The University of Iowa's DEC PDP-8 Problem List

Contents, By System Component

Contents, By Bug Number

Introduction

This is a list of all of the problems encountered with the University of Iowa's PDP-8 computer. Problems are numbered sequentially in the order of discovery. Click on any thumbnail image to see the full-sized image.

Bug 1: Broken handles on switch register

Front panel handle repair

The plastic switch handles at the left end of the row of switches are chipped, as if they collided with something. They feel loose, and it is probable that the plastic ears that hinge some of these switch handles to their switches have broken. Some of the switches may also need rebuilding, as we did with Bug 44.

• Noted Oct. 24, 2013.
• Noted Oct. 25, 2013, with photo.
• Progress, Feb. 11, 2015. Front panel finally opened up.
• Progress, Feb. 16, 2015. Handle repair started.
• Solved, Feb. 17, 2015. Handle repair completed.

Bug 2: Loose cards on side of core memory

Loose cards

There are 8 card-edge connectors facing to the front on the side of the black box holding the core memory. All but one of the circuit cards that should be plugged into these connectors have fallen out.

• Noted Oct. 24, 2013.
• Solved, Feb. 5, 2014.
• Progress, Mar. 19, 2015. Drawings prepared for bracket to prevent recurrance.

Bug 3: Decayed foam strips in cabinet sides

Right side

The front posts of the racks have aluminum channels holding plastic foam weatherstrip against the cabinet skins. Whatever its purpose, the foam in these channels has decayed. It is brittle, and it crumbles when disturbed. We will have to replace it with something less problematic. The problem is similar to Bug 11, involving a foam strip in an aluminum channel, but the dimensions are different.

• Noted Oct. 24, 2013.
• Noted Feb. 28, 2014. Decayed foam removed from sides of PDP-8 rack.
• Noted Mar. 4, 2014. Original dimensions of the foam estimated.

Bug 4: Decayed ribbon cables between half-backplanes

Cable assembly

Cable connector

The ribbon cable assemblies that connect the left and right half-backplanes of the machine are severely decayed. The insulating layer on the two flex-print cable sthat make up each cable assembly have turned to goo. We should find an appropriate replacement material.

• Noted Oct. 24, 2013.
• Noted May. 27, 2014. W034D cables measured and described.
• Noted Feb. 12, 2018. W034D cable connector boards measured.

Bug 5: Stability of the computer rack

Stability

Table plans

• Noted Oct. 24, 2013.
• Noted Oct. 25, 2013. Option of stabilizing leg(s) on attached table-top noted.
• Noted Oct. 28, 2013. Note that the rack has leveling feet on the corners that have a wider stance than the casters. These alone might suffice.
• Mitigated Jan. 23, 2014. Leveling feet lowered to take weight off casters. This provides only a small margin of safety when the computer is extended from the rack!
• Solved temporarily Feb. 05, 2014. A wooden prop was made to support the weight of the fully extended computer while the rack sides are removed for access to the power supply. Replacing the table that was formerly under the computer and had stabilizing feet would be a better solution.
• Progress, Mar. 26, 2015. The table originally on this PDP-8 resembles (but is smaller than) the table on the surviving PDP-7 at the Living Computer Museum. We got photos from them documenting the construction of that table.
• Progress, Sep. 27, 2019. The Computer History Museum in Mountain View has a table from a table-top PDP-8. They took extensive measurements of this table for us. We suspect that the rack-mount table we need used most of the same dimensions.
• Progress, Oct. 29, 2024. Plans for reconstructing the table prepared.

Bug 6: Sticky latch for computer half backplanes

The latch

The latch that holds the half-backplanes of the computer in their closed positions is very difficult to turn. The key is in the latch in the photo.

The solution is to grasp the frames for the two half backplanes near the lock with one hand and squeeze them together while turning the key with the other hand. This releases the friction on the latch making it easy to turn.

• Noted Oct. 24, 2013.
• Noted Oct. 28, 2013. Better photo.
• Solved, Nov. 13, 2013. There is a trick: Squeeze the two half backplanes together before turning the key.

Bug 7: Broken paper-tape reader on Teletype

Broken reader

Repair in progress

The paper-tape reader on the Teletype was broken. The cast ears that attached the frame of the paper tape reader to the frame of the Teletype broke off, probably when someone used paper-tape reader as a handle to move the Teletype. Fortunately, these could be replaced with home-made brackets. Also, the control lever on the paper-tape reader was broken, probably in the same accident.

• Noted Oct. 25, 2013.
• Noted Nov. 25, 2013; plastic covers and frame casting removed from reader.
• Noted Dec. 12, 2013; frame casting taken home for repair.
• Partially resolved Dec. 16, 2013; home-fabricated angle brackets added to replace broken ears on frame casting, the frame casting was re-installed on the Teletype.
• Progress, Jan. 7, 2014; reader re-installed on frame casting. Note that reader control lever is broken; can we glue it?
• Progress, Feb. 19, 2014; replaced broken reader control lever with new one; didn't finish reader reassembly.
• Solved?, Feb. 20, 2014; reader reassembled and mechanism tested.

Bug 8: Teletype power cord and data cables

Before and after

The electrical wires at the back of the Teletype were pulled out of their strain reliefs and needed to be redone before anyone even thinks about plugging it in. Also, there seems to be an extra data cable.

• Noted Oct. 25, 2013.
• Noted Nov. 4, 2013; Bug 14 and Bug 15 should be addressed at the same time.
• Partially resolved Nov. 25, 2013; the power cord was carefully pushed back into its strain relief and the unknown extra cable was folded up into the case. The data cable remains in need of proper strain relief.
• Solved, May. 1, 2014. Repaired data cable installed with new strain relief. Questions about extra data cable remain unresolved.
• Noted Sep. 9, 2014. Created Bug 38; the extra cable is for a power supply.

Bug 9: Teletype print hammer

Old and replaced hammer

The rubber head on the Teletype print hammer was shot. For many years, the standard fix among Teletype collectors has been to replace the rubber with a piece of vinyl tubing, but recently, a serious collector has manufactured new replacement hammer heads.

• Noted Oct. 25, 2013.
• Noted Nov. 25, 2013; with a good photo.
• Solved, Jan. 13, 2015; new hammer head installed.

Bug 10: Reform electrolytic capacitors

Power supplies

The electrolytic capacitors in the power supplies need reforming.

• Noted Oct. 28, 2013.
• Noted Jan. 23, 2014. Necessary tools and supplies acquired.
• Noted Feb. 10, 2014. Decided to remove and partially disassemble PDP-8 power supply, creating Bug 20.
• Progress, Feb. 11, 2014. Removed PDP-8 power supply, began work reforming 10V capacitors.
• Noted Feb. 12, 2014. 10V capacitors reforming well.
• Noted Feb. 13, 2014. 10V capacitors reforming well.
• Progress, Feb. 14, 2014. 10V capacitors reformed.
• Noted Feb. 17, 2014. Begin work on -15V capacitors.
• Noted Feb. 18, 2014. C9 to C11 reforming well.
• Noted Feb. 19, 2014; C9 to C11 reforming well.
• Progress, Feb. 20, 2014; C9 to C11 reformed, begin work on C6 to C8.
• Noted Feb. 21, 2014; C6 to C8 reforming well.
• Noted Feb. 24, 2014; C6 to C8 reforming well.
• Noted Feb. 25, 2014; C6 to C8 reforming well.
• Progress, Feb. 26, 2014; C6 to C8 reformed, begin work on C1 to C4.
• Noted Feb. 27, 2014; C1 to C4 reforming well.
• Noted Feb. 28, 2014. C1 to C4 reforming well.
• Noted Mar. 2, 2014. C1 to C4 reforming well.
• Noted Mar. 3, 2014. C1 to C4 reforming well.
• Noted Mar. 4, 2014. C1 to C4 reforming well.
• Noted Mar. 5, 2014. C1 to C4 reforming well.
• Progress, Mar. 6, 2014. C1 to C4 reformed, begin work on C5, C15 and C16.
• Noted Mar. 7, 2014. C5, C15 and C16 reforming well.
• Noted Mar. 9, 2014. C5, C15 and C16 reforming well.
• Noted Mar. 10, 2014. C5, C15 and C16 reforming well.
• Progress, Mar. 11, 2014. C5 reformed, C15 and C16 reforming, but one may be bad.
• Noted Mar. 12, 2014.
• Noted Mar. 13, 2014. C16 reforming, C15 confirmed bad.
• Progress, Mar. 16, 2014. C16 reformed, start work on capacitors in Teletype Call Control Unit.
• Noted Mar. 20, 2014. TTY CCU caps reforming well, with replacement for C15.
• Noted Mar. 24, 2014. TTY CCU cap reforming well, with replacement for C15.
• Partially solved, Mar. 27, 2014. TTY CCU cap and replacement for C15 reformed! This completes the reforming of the capacitors in the Teletype and PDP-8 power supply.

• Noted Apr. 30, 2014. Extract Type 779 power supply for reforming, creating Bug 32.
• Noted Sept. 9, 2014. Capacitors C1 to C3 reforming.
• Noted Sept. 10, 2014. Capacitors C1 to C3 held at 25V.
• Progress, Sept. 18, 2014. Capacitors C1 to C3 tested, not great but acceptable.
• Partially solved, Oct. 2, 2014. Capacitors C4 to C6 tested, not great but acceptable. This completes the reforming of the capacitors in the Type 779 power supply.

• Noted Nov. 14, 2014. Start work on the capacitor in the Teletype tape-reader supply.
• Progress, Dec. 1, 2014. Change approach to the TR supply to get above 50 volts.
• Progress, Dec. 3, 2014. Reforming voltage idling at 125V.
• Progress, Dec. 4, 2014. Reforming voltage idling at 155V, but job only half done.
• Noted, Oct. 30, 2025. Set up to reform high-speed reader-punch power supply, creating Bug 79.
• Progress, Nov. 3, 2025 Reader-punch supply idles OK at 12.5V.
• Partially solved, Nov. 17, 2025
• Solved, Feb. 12, 2026 I/O rack power supplies all tested.

Bug 11: Decayed foam gasket in cabinet doors

Door gasket

The foam gaskets that seal the vertical edges of the rear plenum door of each cabinet need renewal. The problem is similar to Bug 3, involving a foam strip in an aluminum channel, but the dimensions are different. Each door has two gaskets, one on the latch side that is easy to see, and one on the hinge side that is most apparent only when the door is open and the hinge-side skin of the rack is removed.

• Noted Oct. 28, 2013.
• Noted Oct. 31, 2013. Second gasket on each door discovered.
• Noted Feb. 28, 2014. Decayed foam partially removed from PDP-8 rack plenum door.
• Progress Feb. 3, 2026. Scraped out more bad foam.

Bug 12: Rebuild the Tally reader cable

Tally reader cable

Where is the Tally 424 reader interface that the card-edge connector on the reader cable connects to? And, what slot in the reader interface? As of the identification of this bug, the cable has been plugged into the back of the Tally reader. Mapping of the 34D Scope display interface revealed that this backplane segment at the bottom of the ADC rack includes the high-speed reader interface. Unfortunately, on further investigation, it turned out that the cable, while it may have originally served this reader, had been extensively modified and had to be rebuilt.

• Noted Nov. 4, 2013.
• Solved, Nov. 22, 2013.
• Reopened, June 6, 2014.
• Reopened, June 24, 2025. Reverse engineer old cable.
• Progress, Mar. 5, 2026. Design new Tally cable.
• Progress, Mar. 12, 2026. Start making modified W021 connector.
• Progress, Mar. 20, 2026. Finish modified W021 connector, modify design.
• Solved! Mar. 26, 2026. Cable built, plugged in and tested.
• Reopened, July 21, 2026. Data is inverted.
• Solved!, July 28, 2026. Fixed data inversion, created Bug 87 and Bug 88.

Bug 13: Make 110V power connection for Tally Reader

Dangling Jones plug

The 2-pin Jones plug on the back of the connector to the Tally 424 paper tape reader appears to be intended to bring power to the reader's drive motor. We verified this, and then made the appropriate secondary cable to bring 110V power from the power distribution panel on the ADC rack to this plug.

• Noted Nov. 4, 2013.
• Progress Mar. 26, 2026. New connector and cable assembly for Tally.
• Solved! Mar. 31, 2026.

Bug 14: Repair or replace Teletype data cable

Cable repair

The Teletype data cable was cut between the W070 connector to the computer and the 12-pin Jones plug at the back of the Teletype. The bad parts near the ends of the cable were cut out and the salvaged remainder was reinstalled with a 6-pin Jones plug at the point where the cable had been cut. The resulting cable is shorter than the original, but with the jones plug at the point of the cut, an extension cable can easily be added.

• Noted Nov. 4, 2013, along with Bug 8 and Bug 15.
• Noted Mar. 6, 2014. New 6-pin Jones plugs acquired to repair cut.
• Progress, Mar. 27, 2014. Bad sections of cable cut out and repair started.
• Progress, Apr. 25, 2014. Install reader-run relay.
• Solved, May. 1, 2014. Repaired cable installed to Teletype and reader-run relay.

Bug 15: Inappropriate Teletype data connector

Inappropriate plug

The 12-pin Jones plug in the Teletype cable was not really appropriate. By 1970, DEC was using an 8-pin in-line Mate-n-Lok connector at this point, but (as evidenced by the 12-pin connector here), earlier DEC users who wanted a plug in their cables frequently used Jones plugs. 6-pin Jones plugs are still available, so we installed one.

• Noted Nov. 4, 2013, along with Bug 8 and Bug 14.
• Noted Mar. 6, 2014. New 6-pin Jones plugs acquired to replace 12-pin plugs.
• Noted Mar. 20, 2014. Completed reverse engineering of cable, proposal for connector pinout.
• Progress, Mar. 27, 2014. Old connector discarded; start installing replacement.
• Solved, May. 1, 2014. New connector installed on cable to Teletype and W070 board.

Bug 16: Check and repair 110V rack wiring

Cut power wires

The power distribution wiring harness between the ADC Rack and the PDP-8 rack was cut, close to the terminal strip at the right rear corner of the rack. The power cabling for the both racks will need careful checking.

• Noted Nov. 7, 2013. The cut end on the ADC rack was identified.
• Noted Feb. 10, 2014. The other side of the cut was located in the CPU cabinet and coiled out of the way.
• Progress, Apr. 4, 2014 A bad wire on one of the cooling fans was cut out and replaced. The plug on the PDP-8 line cord was declared unsafe.
• Progress, Dec. 5, 2014. We began work on the cable between the computer and ADC racks.
• Progress, Jan. 23, 2015. Start reverse engineering the Type 834 power control panel.
• Progress, Jan. 27, 2015. Mercury relay tested and documented.
• Progress, Jan. 30, 2015. Unsafe rack wiring documented.
• Solved?, Feb. 2, 2015. Wiring in PDP-8 rack fixed and tested, creating Bug 43.
• Progress, Dec. 11, 2025. Account for total load of all peripherals.
• Progress, Feb. 12, 2026. All but power for paper-tape-reader done.
• Solved! Mar. 31, 2026. Paper-tape-reader powered!

Bug 17: Teletype cover broken

Broken cover, start repair
Repair completed

The transparent plastic insert in the Teletype cover was cracked and entirely broken free from its mounting. It appears that it was thermally riveted to the inside of the cover. The whole cover was filthy, and had a broken mounting ear. There are also cracks and broken mounting fingers in other parts of the cover.

• Noted Nov. 25, 2013.
• Progress, Feb. 21, 2014. New window manufactured.
• Progress, Feb. 24, 2014. Started solvent-welding cracks and broken mounting fingers in the ABS casing.
• Progress, Feb. 25, 2014. Reinforced previously glued cracks with ABS splints.
• Progress, Feb. 27, 2014. Replaced broken window-mounting studs in cover.
• Progress, Mar. 2, 2014. Rebuilt broken left rear mounting ear in cover.
• Progress, Mar. 3, 2014. Rebuilt mounting ear tested by mating cover with Teletype lid.
• Solved, Mar. 13, 2014. Permanently attached new window, installed lid in Teletype cover.

Bug 18: Missing ADC plenum panel and screws

ADC Plenum door

The rear plenum door of the ADC rack is missing one panel (the one below the ADC interface panel) and about 20 screws. This admits dust and degrades the effectiveness of the rack's cooling fan. A somewhat battered replacement panel has been located -- original DEC manufacture, formerly part of PDP-9 serial number 120.

• Noted Dec. 12, 2013.
• Partially resolved Jan. 7, 2014; the old PDP-9 panel was patched and installed (temporarily with the wrong kind of screws).
• Solved, Apr. 14, 2014; missing screws replaced.

Bug 19: Inventory and clean the computer backplanes

Preliminary backplane maps

We need to inventory the boards in both backplanes, then remove them and clean both the boards and the backplanes, and then re-insert the boards where they came from. Wherever there are discrepancies between the boards and the maintenance manuals, we will need to do more to figure out how to put things right, suspecting boards might have been pulled and replugged incorrectly over the years when this machine was not operational.

• Noted Jan. 23, 2014.
• Discrepancy found Mar. 24, 2014.
• Discrepancy found Apr. 29, 2014.
• Progress, Oct. 2, 2014. Reverse engineering of the backplane begun in an effort to resolve discrepancies.
• Progress, Jan. 22, 2015. Exchanged three boards, eliminating all but one discrepancy.
• Progress, Jan. 27, 2015. Resolved final discrepancy; need to document it.

Bug 20: Repair and re-install PDP-8 power supply

The supply out of the rack

Opened for access

To allow reforming of the capacitors in the PDP-8 power supply (see Bug 10), the supply needed to be removed from the rack and partially disassembled.

• created, Feb. 10, 2014. Wiring harness almost completely removed.
• Noted Feb. 11, 2014. Supply moved to table.
• Further damage Feb. 13, 2014. Rivets holding back panel to supply removed.
• Further damage Feb. 14, 2014. Back panel eased out from side panels.
• Progress, Feb. 17, 2014. Installed swage nuts to replace pop-rivets.
• Progress, Mar. 11, 2014. Re-connected C5 and started reconnecting C6-11.
• Progress, Mar. 16, 2014. C6-11 and C16 reconnected.
• Progress, Mar. 27, 2014. New C15 installed, old C17 removed.
• Progress, Apr. 4, 2014. New C17 installed.
• Progress, May 5, 2014. Replacement power connector work begun.
• Solved?, May 6, 2014. Power supply reassembled and installed, creating Bug 33.
• Reopened Oct. 29, 2014, power-OK signal never asserted.
• Progress, Nov. 12, 2014, Transistors Q1 and Q2 verified good.
• Solved?, Nov. 18, 2014, Boards were in wrong slots!

Bug 21: Bad foam rubber pad under Teletype typing unit

Decayed foam

New foam

The foam rubber pad under the typing unit was decayed and crumbled when disturbed. This is Teletype part number 181138, pad, pages 25 (drawing) and 32 (parts list) in Parts Model 32 and 33 Page Printer Set, Teletype Corp. Bulletin 1184B, Nov. 1964, change 5.

• Noted Feb. 11, 2014.
• Solved., Jan. 14, 2015; new foam installed.

Bug 22: Bad vibration isolators in Teletype

Decayed rubber

The rubber grommets that serve as vibration isolators were decayed and crumbled when disturbed. This is part number 181109, mount, vibration, pages 25 (drawing) and 32 (parts list) in Parts Model 32 and 33 Page Printer Set, Teletype Corp. Bulletin 1184B, Nov. 1964, change 5.

• Noted Feb. 11, 2014.
• Ignored!, Feb. 2, 2014. It will work despite this.

Bug 23: Missing Teletype sub-base mounting bolt

Base re-mounted

One teletype sub-base mounting bolt was missing. and two more appear to have been inappropriate substitutes.

• Noted Feb. 11, 2014.
• Noted Jan. 13, 2015; measurements taken.
• Solved., Jan. 14, 2015; all bolts replaced.

Bug 24: Missing Teletype call control unit screws

Two call control unit mounting screws were missing.

• Noted Feb. 11, 2014.
• Solved!, Feb. 4, 2014; replacement screws cut to size.

Bug 25: Clean, and de-gum the Teletype

The print head

The Teletype mechanism was dirty and the old oil and grease has congealed to an adhesive gum. For example, the print head carriage bearings were so thorougly gummed that they would not rotate at all before several soakings in WD-40. Cleaning these parts requirede nearly complete disassembly of the machine.

• Noted Feb. 11, 2014.
• Progress, Feb. 21, 2014. Keyboard degummed.
• Progress, Apr. 4, 2014. Printer carriage degummed.
• Progress, Apr. 14, 2014. Equivalent for KS7470 oil noted.
• Progress, Apr. 20, 2014. Degumming largely complete.
• Progress, Nov. 15, 2016. Degummed ribbon mechanism.
• Solved?, June. 22, 2017; Platen removed, cleaned and reinstalled.

Bug 26: Re-assemble the Teletype

Major subassemblies

Cleaning and de-gumming the Teletype required large-scale disassembly, so we need to put it back together.

• Noted Feb. 11, 2014.
• Progress, Mar. 7, 2014. Keyboard reassembled, but needs lubrication.
• Progress, Apr. 29, 2014. Typing unit reassembly started.
• Progress, Apr. 30, 2014. Reassembly continues.
• Progress, May. 12, 2014. Typing unit reassembled.
• Progress, Nov. 11, 2014. Broke a screw, creating Bug 39.
• Progress, Dec. 3, 2014. Paper tape punch reattached to typing unit.

  Parts re-united

  Reassembled

• Progress, Jan. 14, 2015; All sub-base castings reunited and mounted on pedestal.
• Progress, Jan. 29, 2015; Rebuild keyboard contact block.
• Progress, Feb. 02, 2015; Keyboard reassembled and mounted to base.
• Progress, Feb. 04, 2015; Call Control Unit mounted and wired.
• Progress, Feb. 16, 2015; powered up, creating Bug 48, Loose spring found.
• Progress, Mar. 30, 2015; Loose spring misidentified, keyboard fixed?
• Progress, Apr. 2, 2015; Loose spring correctly reinstalled.
• Progress, Nov. 15, 2016. Reassembled and lubricated ribbon mechanism.
• Progress, Mar. 28, 2016. Note status of reassembly.
• Solved!, May 17, 2019. Reassembly completed!

Bug 27: Teletype Keys are severely crazed

The keyboard, before and after

The Teletype keyboard keys were in very bad shape. The outer grey layer of many keys has almost shattered into multiple fragments that are not strongly attached to the white core of each key. Zoom in on the photo for details. Can we buy or make new keys? Consider the keys here that were made with laser-engraved acrylic filled with contrasting paint, then laser cut and laminated to build up the required depth. We eventually found a source of new-old-stock keys and replaced the worst of the bad ones.

• Noted Mar. 11, 2014.
• Solved?, Jan. 29, 2015.

Bug 28: Clean and reassemble the Tally reader

Tally capstan and take-up reel drive

The Tally 424 paper-tape reader mechanism needs to be properly lubricated. This will require partial disassembly and reassembly of the reader.

• Noted Apr. 14, 2014. Dissassembly under way.
• Noted Apr. 20, 2014. Dissassembly largely complete.
• Progress, May. 27, 2014. Reassembly in under way.
• Progress, June 2, 2014. Reassembly largely complete, reverse engineer connector.

Bug 29: Replace the Tally reader motor run capacitor

The capacitor and replacement

The Tally 424 paper-tape reader motor uses a 3µF paper and oil capacitor. Given the time period when this was made, it is almost certainly filled with PCB oil and reports from others suggest that failures are not uncommon and occasionally explosive. Therefore, it must be replaced.

• Noted Apr. 20, 2014. Capacitor identified.
• Progress, Apr. 25, 2014. Start installing new capacitor.
• Solved, Apr. 29, 2014. Installed and tested with motor.

Bug 30: Replace the Tally reader motor leads

The motor stator

The power leads to the Tally 424 paper-tape reader motor were in bad conditon and needed replacement.

• Noted Apr. 25, 2014. Work started.
• Solved, Apr. 29, 2014. Rebuilt and tested.

Bug 31: Repair and Reconfigure ADC Rack

Installing a rivnut

Removing wiring

When the Psychology department mounted the Tally reader and magnetic tape interface in spare spots in the ADC rack, they drilled out some of the rivnuts DEC had installed and drilled (carelessly) some new holes. Before we remount the Tally reader and any other peripherals in this rack, we should replace the missing rivnuts. We will need to replace other missing parts of this rack, and if we change the configuration of equipment in this rack, we may need to add new rivnuts. We also need to remove the wiring and electronics for the peripherals we no-longer have.

• Progress, Apr. 29, 2014. Missing rivnuts replaced.
• Progress, May 5, 2014. New doorstop for top front doors.
• Progress, Feb. 14, 2018. Remove wiring for mag-tape drive.
• Progress, Feb. 21, 2018. Remove bulk of mag-tape drive interface. Parts remain in row D of the ADC interface.
• Progress, July 17, 2026. Make temporary power and bus connections to test ADC rack.

Bug 32: Reassemble Type 779 power supply

The supply

In order to reform the capacitors in the Type 779 power supply, it had to be extracted from the ADC rack.

• created, Apr. 30, 2014. Extract Type 779 power supply for reforming.
• Progress, Oct. 2, 2014. Capacitors reformed, reassembly started.
• Solved, Oct. 9, 2014. Supply reassembled, tested and put back in the ADC rack.

Bug 33: Test Type 708 power supply

Dummy load

10V at 10A 300mV

With the capacitors reformed and the power supply reassembled, it is time to test it. This involves constructing a dummy load and measuring all of the outputs under this load.

• created, May 6, 2014.
• First testing Aug. 11, 2014.
• More testing Aug. 13, 2014. Created Bug 35 and Bug 36.
• More testing Oct. 28, 2014. Measured +10 and -15 supply ripple with scope.
• More testing Oct. 29, 2014. Measured Marginal Check and regulated outputs. One is bad; This reopens Bug 20.
• Solved?, Dec. 16, 2014. Measured Marginal Check and regulated outputs. All are good!

Bug 34: Reverse engineer the Tally reader interface

Type 750C controller, as found

The Tally 424 paper tape reader interface is a subsystem of the Type 34D Oscilloscope Display backplane. DEC designed this interface as the Type 750C High Speed Reader, but that interface appears to be designed for a photoelectric reader. There is evidence of tinkering in the backplane here, so we need to thoroughly understand what was done to make the DEC interface work with the Tally reader.

• created, June 2, 2014.
• Progress, June 13, 2014. Cable connector reverse engineered.
• Progress, Oct. 2, 2014. Backplane reverse engineering started.
• Reopened, June 24, 2025.
• Major progress, July 1, 2025 Controller, as found, reverse engineered.
• Progress, Feb. 3, 2026 Timing diagram for Tally 424 reader.
• Solved?, Mar. 5, 2026 Annotated schematic from July 1, 2025.

Bug 35: Fix Marginal Check supply panel meter

Trimmer added

The Marginal Check power supply panel meter reads about 10% high compared to the VOM used in power supply testing.

• created, Aug. 13, 2014.
• Progress, Aug. 18, 2014. Trimmer potentiometer installed.

Bug 36: Fix the Inhibit and Read/Write supplies

The Inhibit and Read/Write supplies do not work. These are regulated supplies with outputs that only connect when both voltages are within spec. A relay driver performs the logical and function.

• created, Aug. 13, 2014. Created Bug 37.
• Progress, Oct. 29, 2014. Preliminary measurements.
• Progress, Nov. 12, 2014. Dead-end hypothesis.
• Progress, Nov. 18, 2014. Boards were in wrong slots!
• Solved?, Dec. 16, 2014. Outputs within spec.

Bug 37: Get or build Flip-Chip Module Extenders

W982 board

The Inhibit and Read/Write supplies do not work. These are regulated supplies with outputs that only connect when both voltages are within spec. A relay driver performs the logical and function.

• created, Aug. 13, 2014.
• Progress, Aug. 13, 2014. Purchased two H807 connectors.
• Solved, Sep. 18, 2014. Completed 2 W982 extender boards.

Bug 38: Repair and mount Teletype tape-reader supply

The supply

The power supply for the Teletype mounts either on the Call Control Unit or on in the Teletype pedestal. Our supply was missing, but we obtained a broken one from eBay for a pittance. Unfortunately, it is broken, and it came without a mounting bracket.

• created, Sep. 9, 2014. Broken power supply obtained.
• Progress, Sept. 10, 2014. Missing circuit trace rewired.
• Progress, Nov. 9, 2014.
• Progress, Nov. 11, 2014. Mounting bracket fabricated and installed.
• Progress, Nov. 14, 2014. Fish paper and grounding strap installed.
• Solved, Dec. 1, 2014. Fish-paper shield installed. Now, we need to reform the capacitor, more work on Bug 10.

Bug 39: Repair or replace broken Teletype screw

Old and new screws

Teletype part-number 182891 is a very unusual screw. Unfortunately, the fat screw head invites overtightening. Fortunately, the huge screw head means that this screw can be repaired.

• Created Nov. 11, 2014. Overtightened screw breaks.
• Solved, Dec. 1, 2014. Old screw head drilled and tapped to hold a new screw body.

Bug 40: Repair G007 sense amplifier

The broken board

The repair

The sense amplifier in backplane slot MB30 was damaged during the backplane inventory and cleaning (see Bug 19). The repair was essentially straightforward.

• noted, Dec. 4, 2014. Broken pin-connector socket for sense wire on handle-side of board.
• Solved, Dec. 5, 2014. Repaired using D-subminiature socket parts.

Bug 41: Repair Teletype keyboard cover

Lugs replaced with screws

The cover over the Teletype keyboard was supported and aligned on 3 mounting lugs, all of which have broken off.

• noted, Jan. 22, 2015.
• Progress, Jan. 23, 2015. Screws test fitted to replace the broken lugs.
• Solved, Jan. 27, 2015. Screws cut to length.

Bug 42: Rebuild Teletype keyboard contact block

Original

Rebuilt

• noted, Jan. 22, 2015. New contact block ordered.
• Progress, Jan. 29, 2015. Work started
• Progress, Jan. 30, 2015. Work largely completed.
• Solved, Feb. 2, 2015. Repaired contact block installed.

Bug 43: Test PDP-8

With the power supply tested and the rack wiring fixed, we can turn on the computer. Now we must see if it works!

• noted, Feb. 04, 2015. First power to PDP-8, some front panel lights work.
• Stalled Feb. 05, 2015. Created Bug 44.
• Stalled Feb. 11, 2015. Created Bug 46 and Bug 47.
• Progress, Apr. 21, 2015. Begun functional testing from the front panel; found some bugs.
• Progress, Apr. 22, 2015. Created Bug 49, also note some poor switches.

  Running a test

• Major progress, July 10, 2025 Start programming!
• Progress, July 15, 2025 More programming.
• Progress, July 15, 2025 Video of front panel.
• Progress, July 22, 2025 Video of testing MQ register.
• work, Sep. 29, 2025 First attempt to use TTY interface.
• Progress, Oct 6, 2025 TTY interface does not work. Created Bug 77.
• Progress, Oct 9, 2025 TTY output works.
• Progress, Oct 16, 2025 TTY input works.
• Progress, Oct 30, 2025 Subroutines work.
• Progress, Feb. 19, 2026 Bigger TTY demo works, trouble with BIN loader.
• Progress, Mar. 24, 2026 JMS instruction tested.
• Progress, Apr. 21, 2026 ODT ODT debugger loads and runs, but something is wrong.
• Progress, Apr. 23, 2026 Diagnostic for autoincrement addressing runs.
• Progress, Apr. 28, 2026 Diagnostic for RTR fails, creating Bug 86.
• Progress, May 7, 2026 Successfully use ODT to examine, patch and run some code.
• Progress, July 17, 2026. Start testing I/O bus.
• Progress, July 21, 2026. All IOTs to device 01 seem to work.

Bug 44: Repair PDP-8 marginal check switches

Switch parts during cleaning

After turning on the computer, we found that power was not reaching all parts of the PDP-8 backplane. The problem turned out to lie in the switches that allow switching each row of the backplane from the primary power supply to the variable voltage marginal-test power supply. Repair or replacement of these switches is in order.

• noted, Feb. 05, 2015. Document the switches.
• Progress, Feb. 06, 2015. Begin repairing the memory-side switches.
• Progress, Feb. 09, 2015. Finish memory-side switches, start CPU-side.
• Progress, Feb. 09, 2015.
• Solved, Feb. 10, 2015. All switches reassembled and power wiring tested.

Bug 45: Clean PDP-8 backplane fans

Cleaning the fans

There was considerable dust on the bottoms of the 3 fans at the bottom of each half backplane of the PDP-8, so we removed the fan blades from their motors, cleaned everything, and then opened the motor hubs and oiled them.

• noted, Feb. 10, 2015. Work started.
• Solved, Feb. 11, 2015. All fans cleaned and oiled.

Bug 46: Reassemble PDP-8 front panel

The front panel removed

To deal with Bug 1 and Bug 47, we had to disassemble the PDP-8 front panel. It will need reassembly.

• noted, Feb. 11, 2015. Work started.
• Progress, Apr. 9, 2015. Hard lessons about how to reassemble front panel.
• Solved, Apr. 13, 2015. Front panel reassembly completed.

Bug 47: Fix burnt-out front panel bulbs

The lamps

All lamps lit

When we began testing the PDP-8 (Bug 43) we found that many of the bulbs on the front panel did not light, although other evidence suggested that the bulbs ought to light.

• created, Feb. 11, 2015. Front panel removed and opened up.
• Progress, Feb. 17, 2015. Got replacement lamps and reverse engineered lamp circuitry.
• Progress, Feb. 23, 2015. Replaced a total of 28 lamps, one remains inoperable, a bad transistor?
• Progress, Apr. 3, 2015. Open up front panel to gain access to transistor in question.
• Progress, Apr. 6, 2015. The problem was a bad circuit trace, not a bad transistor.
• Progress, Apr. 9, 2015. Three more lamps failed during front panel reassembly.
• Solved, Apr. 13, 2015. All lamps now light!

Bug 48: Test Teletype

First test

11 weeks later

Having reassembled the Teletype, it is time to see how it works.

• created, Feb. 16, 2015; it chatters away when it should be idling.
• Progress, Oct. 14, 2015; Created Bug 52, adjust the Teletype.
• Progress, July 19, 2016; Tape punch works but punches wrong data.
• Progress, July 20, 2016; Tape punch works, punches as typed.
• Progress, Sep. 13, 2016; Space, CR, LF, work, but types wrong letters.
• Progress, Sep. 27, 2016; Most letters print most of the time, but stutters.
• Progress, Oct. 13, 2016; M misprints as ←, and F, N, V and 6 misaligned.
• Progress, Dec. 1, 2016; S occasionally misprints as ←, T and I also problematic.
• Progress, Dec. 8, 2016; Pretty reliable, but occasional doubles and the bell rings too much.
• Progress, Mar. 7, 2017; Very reliable, but occasional problems with V and W at high speed.

Bug 49: Repair R211 board

Testing the R211

The R211 boards are double-height boards. Each hold one bit of the memory address, program counter and memory data registers, plus the multiplexer logic for loading those registers.

• created, Apr. 22, 2015; The MA register on the board in slots PC8 and PD8 seems to be stuck on.
• Progress, May 19, 2015; Something got hot when the board was powered up. created, Bug 51.
• Progress, May 22, 2015; took board photos, creating Bug 50.
• Progress, June 8, 2015; discovered that tests on May 19 may have damaged board.
• Progress, June 24, 2015; tests on May 19 appear to have done no harm.
• Progress, July 2, 2015; board testing points to bad transistor.
• Progress, Sept. 30, 2015; the problem was a bad diode.
• Problem reopened Apr. 6, 2017; PC carry propagation problem observed, see Bug 61.
• Progress, Apr. 20, 2017; Bad diode found, but a different bit will not load from switches.

Bug 50: Reverse engineer the R211 board

The R211 H board

Reverse engineered

The R211 boards are extraordinarily complex double-height boards, where relating components on the board to the schematic diagram is not easy.

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 part numbers; those prefixed with equals signs (=) reference the 1965 schematic of the R211 H; those without reference the 1966 schematic of the R211 J. Markings on vias relate them to pins on the card edge connector or to the voltages distributed on that circuit trace.

• created, May 22, 2015; took board photos to launch process.
• Progress, June 8, 2015; part numbers related to schematic diagrams.
• Solved, June 17, 2015. revision letters pinned down.

Bug 51: Build Flip-Chip board test rig

Test fixture

Logic probes

Testing boards from the PDP-8 is easier if they can be tested outside the PDP-8 backplane. This required building a test rig and a logic test probe.

• created, May 19, 2015; built a preliminary test rig.
• Progress, May 24, 2015; added framework, on-off switches and pilot lights.
• Done? July 2, 2015; built a pair of logic probes.

Bug 52: Adjust the Teletype

Reader stop latch

Platten height

There are numerous adjustments that must be made after disassembling and reassembling the Teletype.

• created, Oct 14, 2015; adjusted universal lever and reader stop latch.
• Progress, July 19, 2016; can type to punch and typing unit, results are frequently wrong.
• Progress, July 20, 2016; "rangefinder" adjusted, punch reliably outputs what is typed.
• Progress, Sep. 27, 2016; Correctly types most of the time, but stutters.
• Progress, Oct. 13, 2016; Fixed stutter in function clutch (release needed adjusting).
• Progress, Nov. 15, 2016; Fixed tension on selector magnet armature.
• Progress, Dec. 1, 2016; Adjusted selector armature, rear carriage rail height.
• Progress, Dec. 8, 2016; Selector armature, print hammer trip lever, distributor trip linkage.
• Progress, Feb. 2, 2017; Adjust paper-tape punch penetration.
• Progress, June. 22, 2017; Adjust platen height and type-drum centering.
• Progress, June. 29, 2017; Adjust line feed drive link and up stop.
• Progress, June. 25, 2017; Adjust platen height again.
• noted, Oct 13, 2025; Teletype keyboard data has glitch.
• Progress! Oct 16, 2025; Software can echo keyboard input to printer!

Bug 53: Missing felt washers in Teletype punch

Felt washers

The outboard end of one of the pivot shafts in the Teletype is missing several felt washers.

• created, Feb. 17, 2016; made new felt washers, installation looks wrong.

Bug 54: Loose code bar bracket in Teletype

Code bar bracket

The bracket that serves as an alignment comb at the left end of the code bars that serve as the main mechanical data bus in the teletype was not attached. Floating loose on the code bars, it did not seem to be doing any harm, but it should be attached.

• created, July 20, 2016; discovered problem.
• Solved, July 28, 2016; screw and nut installed (screw is in center of photo).

Bug 55: Broken Teletype carriage-return lobe plate

Broken

Replaced

The carriage return lobe plate that mounts on the left side of the carriage was broken. This prevents the spacing mechanism from working after a carriage return because without it, the return lever stays latched, disengaging the spacing pawls.

• created, Aug. 4, 2016; discovered and removed broken plate.
• Solved, Aug. 12, 2016; new lobe plate installed.

Bug 56: Teletype control-character linkage engagement

Most of the control-character actuating levers was not engaged properly with the corresponding linkages, and the corresponding function levers were not properly engaged with their function pawls.

• created, Aug. 12, 2016; discovered the problem.
• Progress, Sep. 8, 2016; CR function lever now decodes, but no actuation.
• Progress, Sep. 13, 2016; CR seems to work correctly.
• Progress, Nov. 15, 2016; BEL and ENQ seem to work correctly, but testing fails.

Bug 57: Teletype bell rings with every character

During reassembly of the Teletype, the automatic codebar was inserted in the wrong slot in the codebar assembly, causing the function bell to ring with every character typed.

• created, Dec. 8, 2016;
• Cause diagnosed Dec. 30, 2016;
• Solved, Jan. 3, 2017;

Bug 58: Teletype bit 4 problem

After enough of the Teletype was repaired that the typing mechanism began working reasonably reliably, there were misprints that focused on incorrect values of data bit 4.

• First evidence Oct. 13, 2016; Looking back through the logs, the evicence of this problem was first recognized and connected to bit 4.
• created, Mar. 7, 2017; Bit 4 only converted from 0 to 1 after a 1, and the problem is in the receiver.
• Diagnosis Mar. 23, 2017; The selector armature is being locked prematurely in the on position.
• Solved, Mar. 28, 2017; A small rangefinder change fixes it!

Bug 59: Teletype platen needs reconditioning

The rubber of the teletype platen is very hard, limiting the print quality. Is there some way to recondition it?

• created, Mar. 7, 2017;
• noted, Mar. 28, 2017;
• Progress, June. 22, 2017; platen removed, cleaned and reinstalled. Looks better.

Bug 60: Teletype stand is missing its back

The back panel of the teletype stand is missing. Can we manufacture a replica of the original?

• created, Mar. 28, 2017;

Bug 61: Debug PDP-8 PC register

When you increment the PC, carry does not propagate from PC bit 2 to bit 1. This is most obvious when you hit RUN with the memory disabled so all fetches get opcode zero (AND). The PC lights up as 00X XXX XXX XXX, where bulbs marked X are on at half intensity because they are flickering between 0 and 1.

• Problem diagnosed Apr. 6, 2017; Moving boards pins the problem on an R211 board, reopening Bug 49.
• Progress, Apr. 20, 2017; Bad diode found, but a different bit will not load from switches.

Bug 62: Fix Teletype linefeed mechanism

Adjusting Height

Adjusting LF link

We broke the linefeed mechanism while adjusting the platen height.

• created, June 22, 2017; We left a screwdriver in the left platen height pry point when we hit LF.
• Solved, June 29, 2017; Adjusting the line feed drive link and up stop compensated for the bending or stretching caused by our error.
• Noted, Oct. 9, 2025; Linefeeds only work when typed slowly.
• Solved, Feb. 24, 2026; Tightened drive link, adjusted upstop.

Bug 63: Fix Teletype reader-run circuit

The problem spring

The paper-tape reader only runs if the clutch-trip magnet's armature at the rear of the Teletype is held down manually. This is evidence that there's something wrong in the wiring of the reader-run circuit.

• created, Mar. 7, 2017; need to hold down clutch-trip armature discovered.
• Progress, July 6, 2017; documents identified that show this should not be needed.
• Solved, July 11, 2017; one contact spring in the reader was improperly reassembled.

Bug 64: Debug PDP-8 Memory

Inhibit and R/W select currents

Memory does not work.

• Problem observed in passing Apr. 6, 2017; memory does not work.
• Problem observed again Nov. 17, 2017; can get data to MB register, but it reads back as zero.
• no progress Feb. 12, 2018; speculate that a W034 cable is missing.
• small progress Feb. 21, 2018; found the memory current limiters.
• small progress Apr. 4, 2018; added test points to simplify current probe.
• Progress? Apr. 11, 2018; preliminary current probe results.
• small progress Apr. 18, 2018; useful current probe results.
• small progress Apr. 25, 2018; check quality of power supply regulation.
• Progress? May 2, 2018; swapping regulators has no effect.
• Progress? Sept. 13, 2018 Memory Strobe and Inhibit signals seem wrong.
• Progress? Sept. 27, 2018 Study B360 board that generates memory stribe.
• Progress? Nov. 8, 2018 Data tracked from the switch register to the Inhibit drivers.
• Progress? Dec. 6, 2018 Strobe pulse may be shorted to ground by a sense amplifier.
• Progress, Mar. 19, 2019 Reverse engineer G007 sense amplifier board.
• Progress? May 17, 2019 Replace open diode in G007 sense amplifier.
• verified status unchanged, Jan. 31, 2023
• found bug? Oct 9, 2024 Clock debugging.
• Progress, Oct 21, 2024 R603 reverse engineer and repair.
• Progress, Oct 28, 2024 Clock debugging.
• Progress! Oct 30, 2024 All memory timing waveforms verified to be correct.
• Progress! Nov. 4, 2024 Memory current settings measured, close to correct.
• Progress, Nov. 13, 2024 Memory current and voltage settings set by the book.
• Progress! Apr. 14, 2025 Memory starts to work!
• Progress! May 5, 2025 Sense amplifier outputs mostly good.
• Progress? May 30, 2025 Repair G208 inhibit driver.
• Workaround found, July 10, 2025 Work around problem, not repair it.
• noted, July 22, 2025
• Progress, Jan. 14, 2026 Explore range of slice voltages that work.
• Progress! Feb. 3, 2026. Adjust voltages to make it work better!
• noted, Feb. 19, 2026 Is there a problem with odd numbered pages?
• progress, Mar. 31, 2026. Problem with a G209 board in MD6 to MD9.
• progress, Apr. 7, 2026 Problem probably a G209 board numbered 9.
• Solved?, Apr. 16, 2026 Two cold solder joints fixed.

Bug 65: Built DEC R-series Logic Probes

First and second generation probes

We found that we needed to watch signals on the wire-wrapped backplane, so we built logic probes.

• initial design, July 2, 2015
• a better probe, Nov. 7, 2018
• yet even better, Dec. 6, 2018
• and more of them, Jan. 31, 2023

Bug 66: Fix marginal sense amplifier

Test fixture

We found that we could only achieve marginally acceptable static balance on the G007 sense amplifier in slot MB27.

• Identify problem, Nov. 20, 2024
• Results of bug, Apr. 14, 2025 Cannot read bit 5 of memory!
• Progress, May 5, 2025 Sense amp seems to work, but no ones to read.
• Progress, May 8, 2025 Rule out a broken inhibit winding.
• Progress, June 5, 2025 Prove that the sense amplifier is bad.
• Progress, June 10, 2025 Diagnose G007 sense amplifier.
• Progress, July 3, 2025 Compare good and bad G007 boards.
• Progress, July 8, 2025 Balance and compare all G007's.
• Workaround found, July 10, 2025
• Progress, Oct 20, 2025 Replace Q2 on weak amplifier.
• Progress, Oct 23, 2025 Redo faulty measurements.

Bug 67: Eliminate noise on Memory Address

MA 11 and its inverse?

We found noise on one of the memory address line MA11(0) with an amplitude exceeding the signal amplitude. We will have to check all the memory address lines.

• Identify problem, Dec. 9, 2024
• Progress, Dec. 11, 2024 Identify G209s as noise source.
• Progress, Dec. 16, 2024 Repair diodes on G209 from slot MD12.
• Progress, Feb. 13, 2025 Repair diodes on second G209, others seem good.
• Solved?, Apr. 7, 2025 Test result of reapir.
• Solved?, Apr. 14, 2025 Memory starts to work!
• reopened, Apr. 7, 2026 Problem probably a G209 board numbered 9.
• Solved?, Apr. 16, 2026 Two cold solder joints fixed.

Bug 68: Fix burnt scope probe ground clips

New clips

We managed to melt the wires in the ground clips of our scope probe by shorting the +10 supply to ground through the scope. We also used the technology we used to solve this problem to make push-on backplane shorting jumpers.

• Create problem, Feb. 13, 2025 We melted the wires.
• Solved, Feb. 20, 2025 We made better ground clips and several shorting jumpers.
• continued, Oct 20, 2025 We made more shorting jumpers.

Bug 69: Combine 2 6-bit BRPE punches to get 8 bits

Parts donor

The PDP-8 documentation says it has an interface for an 8-bit BRPE paper-tape punch. This was evidently modified by the psychology department to support a different punch, which we do not have.

• Create problem, June 4, 2019 We got 2 6-bit BRPE punches. creating Bug 71.
• Progress, Feb. 17, 2025 Start careful disassembly.
• Progress, Mar. 13, 2025 Extract linkages from one.
• Progress, Mar. 20, 2025 Reverse engineer punch block, creating Bug 70.
• Progress, Mar. 31, 2025 Assemble BRPE linkages.
• Progress, Apr. 3, 2025 Reassemble widened BRPE.
• Progress, Apr. 7, 2025 More reassembly.
• Progress, Apr. 10, 2025 Power up and feed tape!
• Progress, Mar. 22, 2026 Installed 8-bit punch block.
• Solved?, Mar. 29, 2026 Reinstalled tape guide.

Bug 70: Make an 8-bit BRPE punch block

The 8-bit block

We need an 8-bit punch block for our BRPE punch. All we have is 6-bit blocks.

• Create problem, Mar. 20, 2025 Reverse engineer punch block.
• Solved! Mar. 22, 2025 Bought a NOS block from eBay, installed it.

Bug 71: Make cables to connect BRPE punch

BRPE sockets

Our BRPE punch came with no data cable and no power cable, just connectors for the cable in back.

• Create problem, June 4, 2019 We noted missing BRPE punch connectors.
• Progress, Apr. 7, 2025 Make a power cord, test the motor.
• Progress, Apr. 17, 2025 Reverse engineer BRPE wiring.
• Progress, Apr. 21, 2025 Start reverse engineering BRPE controller, creating Bug 72.
• Progress, May 21, 2025 Cable design completed!
• Progress, Sep 8, 2026 Punch modified to account for current limits in connector.
• Solved? Mar. 22, 2025 Revised design and made the cable.

Bug 72: Reverse engineer Type 75 punch controller

Type 75 backplane

We have a little documentation for the oscilloscope output option that is part of the multi-device-controller backplane at the bottom of the analog to digital converter rack, but none of this covers more than cursory details of the Type 75 high-speed paper-tape-punch controller. There is evidence of modification to this controller, so we will have to bite the bullet and try to do a complete job of reverse engineering, first determining how it worked after the Psychology department modified it to work with their Tally paper-tape punch, and then determining how to make it work with the BRPE punch it originally supported.

• Create problem, Apr. 21, 2025 Start reverse engineering controller.

  Reverse engineered

• Progress, May 1, 2025 BRPE punch strobe signal conditioning.
• Progress, May 12, 2025 Trace the controller wiring.
• Solved? May 21, 2025 Reverse engineered, creating Bug 73.
• Progress, May 28, 2025 Begin testing punch-strobe receiver.
• Solved? May 28, 2025 Finish testing W501 punch-strobe receiver.
• Progress, Sep 8, 2026 Work out which wires need to be removed.

Bug 73: Fix broken and modified W501 Schmitt Triggers

W501 board

In attempting to figure out how to undo the changes made by Psychology to the Type 75 High Speed Punch controller, we found that both of our W501 Schmitt Trigger boards are bad, one modified, one with a bad transistor.

• Create problem, May 21, 2025
• Progress, May 28, 2025 Fix and test a W501 board.
• Solved, June 5, 2025 Fix and test second W501.

Bug 74: Scan for bad diodes

We have found and replaced many bad diodes in resolving earlier bugs. Notably, in bug 49, bug 61, bug 64 and bug 67. This convinced us to set out on a systematic scan of all diodes on as-yet checked boards in the CPU and peripherals.

• Start work, July 18, 2025 Row PA finished.
• Progress, July 22, 2025 Row PB started, spare boards started.
• Progress, Sep. 18, 2025 Finished CPU and TTY interface.
• Progress, Sep. 29, 2025 Rows A and B on multifuction I/O backplane started.
• Progress, Oct 6, 2025 Rows A and B on multifuction I/O backplane finished.
• Progress, Oct 9, 2025 Found bad W040A board in row C, creating Bug 78.
• Progress, Oct 30, 2025 Row C on multifuction I/O backplane finished.
• Progress, Nov. 17, 2025 Scanned all R210 boards in CPU.

Bug 75: Deal with parts mining

Someone has mined parts from the machine for use elsewhere.

• Create problem Sep. 18, 2025 W050 board in MF25 missing 3 2N527 transistors.
• Solved! Oct 6, 2025 Replaced transistors.

Bug 76: Teletype buzzes when turned off

The Teletype should be silent when the local-off-line switch is in the off position, but instead, it makes a loud buzzing noise.

• Create problem Sep. 18, 2025 Selector magnet buzzes.
• Diagnose Sep. 22, 2025 Bad capacitor in local loop supply.
• Solved!Sep. 29, 2025 Replaced and tested capacitor.

Bug 77: Teletype interface does not work

First TTY output

Attempting to send data to the Teletype, the most we get is one clunk from the mechanism.

• Created, Oct 6, 2025 Create problem, begin diagnosis.
• Progress! Oct 9, 2025; Output works!
• Progress, Oct 13, 2025; Keyboard input does not work.
• Solved! Oct 16, 2025; Software echoes keyboard input to printer.

Bug 78: Fix W040A solenoid driver

On finding a bad diode on a W040A board, we could not find a schematic for the W040A, and it obviously differs from the well documented W040B, so we had to reverse engineer the board to find out the role of that diode in order to infer its specifications to find an appropriate replacement.

• Created, Oct 9, 2025 Bad W040A board in multifuction I/O backplane row C.
• Solved! Oct 16, 2025; Reverse engineering completed, board repaired.

Bug 79: Fix high-speed reader-punch supply wiring

The home-made power supply for the high-speed paper tape reader and punch came with a range of poorly done wiring. This needs to be redone, and in some cases, rethought.

after rewiring

• Created, Oct 30, 2025 Old wiring from supply to backplane removed.
• Progress, Nov. 3, 2025 Internal supply wiring spiral wrapped.
• Progress, Nov. 17, 2025 Reverse engineered supply, planned changes.
• Progress, Dec. 11, 2025 It is safe to wire this supply to 120V rack wiring.
• Progress, Jan. 22, 2026 Rewire supply primary.
• Progress, Jan. 27, 2026 Rewire supply secondary and fuse.
• Progress, July 10, 2026 Start new wiring to backplane.
• Solved!, July 21, 2026

Bug 80: Teletype motor does not spin

During a demo, the Teletype motor did not start when we turned it on.

New capacitor

• Created, Jan. 22, 2026 The Teletype stopped working, initial diagnosis.
• Progress, Jan. 27, 2026 Confirm diagnosis of bad capacitor, order new one.
• Solved!, Feb. 5, 2026 Install and test new capacitor. It barely fits.

Bug 81: Teletype drops bit 2

When we typed ABCDEFG the Teletype printed CBCFGFG.

• Created, Feb. 26, 2026 The Teletype stopped printing correctly, initial diagnosis.
• Solved? Mar. 3, 2026 Blocking lever was hanging up in alignment comb.

Bug 82: Modify Type 750 for Tally 424 reader

Original Wiring