561

The 561 is a member of the 560 series which use plug-ins for both the vertical and horizontal deflection. The mainframes contain no amplifier stages, requiring the plug-ins to directly drive the CRT deflection plates. While promoted by Tek as having the advantage of “not limited by additional circuitry between the plug-in and the deflection plates”, this configuration has a significant disadvantage. Touching up the gain or sweep cal when swapping plug-ins in scopes with amplifiers in the mainframe can be skipped, as doing so only tunes the accuracy to a small amount of additional precision. But in the 560 series, this step is required to even get crude accuracy, as the raw deflection factor of the CRT has a relatively large range, and the mainframe has no circuitry to normalize it.

To maintain good phase match over a wide range of frequency, the mainframe does not contain a delay line in the vertical path. The faster vertical plug-ins contain the delay line, which limits the available volume for amplifier circuitry.

Over its life, the 561 was actually a series of four very different scopes, plus the rack mount versions of each. The original 561 and RM561 were introduced in 1961. Similar to the 560, also introduced in 1961, the 561 has a round CRT. It supports the 50, 60 and 70 series of plug-ins, with the highest system bandwidth of 4 MHz.

The short lived 561 was replaced by the 561A and the rackmount version RM561A in 1962. The 561A were the highest selling model in the 560 series. They introduced a revolutionary new CRT incorporating many “firsts” that would remain through most of Tek's analog scopes to follow – a rectangular faceplate allowing a more compact CRT, a ceramic envelop which provided more mechanical precision than a blown glass tube. This also enabled the use of a separate face plate made from a flat glass panel that could have the graticule markings printed on the phosphor side of the faceplate. This totally eliminated parallax errors, and allows for better edge illumination. The 561A supports all 2 and 3 series plug-ins. The popular 60 and 70 series plug-ins were renamed by adding a “2A, 2B, 3A, or 3B” in front to the two digit model number. New 3 series plug-ins were also added which consume more power than the original 60 and 70 series. Due to the higher power consumption, these newer plug-ins can not be used in the original 561. The highest bandwidth plug-ins gave a system bandwidth of 10 MHz.

The 561B / RM561B were introduced in 1969. The B models actually provided no performance improvements over the 561A, with the change being limited to all solid state design, including the HV rectifiers. While this improves reliability of the mainframe, no all solid state plug-ins were ever designed. The 561B uses the same 2 and 3 series plug-ins used by the 561A.

A special 561S was introduced as a special product, modified from a 561A. The product did not appear in a Tek catalog. With special modified amplifier and time base (3A1S and 3B1S), it achieved a higher 25 MHz system bandwidth. These were the only plug-ins which could be used in the 561S, as the design uses a special CRT with higher deflection sensitivity to minimize the plug-in voltage swings. The higher BW of the 561S came at the expense of trading off some of the advances gained in the original 561A. The CRT is a conventional glass envelop, without internal graticule. Also, the scale was reduced from 8 x 10 divisions to 6 x 10.

Tek 561S
Tek 3A1S and 3B1S

561

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