Signal Generator LSG-17

Manufactured by Leader, Leader Electronics Co. or Ohmatsu Denki Co., Tokyo.

The Leader LSG-17 signal generator is a simple mains-powered portable RF generator that is extremely versatile for testing receivers.

• Principle: signal generator
• Operation modes: CW, AM (1000 Hz)
• Frequency range: LW - FM broadcast band (100 kHz - 150 MHz)
• Frequency display: Analogue dial
• Features: Quartz operation
• Output power: 100 mV

• Mains operation: 110, 115 - 120, 220 - 240 V

• 150 x 238 x 80 mm, weight 2.5 kg

The set is operated from the AC mains, the correct mains voltage is set inside the cabinet. The LSG-17 is switched on using the POWER switch in the bottom left-hand corner of the front panel. The RF voltage is fed to two banana sockets in the bottom right-hand corner of the set (the „hot“ socket is the one marked in red), the output power can be set to LOW (10 % of the output) and HIGH power using the switch above it and can be adjusted using the rotary control.

The signal can be modulated: EXT MOD for external modulation, if no RF signal from the 50 Hz to 20 kHz range is connected to the two sockets on the right, the signal is unmodulated. In the INT MOD position, the signal is amplitude-modulated with a 1000 Hz tone; the 1000 Hz signal can also be tapped at the sockets for testing AF stages of a receiver. In the XTAL OSC position, a crystal in FT-243 format in the 1 - 15 MHz range can be connected. When the XTAL OSC position is in use, the quartz oscillater signal is fed to the output with the output switch set to LOW, the range switch in this mode should be set to 150 MHz and the FINE control to minimum. A 10.7 MHz crystal can be used to check the IF levels of an FM broadcast band receiver, while a 1 MHz crystal can also be used to generate the signals on the crystal harmonics at 1 MHz intervals

The range is selected with the FREQ RANGE switch, the letter designations and limits of the band ranges are labelled on the switch and on the dial. The large rotary knob is used to set the desired operating frequency; the dial is quite coarse, especially in the higher frequencies, and should reach an accuracy of ± 1.5 %.

The band ranges extend from long wave to the VHF range (A: 100 - 300 kHz; B: 300 - 1000 kHz; C: 1 - 3.5 MHz; D: 3 - 11 MHz; E 10 - 35 MHz and F: 32 - 150 MHz), the last range can be used up to 450 MHz thanks to the emission of double and triple harmonics. The typical IFs of 455 kHz and 10.7 MHz are marked on the dial with small black triangles.

The signal can be connected in series to the receiver input with a series resistor of 50 - 100 ohms or coupled to a rod antenna with a few turns of wire. When analysing RF and IF amplifier stages, coupling via a 1 - 5 pF capacitor is recommended. Measuring points with high DC voltages may only be connected via a blocking capacitor.

The use for testing a receiver or a receiver circuit is extremely practical: it is usually sufficient to wrap the cable from the red output socket loosely around the rod antenna base or an unshielded cable to the antenna socket, the coupling of the signal is usually completely sufficient.

Now, with INT MOD switched on, the output switch in the LOW position and the output level turned up halfway, the receiver dial can be tuned through quickly. If the receiver is working, the 1000 Hz tone is immediately audible. Within a few minutes, all bands can be quickly tested for function and contact faults, much faster than this would be possible with a calibrated signal generator. The frequency dial of the LSG-17 is rudimentary, but if necessary you can quickly check a set frequency using a digital frequency counter.

In the RF oscillator, IC102 is used as a Colpitts oscillator, while IC101 is used as an oscillator and modulator for amplitude modulation with a 1 kHz signal. The DC voltage of 20 V from the transformer, the presence of which is indicated by the power LED, is stabilised to supply the two ICs.

The set is solid state.

To gain access to the chassis, three screws are removed: at the front centre on the top and bottom of the cabinet and at the rear on the edge between the power input and the cabinet. The chassis can then be pulled out of the cabinet. I recommend cleaning the contacts on the changeover contacts of the band switch, the HIGH and LOW output stage switches and the FINE potentiometer.

The manual with the circuit diagram of the Leader LSG-17 can easily be found on the Internet.

• Leader LSG-17 on the website of www.radiomuseum.org