Build Standard IC
ROM EPROM
Software Standard Remarks
D6
IC33 - 32k
AH 16k of EPROM mounted pickaback on IC33
IC34 - 16k
D7
IC33 - 32k
JM 16k of EPROM mounted pickaback on IC33
IC34 - 16k
D8
IC33 32k -
AH
IC34 - 15k
D9
IC33 32k -
AH
IC34 16k -
D10
IC33 - 32k
JM
IC34 - 16k
D11-D14
IC33 32k -
JM
IC34 16k -
1.2.2 A newly laid out p.c. board was introduced at Issue 6 to reflect the removal of IC17 and IC27 and the addition of HAL IC38. Resistors R102,
R103 and R104 were added at this stage and trimming capacitor TC1 was replaced with a fixed capacitor C53 of 22pF. The parts list reflects
minor changes to component values. Board layouts are shown in Figures 5.1 and 5.2.
1.3 Mandatory Modifications
Certain component modifications have been introduced to improve performance in specific areas. These components should be checked for on all
units coming in for repair, and if necessary, fitted as a matter of course. These components are listed below:
1. Add resistor R104 (82 Ω) in TR1 collector circuit. It should be noted that this resistor is fitted in a non-standard way in the collector leg
between the transistor and the board.
2. Replace resistor R92 (was 220 Ω) with a resistor of value 390 Ω.
3. Add resistor R105 (1 KΩ) across capacitor C19.
4. Add resistor R106 (1 KΩ) across diode D17.
5. Add diode D22 (1N4148) in series with R100.
6. Add diode D23 (1N4148) in series with R101.
7. Add resistors R102, R103 (33 KΩ) between pins 19 and 21 of IC23 and VM12 (-12V rail). It is convenient to do this by laying the resistors on
top of IC23 and running a flying lead from their common connection to pin 1 (VM12) of IC25.
8. If EPROMs are fitted, upgrade firmware (IC33, IC34) to D11 standard and return salvaged EPROMs to Sinclair Research Limited. Details of
the methods used to upgrade to D11 standard are set out in sub-section 4.
9. Add MDV2 stand-off spacer.
2. FAULT DIAGNOSIS
2.1 Techniques
2.1.1 In a closed loop system such as a computer, because of the interdependance of numerous component parts, fault diagnosis is not
necessarily straight-forward. In addition, because of the high speed cyclic operation, interpretation of any waveforms on control, data and address
lines as being valid depends to a large extent on practical experience of the system. There are however, certain checks with valid waveforms and
levels that can be carried out before substituting any integrated circuits. Experience has shown that the best method of initially checking waveforms
and levels can be to compare with the same point in a known serviceable board. The following pages provide a basic fault-finding procedure and
furnish a list of possible faults along with suggested ways of curing them.
2.1.2 With a densely populated board such as in the Sinclair QL, a careful physical examination of the board can sometimes indicate an obvious
fault. Burst-out discrete components or an overheated track show up immediately, as do the attentions of an enthusiastic amateur. Bearing in mind
the latter, short circuits caused by hairline solder 'splatter' can be of several ohms resistance and can cause some very misleading fault symptoms.
2.1.3 It may be that the label on the faulty unit setting out the customer's assessment of the fault is unreliable. It is therefore usually best to approach
the repair task with an open mind and start the diagnosis with no pre-conceived idea of the fault.
2.1.4 Where the substitution method is used to check a suspect component, the suspect component should be connected into the known
serviceable unit rather than the other way around. The faulty component is less likely to damage the working unit, thus safeguarding the unit and the
known serviceable component.
2.1.5 Provided first principles are adhered to and a common-sense approach is adopted, it will be found after a short space of time that fixing a
faulty QL is very much a routine operation.
2.2 Power Up
NOTE: It may be advantageous to operate the QL with the cover removed, using extension connectors (see para 1.1.1).
2.2.1 At switch-on, the yellow power indicator illuminates and the QL should automatically power-up and produce a clear screen with the following
displayed at the bottom centre of the screen:
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