on indicator wires from the keyboard 
PCB. Undo the 2 screws and separate the mains switch from the keyboard.

The keyboard will need cleaning, they always do. On an HP9810 or HP9820 
(with the 'transformer'  keyboard), pull out all the keycaps and their 
plungers. Slide the springs off, and note that double-width keys have 2 
springs, each somewhat weaker than the one used on a normal key. The 
normal and double-width keycaps can be pulled off their plungers for 
cleaning, but it is better to clean the narrow keycaps on their plungers 
to avoid damage. Remove the screws holding the keyboard PCB to the 
chassis and lift it off. Remove all the plastic key housings.

On an HP9810, there are 10 status LEDs in plastic housings on the 
keyboard chassis. These housings are retained by C-clips. To replace an 
LED, remoce the C-clip and slide the housing out of the chassis. The 
housing will then separate into 2 halves, exposing the LED and its 
connections. It should be noted that the 3 LEDs associated with the ROM 
module keys are in longer housings than the remainder. Therefore if several 
LEDs are removed at once, keep a note as to shich housing parts came from 
each LED.

On an HP9830, pull off all the keycaps. If you want to go further, it's 
necessary to desolder all the keyswitches from the PCB and then remove the 
PCB from the chassis (the fixing screws are easy to spot)., Then unclip 
all the switches from the chassis. When reassembling, it's easiest to fit 
the PCB to the chassis and then to clip the switches in place one at a 
time making sure the pins go through the holes in the PCB before 
soldering that switch in place.

You now have the incredibly boring job of cleaning all the keycaps and the 
other parts of the keyboard. Then reassemble the keyboard unit itself, but 
do not refit the mains switch, front casing strip, bezel or encoder PCB 
at this stage.

On an HP9830, this is a good time to test the power-on bulb. If it is 
burnt-out, solder a new 5V 60mA wired-ended bulb to the terminals on the 
front casing strip.

The keyboard bezel will have turned very brittle with age and it is 
likely to be cracked. If so, remove the display window so it will not get 
damaged during the bezel repairs (it is held on by small screws on the 
bottom flange).

The best way I have found for repairing the bezel is to put the pieces
together and apply a suitable solvent (such as dichloromethane (methylene
chloride), which is sold under the trade name `Plastic Weld') along the
cracks with a small brush (a natural bristle brush is best since it will
not dissolve in the solvent). Then cut a piece of cotton fabric to cover the
cracked area on the underside of the bezel. Put this over the cracks and
`paint' it with the solvent/ When the plastic has softened, force the
fabric into it. When it has dried it should be strong enough for use, but
still treat it with care. 

\section{Dismantling the rear panel}
The rear panel should be dismantled to inspect the mains wiring and 
components and also to clean and lubricate the cooling fan. Again the 
procedure is similar on all models.

Begin by removing both fuseholder caps and the cartridge fuses. Inspect 
and test the latter. If either are blown, particularly if they are 
shattered or blackened, there may be a serious power supply fault.

On an HP9810 or HP9820, the mains filter is likely to be a single module. 
There may be a metal shield in front of it, which should be removed 
first. Then remove the nut and bolt holding the filter to the bracket 
so the filter can be moved clear.

On an HP9830, it is common to have separate filter inductor and capacitor 
units. Undo the 4 screws holding the front shield plate over the filter 
assembly, unclip the inductor and capacitor from their Terry clips and 
put the plate aside. Unscrew its 4 mounting pillars.

On all types of machine, unbolt the power transformer from the rear 
panel. Then undo the 4 nuts at the corners of the connector plate, these 
screw onto studs on the metal bezel. Remove the bezel completely. the 
connector panel is now free, but of course it is wired to the cooling fan.

The original fans used in these machines are very easy to dismantle and 
repair. Take out the 4 screws on the front plate (these are not the 
screws holding the complete fan to the rear panel of the machine, of 
course). Lift off the front plate complete with the fan blades and motor. 
There may be an earth wire on one of the fan housing screws, if so, 
remove that screw. All the electrical parts can now be removed from the 
rear panel.

The fan motor is held to the front plate by 2 screws under the label. Feel
for them with a screwdriver, then remove them and the plate.  Remove the
circlip and washers from the end of the fan spindle then slide the
rotor/blades out from the bearings. There will be spacer washers on the
other end of the spindle too, make sure you don't loose any. 

Back on the rear panel, remove the fan housing and grille. There are
plastic bumpers at each end of the panel, which are held on by screws,
remove them. If they have deteriorated, it is probably best to leave them
off and simply put the screws back in their holes. 

As with the keyboard, these parts will need cleaning. Inspect the parts
for damage, broken connectors, and similar faults. Then reassemble the
complete back panel. When reassembling the fan, put a little light machine
oil on the bearings. Check the setting of the voltage selector switches, 
and fit good fuses of the correct rating in the holders.

It's time for the first electrical tests. If you have an insulation 
tester, check the resistance between the live and neutral pins of the 
mains input plug and between those pins and the earth pin. If you have an 
insulation breakdown, suspect the mains filter components. Connect the 
mains switch, removed from the keyboard, to its in-line connector and turn 
the switch on. If you are worried about the live mains on the switch 
terminals coming into contact with something it shouldn't, put the switch 
in a little plastic bag which you can fix to the cable with a cable tie. 

Now check for continuity -- not a `dead short' though -- between the live 
and neutral pins of the mains connector. If it is open-circuit, suspect 
the mains switch first of all. It is easy to check if the appropriate pins 
on the connector are linked together with the switch is turned on. If not, 
then it's time to replace the switch, I have never managed to 
successfully repair one of these. If the switch is good, check the rear 
panel components including the mains transformer primary windings.

Check the insulation between the live or neutral pins and the ground pin 
again. If you have a breakdown this time, the most likely problem is, 
unfortunately, the mains transformer. But I've never had this.

Time to apply mains at last. Use the switched cable I mentioned earlier 
if you don't have switched mains sockets on your workbench, and if either 
fuse had blown hard, you might want to connect a 100W mains light bulb in 
series with the mains input at this point. Turn on the power. The fan 
should start running and the bulb (if used) should remain dark. If not, 
suspect a short-circuit somewhere, possibly shorted turns in the mains 
transformer. Fortunately, though, this is rare. Using an AC voltmeter, 
check the output voltages of the transformer. 

Assuming everything is correct, disconnect the mains input, unplug the 
switch at the in-line connector, and refit the rear panel to the machine. 
Reconnect the switch so the transformer can be powered up again.

\section{Dismantling a ROM module}
Since the ROM modules contain only custom mask-programmed ROM chips, it 
is unlikely that you will be able to repair a defective one. However you 
may wish to dismantle a ROM module to make one good one from 2 defective 
ones of the same type, to repair the bias potential divider, or just for 
interest.

It is clear that HP never intended these modules to be dismantled. They 
are held together by barbed metal studs forced into the plastic housing. 
But they can be dismantled with little damage as follows : 

Put a suitable tool, such as a flat-bladed screwdriver between the 2 
halves of the case at one corner, near the stud. Heat the stud's head 
with a soldering iron, applying a little solder will aid the heat 
transfer (the solder will not stick to the stud, which is aluminium). 
When the plastic softens, use the screwdriver to force the casing apart 
at that corner, remove the soldering iron and hold it open until the stud 
cools.

Repeat this procedure for the other 3 studs and separate the case halves. 
Remove the extractor handle (if it hasn't already fallen out) and lift 
out the PCB noting which way round it was fitted. 

Push the studs out of the case half. I find it easier to reassemble these 
modules with \#4 * 5/8'' self-tapping screws, which will screw into the 
plastic in place of the original studs.

