A Guide to Buying Tektronix Oscilloscope Plugins

Or ... Plugins can be fun!

The purpose of this guide is to help in the purchase through understanding of the different series of plugins, and what they do. The decision to use the concept of a general-purpose mainframe with a variety of plugins to provide what amounted to a number of different instruments was a difficult one for Howard Vollum, founder of Tektronix, Inc. Many of the managers at the company pushed for the development of a group of single-purpose instruments. Vollum however won out, and today we see a variety of plugins coming up for sale on eBay.  Many of the plugins are simply preamplifiers, trading off bandwidth for sensitivity, or single channels for multi-channels. But others are complete instruments unto themselves, using the oscilloscope as an indicator to display the results of more elaborate testing.

Letter Series

The main series of plugins were the letter- and 1- series for the 530 / 540 / 550 and with adapter the 580 series of oscilloscopes. The second series were the 2- and 3- series for the 560 family. There were other instruments that used plugins, like the 661 sampling oscilloscope and the 580-series, but the variety of units was very limited..The letter series started out with numbers like the 53/54C, and the 53B. Later these became the Type CA and the Type B respectively.  Most of the Letter- series are available on eBay from time-to-time, in a variety of conditions. But it is safe to say that they all show up pretty well used. Fortunately, most are easy to repair, being all vacuum tube, and if in fair mechanical shape can be made to do the job at a price.

The downfall of the letter series is an inability to utilize full bandwidth of the later oscilloscope models, such as the 547, 555 and 556. The CA is limited to about 24 mHz, which does not do justice to a 547. But they are easy to repair and set up. The CA has 15 good tubes in it, and so a lot of them were scrapped for the tubes. Still, buying one at 10 dollars is a pretty good deal. One needs to remember that these plugins are 40 or 50 years old. They have seen a lot of use and then a lot of storage. And so condition can vary from poor to really poor. A nice one is hard to find. Restoration is difficult because there is usually a lot of mechanical damage, especially to the front panel. There are usually a multitude of stickers and other markings that deface the unit. A cherry one is a treasure indeed.

There still is one area where the older Letter-series plugins shines, and that is working in environments with large transient surges. The later plugins can be damaged by surge voltages which might be found around heavy motors, industrial equipment, high voltages and machine tools. The old vacuum tube scopes and plugins are pretty much imune to this type of danger, which would likely wipe later, more sophisticated scopes off the map.

1- Series

The 1- series, such as the 1A1, can be very common, because they were produced in very large numbers. The 1A1 is the definitive dual-channel plugin for these oscilloscopes, followed by the 1A4 and the 1A7. The type M and the 1A4 both produced a 4-channel input, but the 1A4 commands a much higher price because of its better design and scarcity. Generally the 1- series is the better buy, all things considered. The 1- series adopted a letter after the "1" which describes the class the plugin belongs to, an "A" being vertical preamplifier, "L" a spectrum analyzer, and an"S" a sampling plugin.Sampling timebases had a "T" designation. The "1-" series is very cohesive, following a logical progression from 1A1 to 1A7. Fuinctionality is not duplicated, although the 1A2 is sort of a low-cost version of the 1A1, using vacuum tubes and having a lower sensitivity. Early 1A1's had nuvistor front ends, later FET's were used. Some have a cascade output, so one channel can feed into the other for higher gain. I am told that the carbon resistors in these tend to drift and make calibration flaky in older units. The 1- sereis have a mixture of technology. The 1A1 for example has transistors, nuvistors, and maybe FET's (after serial number 20000). This can be a headache when it comes to fixing them, and even board swapping is tricky because each unit requires the correct boards to work together.

2- and 3- Series

The 560 series oscilloscopes were different than the other 500-series in that the entire vertical and horizontal channels were in the plugin. The plugin is not a preamplifier, but drive the deflection plates directly. Tektronix made both vertical and horizontal plugins, giving rise to a confusing variety for sale today.  The number (2 or 3) doesn't matter, but the letter following it does. The "A", "L' and "S" are the same as the 1- series. Because the horizontal channel also was a plugin, a new letter was added, a "B" for a timebase. Another new letter was a "C" for the carrier amplifier plugin.

There is a large and confusing number of "2-" and "3-" series plugins. A Tektronix catalog is a big help with plugins, around 1968 is good. Many of these are difficult to fix, being largely made of unobtainable components. Try to get a wotking one, as it is likely to stay that way. Avoid units that show lots of corrosion and mechanical damage. Manuals for all of them are availble free on BAMA.

Special Plugins

The most common of the "special" plugins were the highly sophisticated spectrum analyser and sampling plugins. These plugins were specialized instruments, using the oscilloscopes power supplies and crt indicator to provide a completely different instrument. These units today command high prices. Unfortunately, the units for sale are often damaged, and are virtually unrepairable. I cannot recommend the purchase of these plugins unless the buyer is learned enough to repair them. Special parts are not available, and have to be made up or removed from units that are working. Still, collecting sampling and spectrum analyzer gear can be a fun and challenging aspect of the whole Tektronix hobby.

It is not necessary to operate the plugin in an oscillosocpe. If you have a Type 134 plugin amplifier, you can operate any plugin as a stand-alone unit. For example, you could use it to drive the horizontal amplifier, making a regular oscilloscope into full X-Y. This is also useful for testing a plugin if you only have one oscilloscope.