Tektronix Type 114 Pulse Generator Repair
I recently completed repairing/refreshing and calibrating two Tektronix Type 114 Pulse Generators. This is a solid state pulse generator with variable pulse width (100ns – 10ms) and period (1us to 100mS). It can produce pulses of both polarities (-10v to +10v into 50 ohms). It was introduced in 1965. The two examples I obtained were SN 001791 and SN 003460. Initally, both units were produced output, but the waveforms were distorted (rounded square waves) and far out of specification in terms of period, pulse width, and risetime.
Visual inspection also revealed damaged components and mechanical damage. A summary of the issues found follows:
- Pulse width and period were out of specification
- Negative pulses and square waves were cut off
- Square waves were rounded and stretched
- Rise and fall times were very slow and far out of specification
- Voltage selector switch was broken
- Wafer switch section on the output attenuator assembly was cracked
- Numerous low value carbon composition resistors were out of tolerance
- Numerous carbon composition resistors showed heat damage (discoloration)
- One resistor in the output attenuator assembly was carbonized (burned by overheating)
- Power supply filter capacitors had excessive leakage
- All the switch contacts were badly oxidized
- Power supply Zener diode reference was the wrong value (from a prior repair)
I tackled the usual problems first: Power supply capacitors, visually bad components, broken components.
The problems with the asymmetrical and out of specification output waveforms were resolved by optimizing the output transistors (2N2501 selection and testing) and performing the calibration:
The calibration procedure in the manual (circa 1965) requires a Tektronix 547 oscilloscope with a 1A1 plug-in, a Tektronix 180A time mark generator and a VOM. A sampling scope or plug-in is also recommended for the rise time measurements. However, I used a modern digital 2Ghz oscilloscope (LeCroy WavePro 960) with an external GPSDO timebase reference and an modern DMM (Keysight U1253B). Measuring the power supply voltages with a lower impedance VOM vs a higher impedance DMM made no difference; circuit loading was not an issue in this case. This made the calibration process very simple.
Tektronix Type 114 Calibration Record Serial: 001791 Date: May 17,2021 Voltage on outside shell of C298 (+25-volt supply): 25.06 v Voltage on positive terminal of C298 (+35-volt supply): 35.1 v Voltage across D209: 2.98 v Regulation of +25-volt supply: 2% Ripple on outside shell of C298: 8.6mv Period accuracy, all settings of PERIOD control, 1 μS: 0.2% 10 μS: 1.3% 100 μS: 0.2% 1 mS: 0.3% 10mS: 0.5% PERIOD—VARIABLE control range: 11.5:1. Width accuracy, all settings of WIDTH control, 100 nS: 0.2% 1 μS: 1.7% 10 μS: 0.1% 100 μS: 0.8% 1 mS: 1.4% WIDTH — VARIABLE control range: 12:1. Square wave period accuracy: 0.8%. Output amplitude into a 50 Ω load, all settings of AMPLITUDE switch, +3 to 10 V: 3.19 to 11.36 v +1 to 3 V: 1.03 to 3.47 v -1 to -3 V: -1.03 to -3.47 v -3 to -10 V: -3.19 to -11.34 v External trigger operation, meets specification: Yes Trigger output amplitude, 50 Ω load: 600mv No load: 1.27v Pulse risetime: 7.8 nanoseconds. Pulse falltime: 6.5 nanoseconds. Tektronix Type 114 Calibration Record Serial: 003460 Date: May 17, 2021 Voltage on outside shell of C298 (+25-volt supply): 25.03v Voltage on positive terminal of C298 (+35-volt supply): 34.60v Voltage across D209: 2.940v Regulation of +25-volt supply: 2 %. Ripple on outside shell of C298: 8.5mv Period accuracy, all settings of PERIOD control, 1 μS: 0.1% 10 μS: 0.7% 100 μS: 0.03% 1 mS: 0.3% 10mS: 0.3% PERIOD —VARIABLE control range: :1. Width accuracy, all settings of WIDTH control, 100 nS :0% 1 μS: 0.5% 10 μS: 0.8% 100 μS: 0.3% 1 mS: 0.5% WIDTH — VARIABLE control range: 14:1 Square wave period accuracy: 2%. Output amplitude into a 50 Ω load, all settings of AMPLITUDE switch, +3 to 10 V: 2.92 to 11.26v +1 to 3 V: 0.910 to 3.47v -1 to 3 V: -0.914 to -3.46v -3 to 10 V: -2.95 to -11.26v External trigger operation, meets specification: Yes Trigger output amplitude, 50 Ω load: 649mV No load: 1.34v Pulse risetime: 8.0 nanoseconds. Pulse falltime: 7.7 nanoseconds.
All specifications for these units have been met or exceeded, (note the fast rise times). As a final summary, here is a 1us pulse from the Type 114 displayed on a Tektronix 2467B analog oscilloscope. The pulse period is 1000ns, the pulse width is 100ns.
I hope this posted information will help others in restoring this valuable vintage test equipment. Thanks for reading.