1401

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Tektronix 1401A Spectrum Analyzer Module

The Tektronix 1401 is a spectrum analyzer front-end covering the frequency range of 1 MHz to 500 MHz, designed to be used with an external oscilloscope, typically a 323, 324 or 326. It has internal rechargable NiCd batteries and can also operate from AC power (using an unusual power cord) or external DC from 6 V to 16 V.

The 1401 is designed to be used with an oscilloscope with 10 horizontal divisions and 6 vertical divisions. Tektronix says it is compatible with any oscilloscope having 0.5 V/div horizontal deflection factor and 1.2 V full screen vertical deflection. Note that most full sized Tektronix oscilloscopes have 8 vertical divisions so use care when doing the initial setup and reading the scale in dBm. Also note the lack of a Z-axis output.

There appear to be two variants of the 1401A, both the manual and catalog specify the resolution bandwidth selector as 1000, 100 and 3 kHz, but the later versions are 1000, 100, and 10 kHz. There is also a 1401A-1 which has a 75 Ω input impedance. The predecessor 1401 did not have the BNC "cal output" or "cal on" pushbutton switch.

Internals

Operation is typical of a swept front end analyzer. The input is attenuated and low-pass filtered to 500 MHz. It is then mixed with the first LO, a varactor tuned oscillator that can sweep 695 MHz to 1195 MHz. The first mixer is a double balanced, four diode star. The signal is DC coupled to this mixer. The manual is not clear as to the damage level; with no attenuation they say that it is non linear above -30 dBm. The first IF output is 695 MHz, filtered to remove the other image. The first IF is mixed with a 720 MHz fixed 2nd LO to produce the 2nd IF at 25 MHz.

The 25 MHz 2nd IF leaves the shielded RF housing and enters the IF board via SMB connector P210. This 2nd IF is then amplified and mixed with a 30 MHz fixed 3rd LO to produce the 3rd IF of 5 MHz. This 3rd IF is then passed through the selectable RBW filters, detected and send to the video out.

Repair issues

See TekScope January 1972 pages 14-15 for some servicing notes that Tek wrote after the manual was published. Service Note

There is one tunnel diode found in the external trigger circuit. It is a 152-0402-00, a 2 mA, 25 pF tunnel diode in a DO-17 package. It is only used when the external trigger input is connected.

There is one Tektronix custom IC, 155-0042-02, listed as a Miller Integrator. It is used to generate the sweep ramp voltage. The package is a 10 pin metal can.

The input attenuator is a fairly custom part due to the IF gain pot that is concentric with the RF atten control. Dissassembly of the attenuator is easy after removing it from the 1401A. Note that the IF gain pot is removed via two allen setscrews. To bench test the attenuator with DC, use a 50 Ω terminator on SMA input and connect the output SMA to an ohm meter. Each of the seven steps should read around 50 Ω. The contacts on the ceramic attenuator elements can wear, they can be repaired with silver solder. Before reassembly, remove the two SMA housings to avoid damaging the contacts and remove the setscrews that hold spring tension on the two ball bearings that align the shaft steps. Reinstall these parts after the housing has been screwed together.

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