Tektronix Type 106 Square Wave Generator Refresh
Introduced in 1966, the Tektronix 106 Square Wave Generator is a signal generator which can produce fast rise (<1nS @50 ohm loads positive or negative going) or high amplitude (120 Volts, high impedance load) square waves from 10Hz to 1MHz. It is used for oscilloscope calibration, risetime testing and for triggering extremely fast rise (<100pS) tunnel diode pulsers. The 106 uses a combination of vacuum tube and discrete solid-state technology. This is the sort of equipment that was used for development and testing during the NASA Apollo Program through the 1960’s.
The cost of the Tektronix Type 106 was USD 590 in 1966 or about USD 4900 in 2021 dollars. I was fortunate enough to obtain one of these units in decent condition and I was able to restore it to better than factory specification. What follows documents some of that process.
The Tektronix 106 originally used GaAs (Gallium Arsenide) diodes in the fast edge output circuit to generate the 1nS (see images below) output square wave. This was in 1966. Now, 55 years later GaAs based semiconductors are enjoying a renaissance because they have better properties than silicon for many applications.
Fortunately, the 7189A output amplifier tubes in my unit were all in VG to EX condition (see curve trace above). Otherwise, they are somewhat expensive to replace (about USD 40/each). There are cheaper alternative solutions which require wiring of the tube sockets. I prefer to keep the equipment original and functional.
For this restoration, I implemented a different protocol regarding aluminum electrolytic capacitors. If a given capacitor tested like its modern replacement, it remained in service. I imagine that someone showed me (sight unseen) test data for a given capacitor (ESR, D, Q, Leakage Current at working voltage) and asked if this capacitor could be used in circuit? I then reformed the capacitor using a voltage/current regulated power supply up to 5-10% over the rated working voltage. Of the large aluminum electrolytic capacitors in the unit (see picture above), two failed this testing and were bypassed; the others were left in circuit after successful reforming.
Given my experience in Materials Engineering, the reforming process makes sense to me. The electrolytic capacitors that I have left as functional (not bypassed/replaced) are as good or better (to best of my testing*) than our modern replacements. I will continue to monitor how this turns out over the next while. I started this process with an ME-26D/U where I could find nothing wrong with the existing electrolytic capacitors in the unit, and so left them as found (after reforming). And this unit has functioned properly for many years.
*I use an HP 4192A, B&K 879B, HP 4263A, Keithley 236 SMU for testing and a regulated high voltage/current limited power supply for reforming.
I consider this Tektronix Type 106 Square Wave Generator to be ready for service and a place on the test bench.
If NASA needs a fast rise square wave generator, I will consider lending #892 to them…..
Thanks for reading.