A  Guide to Tektronix Dual Beam Oscilloscopes

Of all the hundreds of models of oscilloscopes produced by Tektronix up to 1980, my favorites are the dual beam scopes. Five models were produced: the 502, the 551, the 555, the 565, and the 556. Each model was different, yet they all had one thing in common. They all duplicated some or all of a single-beam oscilloscope, right up to the point of writing the traces on the phosphor screen. Each one was different, and it is this difference that can be confusing today. I will review each one in order to help buyers who might find the different specs confusing.

A word about dual trace oscilloscopes. At first, these might seem just as useful as a dual beam at a fraction of the price and complexity. Remember that a dual trace oscilloscope has an electronic switch in the vertical amplifier that displays two traces on the screen. The electronic switch can operate in two modes; chopped and alternate. In chopped mode, the channels are switched at about 1 mHz. This works ok for lower-frequency observations, but at higher frequencies, the chopping waveform itself becomes visible, and actually obscures the desired trace. The alternate setting causes the vertical channel to be switched at the end of each sweep, thus they "alternate" on the screen. This can be used at higher speeds, but works poorly or not at all at low sweep speeds. All in all, the dual trace display is a settings nightmare along with artifacts, lost data points and blurring of the display due to switching. It has its place, but cannot do the job of a dual beam in many projects, such as the combination of an X-Y plot on the upper beam and a time plot on the lower channel, as shown above on the 556. Triggering is also a problem, as the waveform is really two waveforms superimposed on a third, switching waveform.

The 502 and 502A

The 502 was apparently designed for biomedical research and like that. It is a dual beam vertically with a single timebase and single set of horizontal deflection plates. The 502 has no variable control for vertical v/cm nor sweep time, and a maximum sensitivity of 200uv/cm. The 502A added variable vertical gain and sweep time controls, a single sweep feature, and increase the sensitivity to 100uv/cm. A switch allows using one of the vertical amplifiers for the horizontal, allowing high-sensitivity X-Y displays with full bandwidth and attenuator controls. The 502 is a very nice oscilloscope, but limited in bandwidth to 1 mHz. Still, it is very useful to those who require dual beams and high sensitivity. The single timebase means that the sweep is always locked to the signal frequency and cannot display two signals unrelated in time.

The 551 Plugin Oscilloscope

With the 551, we get into the big bad boys of Tektronix! They are the most versatile of the  very versatile plugin series of oscilloscopes. For example, one could use two seperate spectrum analyzer plugins to cover two different frequency ranges on the same display. The 551 has a single timebase like the 502, with a single set of horizontal deflection plates. It has two distributed vertical amplifiers, and bays for two letter- or 1-series plugins. Depending on the plugins used, it has a bandwidth up to 27 mHz, and a maximum sensitivity using the 1A7 or 1A1-cascaded plugins into the low microvolt range. The high post-deflection acceleration (PDA) voltage of 10 kv makes for a very bright and crisp trace.

This oscilloscope has a seperate power supply, connected to the indicator portion by a special cable. Power supplies appear occasionaly on eBay, but cables are very hard to obtain. I am making up these cables and selling them on my eBay store, but the connectors are even getting very scarce. Sometimes one sees one of these oscillosocpes on eBay without a power supply. Be warned that the oscilloscope is useless without the CORRECT power supply. The serial number on the power supply actually matches the serial number of the oscilloscope, and even though the power supply is for the correct model, it may not work with a different serial number. It is almost impossible to get power supplies anyway. So if you want a 551 or 555 then make sure the power supply and cable come with it.

The 555 Plugin Oscilloscope

The 555 is a true example of the "bigger is better" design philosophy. The 555 has two seperate sets of horizontal timebases, arranged in plugin modules for easy servicing. Each sweep drives its own horizontal channel, and can be triggered by either vertical plugin. The 21 and 22 model timebases use conventional trigger circuits, but the 21A and 22A plugins use very high speed tunnel diode triggering, good to well over 50 mHz. Calibrated sweep delay allows precision measurements and display of portions of pulse trains on screen.

Vertically, the 555 has a bandwidth up to 33 mHz, depending on plugins. Vertical sensitivity with cascaded 1A1's is 500 uV/Cm. The 10 kv PDA voltage helps produce a very bright trace, good for high speed pulse work.The 555 is a big scope, with a seperate power supply and that unobtainium cable as well. The notes on these items in the section on the 551 above also apply to the 555. Part of the reason having the correct power supply is so critical is that the power supply really isn't just in the power supply cabinet. It is really split over BOTH the power supply and the indicator. So in order for the two to work together, they must match.

The 556 Plugin Oscilloscope

The 555 allows either timebase to drive either display, but display modes cannot be combined as in the 556. The 556 in fact offers over 50 triggering and display modes It is all contained in one box, no seperate power supply, but it is a very big box indeed! Vertically, the bandwidth is 50 mHz, depending on plugin. With a 6 x 10 cm display per beam, the display area is the largest of any Tektronix dual-beam scope, Accepting all 1- series and letter-series plugins gives maximum versatility for multi-trace displays on two seperate time scales. The integrated nature of this unit eliminates the problem associate with the seperate power supply of the other models.

The 565 Plugin Oscilloscope

The 565 uses two 2- or 3-series plugins from the 560 series catalog. It has two sweeps and features delay and intensified sweep modes. Bandwidth is limited to a maximum of 15 mHz, depending on plugin chosen. It has a large display area, 10 x 10 cm. However, the low PDA voltage of 4kv limits the brightness somewhat.

Parts

Some parts for these oscilloscopes are hard to ontain. CRT's are getting particularily scarce. Many of these oscilloscopes were used for photography, and had P2 or P11 phosphors, ones that burn very easily. Combined with the lesser numbers produced, a crt can be difficult to find.

FEATURE SUMMARY

502

Narrow bandwidth, high sensitivity, inexpensive, compact, one sweep

551

Two plugin vertical channels, wide bandwidth or high sensitivity, one sweep with no delay, seperate power supply

555

Two plugin vertical channels, wide bandwidth or high sensitivity, two sweeps with delay, seperate power supply

556

Two plugin vertical channels, wide bandwidth or high sensitivity, two sweeps , integrated power supply

565

Two 2- or 3- series plugins, two sweeps, delay, bandwidth limited to 15 mHz