This post will documents my recent repair of a Tektronix 284 Pulse Generator. This device was introduced by Tektronix in 1967. It can produce:
- A 50kHz, 200mv fast risetime pulse. Typically, tr < 70 ps
- A 100 kHz, 1 MHz, or 10 MHz square wave at 10, 100, or 1000 mV
- A 100 MHz or 1 GHz sine wave at 100 mV
The Tektronix 284 can be used to check the risetime of oscilloscopes, (both sampling and real time) amplifiers, transistors, transient responses, and for TDR work. The pulses are generated by a tunnel diode which is a semiconductor device capable of changing states very quickly. Tunnel diodes were invented in the late 1950’s in Japan and widely used in test equipment through the 1970’s. Tektronix used tunnel diodes in “pulsers” like the 284 and many oscilloscopes including the 547, 556 and 7000 series. You can read more about tunnel diodes here, and the Tektronix 284 here.
I had already done some cabinet restoration work on the 284:
As well, I cleaned the chassis and circuit boards, switches, and connectors:
The sine/square wave generators were working and within specification apart from the 10 MHz oscillator. The 10 MHz frequency was too high (100 ns period too short). As this oscillator is crystal controlled, there is no specified adjustment for it. In the end, I used a low value silver mica capacitor in parallel with the crystal to pull the oscillator back onto frequency.
The fast rise pulse output was not working, but the trigger pulses were clearly visible on the oscilloscope. The tunnel diode at the front of the unit is connected to the bias/impedance matching network at the rear of the unit by a rigid airline. The contacts between all these parts are compression loaded (spring) metal to metal contacts which had become contaminated over the years. Of particular note is the 54-ohm resistor R184A. This is a special hollow ceramic cylindrical carbon composition resistor into which R184B and R184C fit and the end of which contacts the airline. This resistor is very easily damaged (see images below).
Cleaning all the contacts and carefully reassembling the airline was the first step in restoring the pulse output. Next, the two carbon composition resistors housed inside R184A (R184B and R184C) which had increased in value by 20% (800 ohms total vs 660 ohms) were replaced. This brought the total resistance of R184B+R184C down to 660 ohms. This in turn, pulled the TD bias current back into specification and set up the TD to pulse:
With all the cleaning and component replacement completed, I did a calibration of the 284 as given in Section 6 of the service manual. Rather than describe the entire calibration process here, (to be done in a future post) I will summarize the results graphically, starting with the square and sine wave outputs:
And the fast rise pulse output:
Finally, here is the fast risetime pulse shown on an HP sampling oscilloscope (83480A) with 20GHz bandwidth. The pulse risetime is approximately 47 ps:
One important remark regarding calibration and performance checks on the Tektronix 284. The procedures given in the service manual were written many years ago (late 1960’s to mid 1970’s). Therefore, they reference the test equipment available at the time. This includes nine pieces of test equipment and a considerable number of cables and fittings. With a modern 2GHz oscilloscope (LeCroy 960), a GPS controlled frequency standard (HP Z3801A), and a 20GHz sampling scope (HP 83480A), little else is needed, greatly simplifying the procedures. The important thing is to re-interpret the calibration instructions with the use of more modern test equipment in mind.