PDP-9 Restoration Blog 2023

Go to the earlier restoration blog.


The system booted and ran the "Hello World" program OK.

We replaced the 2N2646 Unijunction transistor Q4, the D-664 diode D14, the 2N4443 SCR D15, and the MDA 942-5 diode bridge on the G850 SCR driver flipchip that goes in slot A12 of the TU55 DECtape drive that is in the Processor cabinet.

The right hub motor works now, but it is always on in the strong torque mode. This could be a problem with the logic that controls the G850, or maybe there are still problems on the G850.


Lots of visitors, so not much debug time.

We had trouble reading the DECtape on the lower-right drive until the system had been powered on for an hour, and then it worked OK. The Hello World program loaded and ran OK.

We swapped the G850 Flipchip for the left and right hubs. The original G850 worked OK in both locations, and the repaired board drive the motor with full torque in both locations. Looks like it isn't repaired yet.


We spent the afternoon experimenting with running the FOCAL interpreter under ADSS. The FOCAL.BIN file needs to be loaded with GLOAD and will link to the RELEAE or RELNON library depending on if the machine is equipped with the EAE option or not. Unfortunately our PDP-9 only has 8k of memory so there is not enough memory to fit the LOADER, FOCAL.BIN, symbol table, and required parts of the library at the same time. For an 8k machine you can use CHAIN (most likely on a larger memory machine) to build FOCAL.XCT and FOCAL.XCU files. These files are prelinked with the required library routines and are loaded and run with the EXECUTE command. We have these files, but it looks like they were prelinked to the math routines from the RELEAE library. Since our machine does not have the EAE option the math in FOCAL doesn't work. We can't find any documentation for the CHAIN command in the PDP-9 or PDP-15 ADSS manuals. The DEC-XV-UCHNA-A-D-CHAIN_XVM_EXECUTE_XVM_UTILITY_MANUAL.pdf PDP-15 manual on Bitsavers has documentation on the CHAIN command for the XVM operating. The CHAIN command under XVM has a lot of options that are specific to the more complicated memory management on the PDP-15 so we will have a lot of trial-and-error work to figure out how CHAIN works on a PDP-9.


Lots of visitors, so nothing accomplished.


We are back to looking at the broken G850 FlipChips for the TU55 DECtape drives. We started by looking at the signal on the base1 of the 2N2646 Unijunction transistor of a working G850. The Unijunction transistor is a little strange, it has two bases and an emitter. You switch it on and off with the emitter. This signal is connected to ground through a 220 Ohm resistor and is  used to fire the SCR and control the torque from the hub motor. Complex logic circuitry in the TU55 controls the torque and brakes on both hub motors to allow for smooth starting, direction change, and stopping of tape motion.

We looked at the base1 of the Unijunction transistor so we could see the current going through the transformer T1. On the base1 of the unijunction transistor we can see about 5mV of noise when the REMOTE/LOCAL switch is on the OFF position. In this case there is no motor drive at all and the brakes are off.

Details of the spike when the Unijunction transistor turns on. The positive spike 80us after the Unijunction transistor turns off is probably inductive kickback from the transformer T1. The diode D14 keeps the positive spike to a minimum so that the Unijunction transistor doesn't get destroyed.

In the LOCAL or REMOTE position, and when making low torque, we see 10uS -20mV spike about 6ms after the zero crossing. The SCR will turn on at the time of the spike, and turn off 2ms later at the zero crossing. Since the SCR turns on very late in the AC cycle very little energy is sent to the motor.

When making medium torque we see the same spike, but it occurs earlier, about 5ms after the zero crossing. This will fire the SCR earlier in the AC waveform, The SCR will stay on for 2ms suppling more energy to the motor, and making more torque.

When making high torque we see 9x spikes, the first occurs about 1.2ms after the zero crossing, and occurs every 800us. I think that the SCR will turn on at the first pulse and stay on for 7ms, making maximum torque.

The low and medium torque settings are adjustable with a Trimpot on the G850 Flipchip. The Trimpot adjusts the turn on time of the SCR, and therefore the amount of energy sent to the motor, and the torque that the motor makes. The low and medium torque of both hub motors need to be adjusted to optimize the tape motion.

We looked at one of the G850 FlipChips that does not drive the motor at all. This one has the expected single spike at low and medium torque, but also has another spike 2.5us later. The high torque spikes look OK, but the extra spike is also present. This one may have a bad D14. The second G850 has all of the correct signals on the Unijunction base1, but no motor drive. Both of these Flipchips may have failed SCRs.

We looked at the GATE signal on the SCR relative to the ANODE. On a functional G850 we see spikes that change for low, medium, and high torque. On the broken G850 we see just about 50mv of ripple at 60Hz and no changes when we press buttons on the TU55. Maybe diode D20 that conducts the pulses from the transformer to the SCR is broken?