hardware, restoration

Bendix G15 Germanium Diodes

In restoring the Bendix G-15 vacuum tube computer, I have uncovered a phenomena which is requiring us to replace 576 germanium diodes. These diodes appear to have lost their hermetic seal and the atmospheric contamination has caused their leakage current to rise to very high levels as they reach a normal operating ambient temperature of approx. 40 degrees C. Because these diodes are used in the clamp circuits that generate the 20 volt logic swing of the computer, the combined low impedance of the approx. 500 diodes ends up shorting out the -20 volt power supply after 5 to 10 minutes of power-on time.
We have replacement diodes on order, and this should resolve the power supply issue.

Interestingly though, the failed diodes exhibit another interesting phenomena which this engineer hasn’t seen before.  Hooking up a diode to an ohmmeter to measure its leakage current, and heating the diode to about 40 degrees C, causes the diode leakage, measured as resistance, to go from a few thousand ohms to a few tens of ohms.  If the ohmmeter remains connected and the diode is allowed to cool to normal ambient, the low resistance measurement persists.  If the ohmmeter is disconnected briefly and then reconnected, the diode leakage current returns to its nominal few thousand ohms.


DEC PDP10 Model 1095 Repair

A few months ago, our PDP10 Model 1095 ( pictured ) had just successfully booted the WAITS operating system and was running an early version of Ethernet.  One afternoon, the PDP11-40 front-end computer ( unit with chassis extended on left ) stopped working and I was tasked to find out what had happened and repair it.  What followed was almost three months of difficult troubleshooting and repair.

What had happened was, one of the peripheral devices ( a TC-11 DECTAPE Controller at the left end of the machine )  attached to the PDP11’s Unibus had had a power supply failure, causing the regulated 15 volt supply to rise to 28 volts.  These supplies have an over-voltage crowbar circuit which is designed to shutdown the supply by blowing a fuse if the power supply ever goes into an over-voltage condition.  This crowbar circuit failed and this resulted in a number of circuit boards in the PDP11 frying.

Once I replaced and/or repaired the failed circuit boards, I upgraded the TC-11 power supply to a modern switcher which doesn’t have the failure mode described above.

With the hardware sorted out ( this is a couple of weeks into troubleshooting ),  I set about trying to boot the WAITS operating system once again.  A further snag cropped up at this point.  WAITS wouldn’t fully start and would complain about a “pointer mismatch”.  This points to the DTE-20 10-11 interface, but no combination of replacement boards would succeed in bringing up WAITS ( except for a number of random times ).  The solution to this problem turned out to be an old bugaboo of the KL-10 processor.  A number of the control devices do not fully initialize at power-on as their reset lines do not go to all of the parts in a particular device.  We have seen this phenomena on the RH-20, but apparently the DTE-20 also has some hardware that doesn’t get initialized.  I determined this by running the -10 side diagnostic for the DTE-20, and then booting WAITS successfully.  This was after weeks of eliminating all other possibilities as myself and others were not aware that the DTE-20 had components that came up in an unknown state at power-on.

One side note: In upgrading the TC-11 power supply, it was found that the power controller that feed line voltage to it, had failed some time ago and been hacked to make it work without it’s contactor.  A new contactor was ordered and installed.