The University of Iowa's DEC PDP-8

Restoration Log

Part of the UI-8 pages
by Douglas W. Jones
THE UNIVERSITY OF IOWA Department of Computer Science



This is a chronological log of the progress restoring the University of Iowa's PDP-8 computer. Entries are added at the end as work progresses. Click on any thumbnail image to see full-sized image.

Mar. 19, 2015, Card Retainer Bracket Plans

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Bug 2: As noted on Oct. 24, 2013 the cards on the side of the PDP-8 core-memory box are prone to falling out. The same problem must have plagued early production models of the PDP-8, because in later production, DEC added a bracket, to prevent this, as evidenced by these photos:

I measured the brackets, using our PDP-8 for comparison, and then sent the resulting drawings to David Gesswein, who checked my dimensions against his brackets and offered many (mostly small) corrections. The drawings given here should allow us to make a bracket that is a reasonable duplicate of DEC's original, although as with all parts that involve foam, we will need to replace the polyurethane foam used with DEC's original brackets with something equally resiliant but less prone to decay.

Mar. 30, 2015, Teletype Reassembly

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The wrong spring!
Bug 26: On Feb. 16 we found a loose spring in the Teletype base pan, behind the paper-tape punch. Searching through the manuals and comparing drawings with what we have, we found a missing spring on the back of the paper-tape punch, the stripper bail spring. This is documented here:

Unfortunately, as the bulletin documents, there were two versions of this design, an early design with a coil spring (not unlike the mystery spring we found) connecting the stripper bail to a hook above, and one using a torsion spring wound around the stripper bail mountin post pushing up on the bail from below. It was only after we mounted the mystery spring in the space between the bail and the hook above that we noticed that the torsion spring was present below the bail. In the photo, our extra spring is shown in the center of the photo (the leftmost of the three springs hanging from the lower extension of the hook), and the torsion spring pushing up on the stripper bail is visible below it (wrapped around the bail post).

As we were leaving for the day, we found a second location for a missing spring, the trip-lever spring on the selector clutch. We must move our extra spring to that location from where we misinstalled it.

We also managed to reassemble the keyboard correctly, so far as we can tell. Our prevous reassembly efforts had left the repeat key with nothing to do; it turned out that the nonrepeat lever and latch levers, while both present, had become hung up on the wrong side of the Universal lever, so were not operative and unable to engage with the repeat key. We think we have these in their proper places now, but it is very difficult to see how they engage and they are impossible to photograph. This is documented here:

Apr. 2, 2015, Teletype Reassembly, Again

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The right spring!
Bug 26: On Mar. 30, we installed the extra spring we had found behind the Teletype's paper-tape punch in the wrong place. I went back and moved the spring to its correct location. In the photo, the leftmost of the two springs near the center of the image was missing. Without it, the friction of the trip lever on the selector clutch prevented it from releasing the clutch upon arrival of a start bit on the data line. This is documented here:

Apr. 3, 2015, Prepare for Front Panel Work

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The circuit board
Bug 47: On Feb. 23, we noted that the MA9 lamp (Memory Address register bit 9), did not light, although the lamp itself is good. With reference to the reverse engineering work reported on On Feb. 17, this implies that either the 15KΩ pull-down resistor for that lamp driver has gone "open circuit" or the transistor is bad. We have a small stock of replacement transistors; the 2N527 is sometimes priced as high as $50 because these germanium transistors are popular in fuzz pedals for electric guitars, but they can also be had for closer to $5 from some surplus outlets.

In order to diagnose the fault and do the necessary repair, I pulled the circuit board from the back of the front panel. The first time we pulled this board, we probably broke the leads for several of the lamps on the bottom row (the Multiplier Quotient register). Since then, we have learned how to replace lamps without removing the circuit board from the masonite body of the front panel, but access to the transistor and pull-down resistor requires pulling the board. As shown in the photo, I pulled the board by tipping is back while the lower rear edge of the board rested on the top edge of the rail holding the switches. Before tilting the board back, I unscrewed the cable clamps holding the wire harness to the chassis, so that the wire harness had sufficient slack to follow the board down and support it in its lowered position. This gives us full access to the solder side and component side of the board.

Apr. 6, 2015, Front Panel Work

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The circuit board
Bug 47: We thought the problem with the MA9 lamp (Memory Address register bit 9) lamp was most likely caused by a bad transistor or pull-down resistor, as discussed on Apr. 3, but close inspection with an ohm meter showed that the printed circuit trace between the transistor and the lamp was an open circuit. Even closer inspection revealed that there was a gap in the trace right where it meets the pad (with through-hole grommet) for the lamp. The best guess is that the gap in the trace was bridged with solder when the board was originally made, and that when the lamp was replaced, the solder bridge melted. We soldered a scrap of 22-gauge wire to the trace and the side of the pad in order to make a more reliable bridge over this gap.

We note that DEC modules from the 1960s do not use a solder mask to limit soldering only to the component mounting pads. Instead, there is a uniform and fairly thick solder layer over all of the circuit traces. We hypothesize that this uniform solder layer served to bridge small defects in the etched copper circuit traces. In the photo to the left, you can see our repair, but you can also see, just to the right of the upper end of our repair, a spot where we heated the adjacent trace enough to melt the solder there, exposing what appears to be a small flaw in the etch. We need to test this before attempting to reassemble the front panel.

We also discovered that we will have to remove the masonite front panel body from the computer in order to reassemble everything. DEC did not design this front panel for easy maintainability.

Apr. 9, 2015, Front Panel Work

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Pre and post assembly tests
Bug 47: I used essentially the same test methodology as we used on February 23, with one change: Instead of unsoldering the -15V lead from the front panel terminal strip, I pulled the Faston connector from -15V terminal of the power supply. As it turns out, there are just 3 -15V connections to the power supply, one for the left (memory) side of the computer, one for the right (CPU) side, and one for the front panel. The wire to the front panel is dark blue, while the other two are light blue, but just to be sure, I checked with an ohm-meter after pulling the dark blue wire to make sure that it was indeed the right wire. As before, I also disconnected all of the wires to the front panel from the backplane before doing the lamp test by connecting the bench supply to the front panel.

As you can see in the left photo above, the MA9 lamp now works. Unfortunately, removing the panel seems to have damaged at least two other lamps that used to work.

Bug 46: We have learned a lesson here: Taking shortcuts with the front panel does not pay because the lamp wires are so delecate. If you need to open up the front panel, remove the glass, then get a 3/4 inch thick board to support the face of the panel so it doesn't rest on the switches, then lower the whole assembly face down onto the table top (even an improvised table will suffice), and only then loosen the screws holding the circuit board. Lift the circuit board straight up to remove it from the (face down) masonite front panel body, and don't even touch the lamps while it is extracted. To reinstall the circuit board, tip it straight down and then lower slowly, again, trying to minimize all contact between the lamps and anything. All lamp replacement should be done with the circuit board mounted on the masonite body so that the masonite protects the lamps that aren't being replaced.

After reassembling the front panel, we repeated the lamp test, and flagged the three lamps that do not light with stickers. Replacement and testing of these will have to wait for another day.

Apr. 13, 2015, Front Panel Work

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All lamps lit!
Bug 47: After the students replaced the bulbs that were marked as non-functional, I tested the front panel again. To recapitulate the lamp test methodology used on February 23 and revised on April 9:

  1. Pull all connectors between the front panel and the backplanes; these are WO26 modules at MA35, PA2-5 and PB3-5 (a total of 8 modules). Use sheets of paper to prevent the pulled modules from shorting to each other or to nearby boards.
  2. Pull the +15 connection to the front panel from the Type 708 power supply. There are 3 +15 wires attached to the supply; 2 are heavy wires supplying power to the CPU and Memory half backplanes, one is lighter, supplying the front panel. Check that you got the right wire after disconnecting it by using an ohm meter to verify continuity from the disconnected supply end to the solder lug on the back of the front panel.
  3. Connect a bench supply to the solder lugs on the back left of the front panel. Connect the positive lead to ground (the lug where the black wires are connected) and the negative lead to the -15 terminal (blue wires). Turn up the voltage to 12V, and all the lights should come on.

Bug 46: Of course, the lights did not all come on, but when I touched the two that did not, they came on. The problem appears to be cold solder joints, so I re-soldered those lamps, tested again, and then put the glass faceplate on the front panel and tested again before taking the photo shown here.

Note: The DATA FIELD and INST FIELD lights do not light because this PDP-8 was purchased without the Memory Extension Control option, and no lamps are installed behind these two 3-bit displays.

Apr. 21, 2015, Front Panel Functional Tests

Bug 43: With the front panel lights all working, I resumed testing of the function of the PDP-8. Before powering it up, I checked that all the margnial check switches were in the normal (not the test) position and that all the wiring that had been disconnected for front panel work was reconnected. With that taken care of, I powered up the machine and performed the following tests:

The next thing to test is the switch register and the ability to set the program counter from the switches. The following table shows the values set on the switch register and the values observed in the program counter after pressing the momentary load address switch:

Clearly, there is a problem. Bits 3, 8 and 9 cannot be reliably set from the front panel. I stopped this test several times to press the momentary examine switch. It always reliably incremented the program counter, so the problem is not storage of bits 3, 8 and 9, but loading them from the switches.

Also note: The load address switch did not always work. Sometimes, it took several presses of this switch to get the computer to react. We need to find out why.

I turned off the computer and I looked at the processor map in the CPU maintenance manual to find out what boards hold what bits of the program counter. I then swapped the boards that hold bits 3 and 8 with the boards that hold bits 10 and 11, trying to put all the bad bits at the bottom of the program counter.

Powering up the machine and repeating these tests, swapping boards had no impact. Therefore, problem must be in the path from the switches to these boards, while the boards themselves are probably good. The problem could be as simple as bad electrical connections to the switches.

Pressing examine transfers the program counter to the memory address register before it increments the program counter. As a result, these tests also tested that register. Bit 0 (the most significant bit) of the memory address register appears to be stuck on (1).

Apr. 22, 2015, Front Panel Diagnosis

Bug 43: Continuing the testing from April 21, Zimu Zhang and I investigated the wiring between the switch register and the R211 boards holding the bits of the program counter. As it turns out, each switch is directly wired to pin DF of the corresponding R211 board. The board itself is documented in Maintenance Manual Drawing RS-D-R211 MA, MB, PC. The wiring of the switch-register to the program counter and memory address register is documented in Maintenance Manual Drawing BS-D-8P-0-4 PC and MA Registers.

With an ohm meter, we determined that lifting each switch connects pin DF of the corresponding R211 board to the ground bus soldered to the back of the switch register through one of the 100Ω resistors on the W026 boards that connect the front panel lights and switches to the backplane at backplane positions PA4 and PB4. Most of the switches reliably grounded the corresponding pin DF when raised, but the switches for bits 3, 8 and 9 were not reliable. The problem appears to be in the switches themselves. The problem became less severe with repeated toggling, so we suspect that getting a bit of oil (grease solvent) into the switches will alleviate the problem. If that does not work, we may have to disassemble and clean these switches the way we did the Marginal Check switches in our response to Bug 44.

On April 21, I noted that bit 0 (the most significant bit) of the memory address register is stuck on. To diagnose this problem, we exchanged the R211 board in slots PC7 and PD7 with the board in PC8 and PD8 (these are double-high boards). These boards hold bits 0 and 1 of the PC, MA and MD registers.

We then turned on the machine and repeated the basic front panel tests. Exchanging the boards moved the bad bit from bit 0 to bit 1, so we now know that the R211 board in slot PC8-PD8 has a problem. This creates Bug 49.

May 19, 2015, Board-Level Debugging

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The board-level test rig.
Bug 51: In order to work on the bad R211 board isolated on Apr. 22, we had to build a test platform consisting of an H800-W connector block, two large capacitors (with 1K bleeder resistors across them), and a pair of unregulated bench supplies. This test setup is shown in the left photo.

Bug 49: After verifying that the power supplies were adjusted for the correct voltages, I let the bleeder resistors pull the voltage down, and then plugged in the questionable R211 board, powered everything up, and began exploring it with a voltmeter. The initial focus of this exploration was on the MA (memory address) flipflop (Q4 and Q6), its output buffers (Q10 and Q11), the buffered MA outputs (pins CD and CH), and its set (pin CK) and reset (pins CJ and CL) inputs. This is an extraordinarily complex circuit board by the standards of the time, with a component density much higher than most other boards in the machine. As a result, merely relating the components on the board to those on the R211 schematic diagram in the maintenance manual was time consuming.

While I was just starting to experiment, I began to smell hot electronics, so I turned off the power and quickly scanned the board for hot spots using my thumb. A pair of transistors other than the ones I had been focusing was quite warm. It will take some time to relate those to the schematic diagram, so this work will have to wait for some other day.

May 22, 2015, Reverse Engineer R211 Board

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The R211 board, top and bottom.
Bug 49: As I noted on May. 19, the R211 board is extraordinarily complex, and it is quite difficult to relate the components on the board to those on the R211 schematic diagram in the maintenance manual. To solve this, I took a photo of each side of the board in preparation for the task of reverse engineering the board. This creates Bug 50.

June 8, 2015, Reverse Engineer R211 Board

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R211 reverse engineered.
Bug 50: Reverse engineering the R211 board was labor intensive and it revealed a few surprises. First, the R211 schematic diagram given on page 10-18 of the Maintenance Manual (F-87 2/66, copyright 1966) shows what turns out to be a revised board (Vincent Slyngstad says that this is Rev. J; he has more documentation). The board in our machine differs from this in the memory address register output buffers centered on transistors Q10 and Q11. Some of the resistor values elsewhere on the board also differ from this schematic.

The Preliminary Maintenance Manaual (F-87P2, copyright 1965) shows a schematic diagram on the top half of page 10-12. This schematic diagram has only scattered part numbers, but the area around the memory address output buffers is fully numbered and corresponds to our circuit boards.

The image showing the result of the reverse engineering shows both sides of the board, superimposed (with traces on the solder-side shaded red). The text gives:

Bug 49: In the process of reverse engineering, I discovered the reason for the hot electronics smell I noted on May. 19. The R211 board has non-standard wiring on pins DA and DB (pins A and B of the lower card-edge connector). These are used as data inputs, so plugging this board into a generic backplane that has pin A of every connector wired to +10 and pin B of every connector wired to –15 will cause trouble. We need to investigate what we might have burned out by applying power supply voltages to these two inputs. The most likely victims are the diodes that clamp the inputs within the -3V to 0V range.

June 17, 2015, Reverse Engineer R211 Board

Bug 50: Vincent Slyngstad asked what the revision letter is on the R211 boards from our machine. The revision letter is totally illegible on the board we pulled, but when we pulled other boards from the machine, we found that all of them have the same revision data etched into the solder side of the board: "9-65 MA MB PC R211 H".

Neither edition of the maintenance manual lists revision numbers, but as already noted, Vincent Slyngstad concluded that the 1966 schematic diagram appears to match his work reverse engineering an R211 J board. We now know that the 1965 schematic diagram corresponds to the R211 H board.

June 24, 2015, Board-Level Debugging

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The board test rig improved.
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R211 board in test rig.
Bug 51: After possibly damaging the R211 board by careless testing, I decided to improve the test rig with on-off switches for each row of boards and pilot lights to warn when a row was powered up. The switches allow the power to be turned on and off instantly, while under the old test rig, the filter capacitor voltage took several seconds to stabilize on power up and even longer to discharge on power down. Aside from the LED pilot lights and their series resistors, all the parts used to improve the test rig are recycled scrap.

Bug 49: Having improved the test rig, I plugged in the R211 board and began poking around with a voltmeter. The first things I tested were the on-board -0.6V and -2.4V busses. These voltages are generated on board by a cascade of diodes (D110 to D113) and a series resistor (R54) (See the 1966 schematic diagram). Applying -15V to pin DB would have reversed biased the diodes D92 and D93 on the board. This is unlikely to have caused any damage (these are a DEC D-664 diodes, 1N3606, rated at 50V).

Applying +10V to pin DA is more problematic. This puts D91 and R47 in series across 25V. R47 is 15KΩ, so this pushes 1.6mA through D91. This is not a problem; the 1N3606 can handle 100mA. This also puts 25V across D84, D112, D111, D110 and R54. R54 is 1.5KΩ, and the forward voltage drops of the diodes reduce the voltage to about 23V, so this pushes 15mA through D84. Again, this is not a problem, being under 100mA. At this current, however, R54 is dissapating 0.35W, and it is a 1/4W resistor. That explains why the board started to smell of hot electronics! The loction of R54 on the board is in the vicinity where things seemed very hot when I scanned the board with my thumb after smelling hot electronics and turning off the power on May 19.