Rohde & Schwarz SMAF (BN 41402)

Signal generator for AM and FM, type SMAF, BN 41402, manufactured by Rohde & Schwarz, Munich.

Technical data

Principle]]: Signal generator,
Operation modes: AM, FM
Frequency range: SW, VHF (10-17 / 16-28 / 26-47 / 45-80 / 78-135 / 132-216 MHz)
Frequency display: Analogue dial
Frequency memory: none
Features:
Output power: 0.05 μV - 50 mV

Power supply

Mains operation: 110, 125, 150, 220 V

Dimensions

450 x 320 x 360 mm, weight 28 kg

Accessories

Operation

First, the correct mains voltage must be applied; the sets used in German-speaking countries are usually set to 220 V alternating current. The set can be set to an alternative mains voltage using the voltage selector on the mains transformer, which can be accessed by removing the perforated cover on the back.

The main switch (9) is used to select the operation mode, in most cases this will be AM or FM. The SMAF can also provide a video signal, but the picture blanking synchronisation signal must be fed to the set via the television modulation socket.

Frequency modulation is possible with a 1000 Hz tone („FM internal 1000 Hz“ setting) or externally supplied external modulation („FM external“), amplitude modulation on my BN 41402 set is strangely only possible with external modulation („AM external“). Various combinations of AM and FM signals are also possible.

In the FM mode („FM internal 1000 Hz“), the frequency deviation can be set with the control (12) and read on the left-hand instrument (13), the „Display times“ switch switches between a display range of 0 - 10 kHz (left-hand position 0.1) and 0 - 100 kHz (right-hand position 1), a frequency deviation of 75 kHz is usually used in FM broadcasting.

The operating frequency is selected with the range switch (3), the BN 41402 covers the ranges 10-17 / 16-28 / 26-47 / 45-80 / 78-135 / 132-216 MHz, i.e. the higher shortwave bands, FM broadcast band and the VHF range. The large centre frequency dial on the frequency dial is used for coarse adjustment of the operating frequency, while a fine tuning knob on the right with a log scale and adjustable zero point is used for fine frequency adjustment.

The output level can be varied; the HF control (4) is used to set the level so that the instrument pointer is at 1. The correct output voltage levels are then read at the voltage dividers A and B. The regulator (6) can be used to regulate the output voltage between 0.5 and 5, and the level can be switched using the step switch of voltage divider B (7). Depending on the range, the output voltage in the 0.1 range can then be selected between 0.05 and 0.5 μV, in the 1 range between 0.5 and 5 uV and so on further up to the 10^4 range where the output level is between 5 and 50 mV.

The output signal is feed to a typical Rohde & Schwarz Dezifix connector with an impedance of 60 Ω. These special connectors are no longer easily available.

Inspection

After a long period of storage, my set is carefully powered up with the regulating transformer, with the main switch (9) in the „AM external“ position, the set needs a few minutes to warm up, for good frequency stability Rohde & Schwarz recommends a warm-up time of 30 minutes.

Fortunately, I have a Dezifix to BNC adapter to make the connection to the Dezifix B RF output socket, the signal generator output is connected to the oscilloscope.

To test the generator with an output power of 10 mV, the output voltage regulator A (6) is set to 1.0, the output voltage switch B remains in the 10^4 position fully clockwise, with this setting the highest level is selected.

The main tuning is set to 100 MHz with the 78 - 135 MHz range active.

Use the HF control (4) to increase the oscillator voltage until the pointer is at the 1 mark in the red field. Now set the X deflection on the oscilloscope to approx. 5 mV/cm, the trigger responds and a stable sine wave appears on the screen when the time deflection is set to approx. 10 nsec/cm (and my oscilloscope goes up to 100 MHz).

The output voltage can now be varied using the output voltage regulators A and B and the change in level can be observed on the oscilloscope. If the frequency is increased, the sine wave becomes narrower, with lower frequencies it becomes wider. Thanks to the measuring function, the duration of a sine wave is measured at 10.1 nsec. when tuned to 100 kHz and at 100 nsec. on 10 MHz in the 10 - 17 MHz range. Apart from slight contact problems at the output level switch and the level control 4, the 75-year-old SMAF is pleasantly stable and surprisingly precise.

To access the inside of the device, the eight outer screws (four at the top, four at the bottom) are removed, then the front with the chassis can be pulled out of the cabinet. This is still very heavy, now it can be seen that the power supply unit is installed in the rear part of the cabinet and the voltages are fed to the chassis via a multipole connector. Unfortunately, it is not possible to take live measurements on the chassis during servicing without extending the multipole connector.

Technical principle

RF oscillator, the frequency modulation is generated via a crystal diode, the FM modulation level is displayed with a two range AF tube voltmeter. The internal modulation of 1000 Hz is processed in an AF oscillator.

A diode voltmeter displays the RF voltage with the adjustable control (49).

Two tubes are used as broadband amplifier for the RF signal, the capacitive voltage divider A and the capacitive and resistive voltage divider B allow the setting of the RF output level with high precision.

The power supply is electronically stabilised.

Components

Rö 1 EC81, oscillator
Tube 2 EB41, broadband amplifier
Tube 3 EF80, broadband amplifier
Tube 4 EAF42, cathode amplifier FM modulation
Tube 5 EF42, display amplifier I2, modulation level
Tube 6 EF42, AF oscillator 1000 Hz
Tube 7 EF80?, amplifier oscillator voltage instrument
Tube 8 EL41?, current control tube
Tube 9 EL42?, control tube of tube 8
Tube 10 STV280/80, stabiliser tube for grid bias voltage Tube 9
Tube 11 / 12, two AZ41, rectifier

The emission of the final tube can be checked by pressing a black button, the pointer of the right-hand instrument must be in the red sector. According to the manual, it must be checked whether it reaches the red mark after a tube change, as the accuracy may not be perfect after a tube change and the set may need to be recalibrated.