In restoring the Bendix G-15 vacuum tube computer, I have uncovered a phenomena which is requiring us to replace over 3000 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. 3000 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.
