PDP-8 Troubleshooting

PDP-8 serial 85, University of Iowa (Douglas W. Jones, CC0)

Fault-finding on the original PDP-8 (the Straight-8), drawn from the corrective maintenance chapter of DEC's PDP-8 Maintenance Manual (F-87) and from two restorations: David Gesswein's, which lists every failed card with the parts replaced, and the University of Iowa's on serial number 85. Check the power supply first, using PDP-8 Power Supply Restoration; most of what follows assumes the 708 is delivering good voltages.

DEC's method

DEC splits corrective maintenance into six phases: a preliminary investigation of the fault and of the machine's physical and electrical security; system troubleshooting to find the faulty module; circuit troubleshooting within the module; repair; a validation test; and an entry in the maintenance log. In the preliminary investigation DEC asks for the power supply checks and an inspection of the air filter, since overheating from a clogged filter can damage marginal semiconductors. It also warns that faulty ground connections between the computer and its peripherals are a common source of trouble that looks like a computer fault.

Diagnostics

The MAINDEC programs are on paper tape in RIM format.

Straight-8 MAINDECs (F-87, Table 9-1 and pp. 9-23 to 9-24)

MAINDEC	Tests
801	Instruction test: processor registers, controls and PC, plus the Type 182 EAE and Type 183 memory extension
802	Memory checkerboard: worst-case noise patterns in the core
803	Memory address test: MA and MB registers, memory selectors and their controls
810, 812	Paper tape reader and punch
814	Teleprinter and interrupt system

DEC calls 801 the basic troubleshooting tool for all but the most obvious faults; its printouts and halts point to a pair of modules. Gesswein found the later MAINDEC-08 series useful too, and disassembled tapes whose write-ups he could not find to work out what had failed.

Order of work on a dead machine

Gesswein brought his machine up one function at a time and fixed each fault before moving on:

1. Run light stuck on.
2. Load address.
3. Read and write a small area of memory.
4. All front panel lamps and functions.
5. Short programs toggled in from the console.
6. Teletype interface.
7. MAINDECs in their suggested order.
8. Operation over a range of margin voltages.

He later suggested leaving the core memory disconnected until the processor had been tested as far as possible, because a stepping fault on his machine cleared the core and the last program in it was lost.

Symptoms and causes

Straight-8 faults with their causes

Symptom	Cause found	Source
Rows of modules without power	Corroded marginal-check switch contacts	Iowa, Gesswein
Program halts; processor will not run; power OK never asserted	OK level from a G808 or the G809 missing (the processor halts whenever any of the three OK levels is lost); on serial 85, G808 and G809 cards in each other's slots	DEC, Iowa
Run light stays on with the machine halted	Bad S603 pulse amplifiers at PB36 and PF28 holding the run flip-flop set	Gesswein
Deposit does not work; instruction register shows AND	S203 at PB28 holding the instruction register clear (4 diodes)	Gesswein
Some memory locations always zero	Memory selector card at MD8: one core X line not driven (transistor Q15)	Gesswein
MAINDEC halts at 1001 with the PC jumping back to 1000	No carry into PC bit 5. Gesswein's text puts the fix on the R210 at PA13; his repair table lists the PC 5 to PC 4 carry on the R211 at PC11	Gesswein
No Teletype output; TTO flip-flop does not stay set	R220 at ME22 (two diodes)	Gesswein
Wrong Teletype baud rate or no Teletype clock	S202 clock divider at MF14 (wrong baud rate, four diodes) and S202 at MF15 (no clock, two diodes)	Gesswein
RAR instruction does not work	S603 at PA19	Gesswein
Bit 10 always written as one to the DF32 disc	Break in the MB10 conductor of the processor-to-memory ribbon cable	Gesswein
Deposit and examine unreliable; PC jumps by more than one	Console switch bounce: the logic uses the switch state unlatched	Gesswein
Intermittent wrong execution after hours of running	Ringing in W300 delay lines producing double memory strobes; heat had only temporarily repaired one W300	Gesswein

Gesswein's page lists the card, slot and parts for every repair on his machine.

Memory

DEC's procedure when the whole memory fails:

1. Measure the read/write and inhibit outputs of the 708 against its Table 9-3. Do not adjust them; if the supply is faulty, repair it and then set it through the memory current check.
2. If the supply is good, make the memory cycle continuously through sequential addresses: power down, jumper PB32U and PC20U to ground, power up and press START.
3. With a current probe on one of the left-hand G209 selector terminals (M, H, N or J), all read/write pulses for one axis appear as a train of spikes; a missing spike is a failing address.
4. Trace the read/write gating pulses from the BD module at MC16 to the E and D terminals of every G209; a G209 cannot select without them.
5. Run MAINDEC 803 to the end, list every failing address and look for common address bits. Check the selectors that decode those bits.

For single addresses:

• An address dropping bits: deposit all ones, examine it, and check the sense amplifier, inhibit driver and inhibit resistor for the bit that reads 0, and the inhibit current.
• An address picking up bits: deposit all zeros and check the same parts for the bit that reads 1.
• Bits dropped or picked up at every address: adjust the slice level (R3 on the G008 master slice control at MB31). If the outputs of several sense amplifiers are clipped, adjust the second-stage clamp, R13 on the G008.

Gesswein's alignment of a restored memory: he set the bottom potentiometer on the G008 for 8 V at the differential amplifier operating point (it had drifted to 7.3 V), nulled each G007's balance potentiometer as close to zero as the potentiometer allowed, and then set the slice threshold midway between the failure points, about 7.67 V on his machine.

Logic tracing on R-series cards

• R-series logic is diode-transistor logic: a transistor pulls a signal to ground and a resistor pulls it to about −3 V. Several outputs can be wired together, so one signal may have many drivers and many loads, and the schematics carry no cross-references.
• To find which card is pulling a signal, swap spares where available; otherwise cover one driver's output contact with tape and see whether the signal recovers. Never disconnect a collector clamp to −3 V while leaving a pull-down to −15 V, or the transistor will be destroyed.
• Put the suspect card on a W980 extender and trace it with an oscilloscope while a short program loop exercises it.
• For intermittent faults, tap the module handles with a wooden rod while a test runs, narrowing down to one or two modules, then check their seating, connector wear and solder joints.

Component faults

DEC's view is that the delay lines on W300 modules seldom fail and that a faulty one should go back to DEC, not be replaced on the board. In practice the commonest failures are diodes. Gesswein's card repairs were mostly DEC D-664 diodes, with two DEC-3639B and one DEC-1008 transistor; the bad transistors read dead on a tester, while bad diodes showed a low or high forward drop, a short, or leakage only at higher reverse voltage. On most cards good diodes read above 0.58 V forward and above 1.7 V reverse in circuit, but readings depend on what else is connected, so compare against the same position on another card before removing anything. The Iowa restoration found enough bad diodes to start a systematic scan of every board.

Testing a single module

DEC's in-line marginal check uses a W980 extender modified so that terminals A, B and C are cut from the backplane and brought out on 3-foot leads. Connect A to the orange +10 V marginal-check terminal (or B to the green −15 V one), run a routine that exercises the module, and vary the marginal-check supply until it fails, recording each failure voltage and the console state. DEC treats the module as good if it holds ±5 V on the +10 V supply and works with −15 V varied between −12 V and −15 V.

Related pages

• PDP-8
• PDP-8 Power Supply Restoration
• PDP-8 General Maintenance
• PDP-8 Documentation