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Sinclair QL Maintenance Guide
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== Mandatory modifications == Sinclair specified a set of modifications to be checked for, and fitted if absent, on every machine coming in for repair. On an unrestored QL these are worth verifying:<ref name="sm" /> # '''R104''' (82 ฮฉ) added in the TR1 collector circuit โ fitted non-standard, in the collector leg between the transistor and the board. # '''R92''' changed from 220 ฮฉ to 390 ฮฉ. # '''R105''' (1 kฮฉ) added across capacitor C19. # '''R106''' (1 kฮฉ) added across diode D17. # '''D22''' (1N4148) added in series with R100. # '''D23''' (1N4148) added in series with R101. # '''R102, R103''' (33 kฮฉ) added between pins 19 and 21 of IC23 and the โ12 V rail; conveniently done by laying the resistors on top of IC23 with a flying lead to pin 1 of IC25. # EPROM-fitted machines upgraded to the D11 firmware standard. # The MDV2 stand-off spacer fitted. The R102/R103 pull-ups are the same fix the repair community reaches for when the serial ports will not transmit.<ref name="tfs" />
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