Virtual Museum of Vintage Electronic Counters

This is a unique collection of early test instruments of the type known as electronic counters (or frequency counters). These 1950s instruments implemented digital circuits using vacuum tubes, and since this requires many tubes (and also many related components, and an adequate power supply), the instruments were large, heavy, and expensive (the equivalent of $5.000 to $40,000, perhaps, in today's US dollars, depending on model). Moreover, while other test instruments had also simpler versions, which were perfectly useful and reliable at less than one tenth the cost, there was little that could be simplified in counters to reduce their cost.

chassis top

As a result, few counters were built compared with other instruments (fewer than a hundred for some models, and just a few thousand for the popular models). Also because of their large size, most of them were discarded once retired. They were displaced by transistorized counters starting in early 1960s, having dominated this field, thus, for only about ten years. (It must be noted that some tube counters were already using solid-state components: crystal diodes in the counting circuits to permit higher-frequency signals, or silicon diodes in the power supply to reduce the size and heat.)

This museum is an attempt to preserve and to explain visually that brief period in the history of digital electronics. At the same time, it serves as an illustration of complex 1950s electronic hardware in general.

There are 37 models, representing several manufacturers, and depicted with hundreds of high-resolution pictures. Each counter is shown from many different angles, with pictures of various sizes, so you can stop at overall views or examine small details, both above and below the chassis. The bottom views, in particular, reveal the complexity of these hand-built instruments — probably the most intricate single-cabinet devices created with vacuum-tube technology.

Basic principles of the early counters

The name "counter" derives from the fact that these instruments count the number of transitions (level changes) in the input signal. The transitions are converted to pulses, which trigger the counting circuits.

An internal time-base oscillator generates its own pulses, usually at 100 KHz. This frequency is repeatedly divided by 10, down to 1 Hz, and often to 0.1 Hz. The frequencies from these stages are selected by the operator to control an electronic "gate." The gate is opened and closed by two consecutive pulses, thus allowing the input pulses to pass through and be counted for a specific length of time (10 seconds, or 1, 0.1, 0.01 second, etc.).

function selection knob

Since the accuracy of all measurements is determined ultimately by the precision of the original time-base frequency, counters employ a crystal-controlled oscillator, and the crystal is usually kept at a constant temperature in an "oven." For special measurements, the time-base can be driven by an external oscillator, or the gate can be opened and closed manually.

If the input signal is a regular waveform, its transitions indicate cycles, and the instrument will measure the signal's frequency (usually as cycles/second — what we call today Hertz, or Hz). But the signal can represent many other phenomena, so the transitions are generally known as events. (Thus, Beckman called these measurements Events Per Unit Time, or EPUT, or E/UT.)

Many counters can measure time intervals. This function is implemented by reversing the input and the time-base signals: consecutive input pulses are used to open and close the gate, and the time-base pulses are the ones counted. Thus, if the input signal is a regular waveform, the gate is open for the duration of one cycle, so the number of pulses is the signal's period. For example, if the selected time-base frequency is 1 KHz, its pulses are spaced 1 msec, so the number of pulses displayed is the period in milliseconds. Some counters can measure arbitrary time intervals, by using the pulses of dedicated start and stop signals to open and close the gate.

A related function is to measure the ratio of two frequencies. The pulses of the lower-frequency signal are used to open and close the gate, and the pulses from the other signal are treated as input and are counted. Thus, the counter will display the number of pulses of the latter occurring between two pulses of the former, or the ratio of the two frequencies.

The check function lets the operator test the counter without any input signals. The time-base pulses open and close the gate, and the time-base oscillator pulses are treated as input and are counted. If the latter are at 100 KHz and the time-base dividers are properly adjusted, the counter should display 100, 1,000, 10,000, etc., when the time-base frequency selected is 1 KHz, 100 Hz, 10 Hz, etc. Most circuits are checked by this function, but the counter cannot verify the accuracy of its own oscillator (so it will display those values even if the oscillator's frequency is wrong, because a deviation will affect equally the frequency counted and the time the gate is open, thus self-cancelling).

Some counters have decimal points — small lamps situated between the display digits and lit according to the position of the selection knobs. They help the operator interpret the number of pulses in familiar units (Hz or KHz or MHz for frequency, sec or msec or µsec for time).

The counter can be instructed to repeat a measurement. The operator can do it by pressing the reset button, or by limiting the display time: after displaying the result for a brief period defined by the operator, the counter resets itself, performs the measurement, and returns to the display step, repeating this cycle indefinitely. For regular waveforms, the values repeatedly displayed should be identical, or very close.

Decade counters

The counting operation in most counters is performed by plug-in modules known as decade counters, or decade (or decimal) counting units, or DCU's. The term "decade" is due to the fact that these modules divide the signal's frequency by 10: every tenth input pulse generates an output pulse.

Decade counters are connected in series, the output of the first module to the input of the second one (which is on its left in the instrument's front panel), and so on, for as many digits as the counter requires. Watching the displayed count, each digit starts at 0 and increases by 1 every time the one on its right changes from 9 back to 0. At any moment, therefore, the value displayed is the number of input pulses since the counter was last reset. In most applications, though, the first few digits change too fast to follow, and it is only the final measurement that matters.

decade counter

Most decade counters have 4 tubes, each one consisting of 2 triodes in a circuit that can be in one of two states, like 0 and 1. These states are flipped by the input pulses. The tube generates an output pulse when its state returns to 0, at every second input pulse. Each tube drives the next one, dividing the pulse frequency by 2 and at the same time doubling the number of states possible for the module as a whole. In digital terms, each tube is one bit, and the 4 tubes are one hexadecimal digit with 16 states, 0000 to 1111. The input pulses cause the tubes to display these states in binary sequence, and then return to 0000. But 6 states (in the middle of the sequence) are skipped electronically, so the module is reduced to the 10 states needed for a decimal digit.

In some counters the decade counter for the first (and sometimes also the second) digit is more complex than the others, because it must support the highest frequency (the following ones receive lower and lower frequencies). In 10 MHz counters, the circuit for the first (and sometimes also the second) digit is so complex that it requires a separate section, and only a small display module is placed in the front panel. Conversely, in some simple counters the frequency is reduced even by the first decade counters to 10 - 15 Hz, so the following digits can be implemented with a small mechanical register.

decade counters for first 4 digits

In addition to counting, the decade counter displays the decimal digit corresponding to the current state, 0 - 9. A decoding circuit translates the 4 states into 10 output voltages, which light 10 neon lamps arranged in a column behind a mask with digit-shaped cutouts. The decoding requires logic operations, as usual, but instead of logic gates, it uses resistors and the tubes and neon lamps themselves to implement these operations.

Starting in 1959, the display tubes known as Nixie were used as an alternative display method. These gas-filled tubes have 10 electrodes made of thin wire, shaped as digits and stacked behind one another. High voltage applied to an electrode produces a glow around it, and hence a nice image of that digit alone when the tubes are behind a dark-red filter. The counting and decoding are the same as in the columnar decade counters, but the light of each neon lamp controls a photoconductor switch which supplies the high voltage to the corresponding digit in the Nixie tube.

columnar display Nixie display

List of counters

Museum instructions

• The counters are listed by model number within manufacturer.

• In the list of counters, hover with the mouse in the top left corner of a thumbnail picture to display a larger image. Click on a picture or on the model number to go to the counter's views window. The list of views is displayed there as thumbnails in the left pane, while the selected pictures are displayed in the right pane. Under each thumbnail view there are 3 buttons, S M L (Small, Medium, Large), which let you select the picture size. With Medium and Large you can see smaller details. (You may be able to further enlarge details with the browser's zoom feature Ctrl +.) The button you select stays black until you click another one, to help you find the current view if you scroll the views list up or down.

• If the browser's window is large enough, Small fits in its entirety. Medium and Large are 2.5x and 6.4x larger than Small, and they must be scrolled in order to bring the portion of interest in the window. Scroll the image directly with the mouse, or using the scrollbars, similarly to scrolling a map. The browser will generate scrollbars for Small too if the window is not large enough to display it in full. But then you will have two sets of scrollbars for Medium and Large, which can be confusing.

• Click Back to list at top or bottom to return to the list of counters.

• Click Gallery for a quick scan of all views, forward or backward (in Small size).

• Click Add tab to open a second views window, where you can select other views. This lets you compare views by clicking on the tabs or by flipping them with the browser's feature Ctrl Tab. More than two tabs can be opened, by clicking Add tab in any one of the open tabs. To compare views from two or more different counters, click Add tab and then Back to list, where you can select another counter.

Note: The pictures are licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives International License 4.0. They are copyrighted material; you may download and share them, but you may not use them commercially or distribute modified versions.

Hewlett-Packard Company (HP)

HP 521A Electronic Counter HP 521A Electronic Counter

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HP 521A Electronic Counter

• Counts events • Measures frequency • Input signal to 120 KHz • Optional crystal time-base plug-in module • 33 tubes • 4‑digit display with columnar decade counters • Width/height/depth 25/35/34 cm, weight 13 Kg • Introduced in 1955 (this unit, serial #270, is a very early example) • Price US$475

HP 521C Electronic Counter HP 521C Electronic Counter

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HP 521C Electronic Counter

• Counts events • Measures frequency • Input signal to 120 KHz • Includes crystal time-base plug-in module • 44 tubes • 5‑digit display with columnar decade counters • Width/height/depth 25/35/34 cm, weight 13 Kg • Introduced in 1957 • Price US$650

HP 521CR Electronic Counter HP 521CR Electronic Counter

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HP 521CR Electronic Counter

• Rack-mount version of model 521C; see that model for details • Width/height/depth 48/22/32 cm, weight 12 Kg • Price US$655

HP 521D Electronic Counter HP 521D Electronic Counter

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HP 521D Electronic Counter

• Counts events • Measures frequency • Input signal to 120 KHz • Optional crystal time-base plug-in module • 34 tubes • 4‑digit display with Nixie decade counters • Width/height/depth 25/35/36 cm, weight 13 Kg • Introduced in 1959 • Price US$675

HP 521DR Electronic Counter HP 521DR Electronic Counter

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HP 521DR Electronic Counter

• Rack-mount version of model 521D; see that model for details • Width/height/depth 48/22/36 cm, weight 12 Kg • Price US$680

HP 521E Electronic Counter HP 521E Electronic Counter

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HP 521E Electronic Counter

• Counts events • Measures frequency • Input signal to 120 KHz • Includes crystal time-base plug-in module • 45 tubes • 5‑digit display with Nixie decade counters • Width/height/depth 25/35/36 cm, weight 13 Kg • Introduced in 1959 • Price US$875

HP 521G Electronic Counter HP 521G Electronic Counter

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HP 521G Electronic Counter

• Counts events • Measures frequency • Input signal to 1.2 MHz • Shown with crystal time-base plug-in module (optional in this model) • 42 tubes • 5‑digit display with columnar decade counters • Width/height/depth 25/35/34 cm, weight 13 Kg • Introduced in 1957 • Price (with crystal module) US$800

HP 522B Electronic Counter HP 522B Electronic Counter

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HP 522B Electronic Counter

• Same model number for cabinet and rack-mount versions (just different case and air flow) • Counts events • Measures frequency, period, time interval • Input signal to 120 KHz • 49 tubes • 5‑digit display with columnar decade counters; 4 decimal points • Width/height/depth 50/31/35 cm, weight 23 Kg (rack-mount version 48/27/36 cm, 20 Kg) • Introduced in 1953 • Price US$915

HP 522B Electronic Counter (early version) HP 522B Electronic Counter (early version)

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HP 522B Electronic Counter (early version)

• Early version of model 522B; see that model for details • Differences: front panel appearance, chassis appearance, decade counters (Beckman), plug-in modules, some components and circuit details

HP 523C Electronic Counter HP 523C Electronic Counter

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HP 523C Electronic Counter

• Counts events • Measures frequency, period, time interval • Input signal to 1.2 MHz • 64 tubes • 6‑digit display with Nixie decade counters; 5 decimal points • This unit includes the Digital Recorder option (which drives a printer) • Width/height/depth 46/26/47 cm, weight 24 Kg • Introduced in 1959 • Price US$1,575 (without options)

HP 523CR Electronic Counter HP 523CR Electronic Counter

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HP 523CR Electronic Counter

• Rack-mount version of model 523C; see that model for details • Differences: front panel width and appearance, case • Width/height/depth 48/22/47 cm, weight 22 Kg • Price US$1,550

HP 523D Electronic Counter HP 523D Electronic Counter

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HP 523D Electronic Counter

• Counts events • Measures frequency, period, time interval • Input signal to 1.2 MHz • 63 tubes • 6‑digit display with columnar decade counters; 5 decimal points • Width/height/depth 46/26/47 cm, weight 24 Kg • Introduced in 1959 • Price US$1,350

HP 523DR Electronic Counter HP 523DR Electronic Counter

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HP 523DR Electronic Counter

• Rack-mount version of model 523D; see that model for details • Differences: front panel width and appearance, case • Width/height/depth 48/22/47 cm, weight 22 Kg • Price US$1,325

HP 524B Electronic Counter HP 524B Electronic Counter

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HP 524B Electronic Counter

• Precursor of models 524C and 524D • Same model number for cabinet and rack-mount versions (just different case) • Counts events • Measures frequency, period • Input signal to 10.1 MHz • Optional external plug-in modules for higher frequencies, time interval, and other measure­ments; shown with 526B Time Interval Unit • 84 tubes (excluding the plug-in module) • 8‑digit display with 6 columnar decade counters and separate counting circuits for first 2 digits using display meters; 5 decimal points • Width/height/depth 50/53/59 cm, weight 53 Kg (rack-mount version 48/49/56 cm, 49 Kg) • Introduced in 1954 • Price (without plug-in modules) US$2,150

HP 524C Electronic Counter HP 524C Electronic Counter

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HP 524C Electronic Counter

• Same model number for cabinet and rack-mount versions (just different case) • Counts events • Measures frequency, period • Input signal to 10.1 MHz • Optional external plug-in modules for higher frequencies, time interval, and other measure­ments; shown with 525B Frequency Converter Unit • 90 tubes (excluding the plug-in module) • 8‑digit display with 7 Nixie decade counters and separate counting circuit for first digit; 5 decimal points • Width/height/depth 50/53/59 cm, weight 53 Kg (rack-mount version 48/49/56 cm, 49 Kg) • Introduced in 1959 • Price (without plug-in modules) US$2,300

HP 524D Electronic Counter HP 524D Electronic Counter

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HP 524D Electronic Counter

• Same model number for cabinet and rack-mount versions (just different case) • Counts events • Measures frequency, period • Input signal to 10.1 MHz • Optional external plug-in modules for higher frequencies, time interval, and other measure­ments; shown with 526B Time Interval Unit • 89 tubes (excluding the plug-in module) • 8‑digit display with 7 columnar decade counters and separate counting circuit for first digit; 5 decimal points • Width/height/depth 50/53/59 cm, weight 53 Kg (rack-mount version 48/49/56 cm, 49 Kg) • Introduced in 1959 • Price (without plug-in modules) US$2,150

HP FR-38A/U Frequency Meter HP FR-38A/U Frequency Meter

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HP FR-38A/U Frequency Meter

• Alternative model # AN/USM‑26 • Military version of model 524B rack-mount; see that model for details • Shown with MX‑1637/U Frequency Converter Unit • Main differences from 524B: front panel appearance, shallow case (large fan at rear replaced with small DC fan at bottom), some components and circuit details • Width/height/depth 48/49/46 cm, weight 52 Kg

Beckman Instruments - Berkeley Division

Beckman 554F-1 EPUT Meter Beckman 554F-1 EPUT Meter

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Beckman 554F-1 EPUT Meter

• Measures frequency • Input signal to 100 KHz • 45 tubes • 5‑digit display with columnar decade counters • Width/height/depth 48/22/37 cm, weight 25 Kg • This model (554F‑1 or military FR‑67/U) is a version of model 554, which was introduced ca 1952

Beckman 5001 Electronic Counter Beckman 5001 Electronic Counter

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Beckman 5001 Electronic Counter

• Counts events • Input signal to 125 pulses/second, resolution to 25 µsec • 8 tubes • 7‑digit display with one columnar decade counter and 6‑digit mechanical register • Designed for industrial use: power and signal cables connected to internal terminals through side holes • Width/height/depth 16/27/20 cm, weight 7 Kg • Introduced ca 1954

Beckman 5230 Universal EPUT and Timer Beckman 5230 Universal EPUT and Timer

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Beckman 5230 Universal EPUT and Timer

• Counts events • Measures frequency, period, time interval, frequencies ratio • Input signal to 100 KHz • 31 tubes • 4‑digit display with columnar decade counters • Width/height/depth 20/30/37 cm, weight 12 Kg • Introduced ca 1956 • Price US$575

Beckman 5500 Universal Counter and Timer Beckman 5500 Universal Counter and Timer

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Beckman 5500 Universal Counter and Timer

• Counts events • Measures frequency, period, time interval • Input signal to 100 KHz • 50 tubes • 5‑digit display with columnar decade counters; 6 decimal points • Width/height/depth 53/31/40 cm, weight 38 Kg • Introduced ca 1954

Beckman 5571 Frequency Meter Beckman 5571 Frequency Meter

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Beckman 5571 Frequency Meter

• Counter and heterodyne frequency converter in one cabinet • Counts events • Measures frequency, period, time interval, frequencies ratio • Input signal to 2.1 MHz (to 42 MHz using the converter) • 64 tubes (excluding the converter) • 7‑digit display with columnar decade counters • Width/height/depth 53/49/40 cm, weight 48 Kg • Introduced ca 1955 • Price US$1,745

Beckman 5691 EPUT Meter Beckman 5691 EPUT Meter

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Beckman 5691 EPUT Meter

• Measures frequency • Input signal to 100 KHz • Uses subminiature tubes and dense design to attain a very small size • 36 tubes • 5‑digit display with small columnar decade counters • Width/height/depth 27/20/22 cm, weight 9 Kg • Introduced ca 1956

Beckman 7350A Universal EPUT and Timer Beckman 7350A Universal EPUT and Timer

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Beckman 7350A Universal EPUT and Timer

• Counts events • Measures frequency, period, time interval, frequencies ratio • Input signal to 500 KHz • 46 tubes • 6‑digit display with columnar decade counters • Width/height/depth 51/26/40 cm, weight 22 Kg • Version of model 7350 (which was only 100 KHz, introduced ca 1956, price US$890)

Beckman 7360A Universal EPUT and Timer Beckman 7360A Universal EPUT and Timer

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Beckman 7360A Universal EPUT and Timer

• Counts events • Measures frequency, period, time interval, frequencies ratio • Input signal to 2 MHz • 57 tubes • 7‑digit display with columnar decade counters • Width/height/depth 51/26/40 cm, weight 23 Kg • Version of model 7360 (which was only 1 MHz, introduced ca 1956, price US$1,175)

Beckman 7360R Universal EPUT and Timer Beckman 7360R Universal EPUT and Timer

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Beckman 7360R Universal EPUT and Timer

• Rack-mount version of model 7360 • Counts events • Measures frequency, period, time interval, frequencies ratio • Input signal to 1 MHz • 52 tubes • 7‑digit display with columnar decade counters • Width/height/depth 48/22/39 cm, weight 19 Kg • Introduced ca 1956 • Price US$1,175

Beckman 8361LT Preset Universal EPUT and Timer Beckman 8361LT Preset Universal EPUT and Timer

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Beckman 8361LT Preset Universal EPUT and Timer

• Counts events • Measures frequency, period, time interval, frequencies ratio • Option to divide or multiply the measured entity by a preset value • Input signal to 1 MHz • 66 tubes • 6‑digit electroluminescent display panel with 7‑segment figures, driven by separate decade counters using optical conversion logic • 4‑digit preset count, selected with thumbwheels and implemented with 4 special decade counters • Width/height/depth 48/22/39 cm, weight 21 Kg • Introduced ca 1960

Computer Measurements Company (CMC)

CMC 200B Frequency Counter CMC 200B Frequency Counter

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CMC 200B Frequency Counter

• Counts events • Measures frequency • Input signal to 220 KHz • 36 tubes • 4‑digit display with columnar decade counters • Width/height/depth 41/18/32 cm, weight 14 Kg • Introduced ca 1957 • Price US$425

CMC 201C Frequency Period Counter CMC 201C Frequency Period Counter

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CMC 201C Frequency Period Counter

• Counts events • Measures frequency, period • Input signal to 220 KHz • 41 tubes • 5‑digit display with columnar decade counters • Width/height/depth 43/18/32 cm, weight 14 Kg • Introduced ca 1958 • Price US$640

CMC 225B Universal Counter-Timer CMC 225B Universal Counter-Timer

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CMC 225B Universal Counter-Timer

• Counts events • Measures frequency, period, time interval, frequencies ratio • Input signal to 220 KHz • 56 tubes • 5‑digit display with columnar decade counters; 4 decimal points • Width/height/depth 43/23/36 cm, weight 19 Kg • Introduced ca 1958 • Price US$840

CMC 225C Universal Counter-Timer CMC 225C Universal Counter-Timer

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CMC 225C Universal Counter-Timer

• Counts events • Measures frequency, period, time interval, frequencies ratio • Input signal to 220 KHz • 54 tubes • 5‑digit display with columnar decade counters; 3 decimal points • Width/height/depth 48/22/43 cm, weight 21 Kg • Introduced ca 1959 • Price US$900

CMC 303A Totalizing Counter CMC 303A Totalizing Counter

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CMC 303A Totalizing Counter

• Counts events • Input signal to 15,000 pulses/second, resolution to 5 µsec • 19 tubes • 8‑digit display with 3 columnar decade counters and 5‑digit mechanical register • Width/height/depth 29/19/24 cm, weight 7 Kg • Introduced ca 1956 • Price US$325

Sentinel Electronics

Sentinel FR-114A/U Frequency Meter Sentinel FR-114A/U Frequency Meter

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Sentinel FR-114A/U Frequency Meter

• Alternative model # AN/TSM‑16 • Made for the U.S. military • Counts events • Measures frequency, period, time interval • Input signal to 1 MHz • 39 tubes • 6‑digit display with Nixie tubes, driven by separate decade counters using beam switching tubes • Width/height/depth 48/22/44 cm, weight 25 Kg • Introduced in 1959

General Radio Company

General Radio 1130-A Digital Time and Frequency Meter General Radio 1130-A Digital Time and Frequency Meter

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General Radio 1130-A Digital Time and Frequency Meter

• One model for both bench and rack use • Counts events • Measures frequency, period, time interval • Input signal to 10 MHz • Optional external plug-in modules for various time-base oscillators (at rear, shown with 1130‑P2 oscillator/multiplier) • 100 tubes • 8‑digit display with 7 columnar decade counters and separate counting circuit for first digit; 3+3 decimal points • The display can be divided into two parts, with the left half used to show a stored value • Circuits implemented as plug-in boards, densely filling the internal space • Width/height/depth 49/42/49 cm, weight 39 Kg • Introduced ca 1961 • Price US$2,750 (with time-base module 1130‑P2)

Hamner Electronics Co.

Hamner N-277 Decade Scaler Hamner N-277 Decade Scaler

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Hamner N-277 Decade Scaler

• Counts events up to a preset number, or up to a preset time length if used with the N‑803 timer (see that model for details) • Input signal to 1 MHz • 43 tubes • 7‑digit display with columnar decade counters • Designed with a very low profile for minimal vertical space in a rack • Width/height/depth 48/9/35 cm, weight 11 Kg • Introduced ca 1965

Hamner N-803 Electronic Timer Hamner N-803 Electronic Timer

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Hamner N-803 Electronic Timer

• Used with the N‑277 scaler (see that model for details) to preset a time length, or to measure the time elapsed, for the events counted • 40 tubes • 6‑digit display with columnar decade counters • Designed with a very low profile for minimal vertical space in a rack • Width/height/depth 48/9/35 cm, weight 11 Kg • Introduced ca 1965

Potter Instrument Company

Potter 432 Interval Timer Potter 432 Interval Timer

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Potter 432 Interval Timer

• Counts events • Measures time interval • 26 tubes • 3‑digit columnar display with separate decade counters; 3 decimal points • Most circuits implemented as plug-in modules • Width/height/depth 30/27/37 cm, weight 16 Kg • Introduced ca 1953

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