QL Computer Hardware
The QL has undergone only two design changes during its
varied fortunes from a hardware point of view leading to the
conclusion that either it was nearly right from the word go, or it
is so awful that very little can be done with it. Events taking
place susequent to its launch confirm that it is a good machine
that has many advanced features as well a microdrives which are OK
but not everyones cup of tea. It is significant that a fairly
major design change took place at build no.14 moving from Issue 5
to Issue 6 boards. Issue 0,1,2,3 & 4 boards have not been
encountered leaving one to assume they were R & D material.
Reference will simply be made to Issue 5 or Issue 6 boards as
build numbers such as D05, D11, D16 etc. also AH,JM,JS ROM's are
not significant from the hardware aspect. The only certain way to
ascertain what is inside the black box is to open it up and
examine the board. This is achieved by turning the QL over to gain
access to eight cross-head screws. There being four short ones
under the front lip and four long ones in line with the legs. Do
not remove screws from under the microdrives at this stage. Use a
new screwdriver with the point covered in cling film to obviate an
amateur chewed screw look.
Lift the keyboard up gently noting two wide flat strips which
are the membrane connector tails. Grasp these between finger and
thumb pulling gently but firmly vertically upwards. There is no
need to disconnect anything else at this stage.
Re-assembly is simple, with the membrane tails being
carefully inserted by a gentle downward movement ensuring that
they are not creased which could break the internal track. Also
check that the tails are not folded over in the socket as this
will prevent some keys from functioning.
The issue number is on the board in three places, but the two
different boards are easily identified by looking to the right of
the ROM chips. If it contains a HAL 16L8 chip, it is Issue 6. Issue
5 boards employed a 74LS03 chip.
The various ic's (chips) will be dealt with purely from a
servicing aspect laying particular stress on practicality. No
attempt will be made to discuss the technical merits of a
particular ic or to propose extensive modification to the circuit
invloving spaghetti wiring.
Purchase of the QL Service Manual is strongly advised and at
least some practice made on soldering small components before
embarking on any updates or projects. It is essential that correct
means be employed in extracting ic's as levering with screwdrivers
etc. will damage the board as well as the ic. Pcb's are not
available as replacement parts consequently damage of a serious
nature will scrap the machine. As always it is advisable to work on
baking foil but most of the ic's appear to be fairly robust.
Two ULA's are used on the main board and one on each
microdrive. The microdrive ULA is the same as the SPECTRUM
microdrive ULA. It goes off pop occasionally probably due to it
being fed large doses of heat from the 5 volt regulator heatsink,
its own heat and heat from the motor. It is not easy to replace on
mdv1 and a socket cannot be used as the ULA is jammed up to an ic
on the board with a insulated screen in between. Leaving the screen
off produces more heat, smoke, damage and anguish. The microdrive
pcb is not available as a replacement part which is a pity as chip
failure can damage the board.
Fastened to the board is also the microdrive READ/WRITE head.
Not unknown to fail but expensive to replace. It is only available
as a 'sub-assembly' being a bog standard stereo tape head on a
plastic mounting. Thus two parts become a sub-assembly !!!.
page 1
Microdrive ULA's arrive with adorned with a variety of
coloured spots, white ones, pink ones etc. They must mean something
but do not appear to have any performance significance.
Rattles heard whilst microdrive are spinning indicate the
rubber drive rollers are not perfectly true or more probably the
roller in the cartridge is not true. Replacement of the microdrive
rubber roller is easy with a bit of glue to make sure they stay in
position.
Microdrive 2 has a useful space under it which will be
later utilised when the battery back-up is explained.
Turning to the main board, one of the most frequent
replacements is the ROM chips. There are two which must be replaced
as a pair. You can't get away with updating the one chip. Usually
owners want to update to a JS ROM which is easy even if you have an
AH. Just extract the two chips and insert the new ones the correct
way round. Have a look at the JS ROM chips as the colour spots
again appear (blue ones usually) and a leg disappears from the 128k
chip. It is one of the 5 volt supplies. Don't worry it works OK.
Whilst the top is off the QL it is worth considering how much
keyboard bashing you do and the consequences of a keyboard failure
at a critical time. Murphy's law will cause it to fail on something
important. Replacement of the switch contacts is by means of
fitting a new keyboard membrane being both cheap and easy, after
the aluminium plate has been removed. A bubble mat provides the
'springs' for the keys so there may be good reason for fitting a
new one of these at the same time. The ultimate I suppose is to
invest in a proper PC type serial keyboard plugged into SER2. Be
careful when refitting the upper case to the lower, as any poor
contact on the six LED connections stops the appropriate
microdrive.
Back to the main board to have a look at the ULA which
controls the serial ports and the microdrives. It is a ZX 8302
having a 5 volt supply on pin 40 & pin 11 with pin 40 being fed
through a link.
Links in the QL are zero ohm resistors. That is, they look
exactly like a resistor making it easy for automatic mechanical
insertion but with a single black band to indicate the value. Even
though the circuit diagram shows a link for both Issue 5 & Issue 6
boards, in practice, only Issue 5 has this present at the side of
SER1 port marked D22. That's right. Issue 5 board has a link which
looks like a resistor, in a position marked for a diode, with
another position at the side for another diode D23.
For some reason Sinclair decided to redesign this area on
Issue 6 boards.
Part of the ZX 8302 provides the clock for the QL with all
the hassle of having to reset it frequently, but if the chip can
be permanently powered then the date and time would remain correct.
To implement this change, replace the link in position D22
with a small diode together with a further diode in D23. A small
3.8 volt rechargeable battery connected to the points marked
BATTERY (under the heatsink) and fastened with sticky pads under
microdrive 2 does the trick. For purists, something like a 270 ohm
resistor in the positive lead will restrict the charging current.
On Issue 6 boards this simple approach is not possible due to
changes in the track layout. The only practical solutions being to
isolate pin 40 either by bending it out, cutting it or the track,
after which the same type of battery can be connected by soldering.
POS (+) to pin 40, NEG (-) to regulator common.
Staying with the ZX 8302 provides a further item of interest
in that colour spots are not used but marking codes are.
Example:-
ZX 8302 NCRO371275 USA F812119 F8416
ZX 8302 NCRO371275 USA F812877 F8431
page 2
In my experience, a change of chip significantly improves
microdrive handling, but it pure conjecture as to whether the ZX
8302 is faulty or if chips of later manufacture are of an improved
quality.
Moving on to the next ULA, namely the ZX 8301, a similar
situation arises in that different markings appear to coincide with
differing performance.
ZX 8301 markings:-
PS P 02 CLA2310 Y8408B
PS P B CLA2345 8544C ZX8301 Sinclair © 1983
Jittery monitor displays usually respond to the replacement
of the ZX 8301 with a loss of colour requiring a new MC 1377p.
Squirt the MC 1377p with freezer to restore your QL to full colour
until it warms up again. This trick also produces some amazing
results on other chips as does running the machine with the top
off. Indications therefore are that it gets a bit hot inside and
some ventilation would help in conditons of high ambient
temperature.
The reported machine crashes can be reduced by cooling the
machine but for real improvement the 7805 5 volt regulator requires
attention both in the QL and in any add-ons. A 100 nf capacitor
must be soldered between input and common on the chip not some
distance away. It is part of the specification and is chearfully
ignored by some manufacturers of computers to the detriment of the
performance. The knittng of three platefuls of spaghetti hanging
out the back is asking for trouble. Keep wires neat and well away
from each other, especially the power lead, mains lead and VDU
lead. I sugest you use a curly VDU lead and run the power lead to
the right of the machine. Running the TV or monitor slap up against
the QL is bad practice not only for the QL but it is in a poor
position to be viewed without strain. If possible the interior of
the QL casing should be sprayed with elecrically conductive paint
and two 7805 regulators paralleled up with the use of liberal doses
of heat-sink compound. Any regulators in add-on boards must be
checked to make sure they are contacting the heat-sink and again
use heat-sink gung. Do not forget the capacitor across the input to
the regs to remove any possibility of oscillation.
The two remaining large chips are the 68008 CPU and the MAB
8049 co-processor both of which appear to be reliable. The 8049
receives the RS 232 interface signals,monitors the keyboard,
controls the loudspeaker & controls the joystick. A simple but
effective test is to plug SER1 into SER2 then get the computer to
talk to itself by some BASIC software. A joystick test can also be
incorporated as well as a colour and sound test. This pretty-well
proves the system. A RAM test requires a m/c routine but if there
is a RAM failure it is usually visible as a black line on the tweed
start-up screen or with a completely green or white screen.
MODIFICATIONS
1. The work involved in having your clock running
continuously is quite small and fairly easy. If possible check the
current consumption which should be less than 2 microamp with the
mains power OFF. If the consumption is much greater then examine
the back of the board for a resistor which should not be there.
If the clock does not work from a battery, check for
continuity between the battery and the chip. It is not unknown for
tracks having been cut.
2. Some QL's have 33K resistors soldered to pin 21 & 19 of
the 8302 then connected to -12 volts. Some machines don't have them
and some have different value resistors. It is a nifty piece of
soldering but the manual states it is mandatory. It looks to me
that the factory have never read the manual.
3. Own up all those who kept the dongle !. Now is the time
to put it to good use by opening it up to reveal a 16K EPROM. Have
the EPROM reprogrammed with what you need then insert it in the
socket you have soldered into the vacant location at the side. If a
switch is used to pull the appropriate chip select line low, then
two EPROMS can be used but only one at any one time.
page 3
Semiconductor Function Issue 5 Issue 6
74LS00 2-in NAND Gates * *
74LS03 3-in NAND Gates * -
74LS245 Bus-transceiver * *
74LS257 Address Multiplexer * *
7805 5v Regulator * *
7812 12v+ Regulator * *
7912 12v- Regulator * *
HAL 16L8 Hard Array Logic - *
MA 8049 Peripheral controler * *
MC 1377P PAL/TV Signal Generator * *
MC 68008 Central processor * *
SN 1488 RS 232 Buffer * *
SN 1489A RS 232 Buffer * *
ZX 8301 ULA Controls display * *
ZX 8302 ULA Controls MDV's, SER & clock * *
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File: C:\Users\leepr_000\Dropbox\QL Preservation - Unsorted\Sinclair Research Limited\System Test Tape\QLHARDW_DOC
Translated by Quill-View 0.5 Beta (compiled Oct 13 2008)
Copyright 2008 Mikael Strom