Sequential Circuits Prophet-5 Service Manual - Audiofanzine

For troubleshooting, it should be emphasized that most computer malfunctions are caused by failures ofdevices connected to the Data Bus. For example, any shorted latch input can prevent an entire data linefrom ever being high. Shorts between data lines will also confuse the computer terribly. Shorts mayoccur within a device or between the PCB traces. Ifyou suspect a data bus problem try to pick out theline(s) with questionable levels: low should be 0-500 mV^ high 4-5V (3.5V min for CMOS), as shown inWaveform 2-OA. Readings of IV or 1.8V, for example, indicate a failure. The general procedure is to firstremove socketed PCB 3 devices: CPU, EPROM, SPAD RAM, and NV RAM—WHICH WILL CLEAR THEPROGRAM MEMORY! If this doesn't locate the problem you may have to cut traces. REMEMBER-SINCE THE COMPUTER WAS OBVIOUSLY RUNNING AT ONE TIME, THE SHORT IS MORE LIKELY TOBE DEVICE FAILURE THAN A PC-SHORT, SO BE SURE TO CHECK ALL POSSIBLY BAD COMPONENTSBEFORE CUTTING TRACES. The customary technique is to make the first cut at the electrical center of abus line to isolate the problem to one half or the other. Then halve the trace again, and so on, untilcorrect levels are restored. To prevent other malfunctions, cut traces should usually be repaired just afterthey have yielded information on the direction of the short. Note that a short on the high-voltage databus (DBH) will not usually degrade computer operations since DBH is buffered from DB. However it willbe easy to check DBH for malfunctioning switch bits, GATE bits, or S/H addresses.A DBO, U311-14B2.5 MHz CLOCKV:H:2V/div1 us/divWaveform 2-0DATA BUS AND CLOCK2-14