LEADER LSG-11 Signal Generator


In 2015 I acquired an LSG-11 (circa 1960's) to add to my suite of test-gear.
An uncomplicated valve based circuit design.
When received, although partly working, some servicing was needed.
The 400/1000Hz audio oscillator didn't work.
C2 & C3 were found 'leaky' and replaced.
Replace the power supply filter electrolytics.
Upgrade the selenium rectifier to a (1N4007 1000v/1A) silicon diode.
Upgrade to a 3 core mains lead to improve electrical safety.
Metal case is now earthed.
V1 the 6AR5 is an oddball valve to me, and I have no spare.
Its used as a triode connected Pentode anyway ?
By connecting valve socket pin 2 to pin 7, (cathode to suppressor grid)
allows a 6AU6 to work as a plug-in substitute!

Frequency Range 120Kc to 390Mc
Band A 120Kc to 320Kc
          B 320Kc to 1000Kc
          C 1.0Mc to 3.2Mc
          D 3.2Mc to 11Mc
          E 11Mc to 38Mc
          F 38Mc to 139Mc
Calibrated Harmonics 120Mc to 390Mc
RF Output over 100,000uV 120Kc to 38Mc
RF Controls High-Low switch, and Fine
Modulation Frequencies 400 and 1,000 cps
Crystal Oscillator 1-15Mc (FT243 type)
AF Output 3 to 4 volts
AF Input 4 volts, approx
Tube Complement 12BH7 RF Oscillator-Buffer
6AR5 AF osc or Amp and Xtal Osc
Rectifier half wave selenium
Accessory 75 Ω coaxial cable
Power Source 100, 117 or 230 volts as specified,
AC 50/60 cps; 13VA approx
Size, H x W x D 190 x 275 x 115mm
7 ½ x 10 ¾ x 4 ½ in
Weight, net 2.7 Kg (6 lb)

Schematic redraft:

From manufacturers handbook:
The diagram shows the RF, mixer and combination AF oscillator or amplifier and crystal oscillator.

RF oscillator
One section of a 12BH7 (V2) duo-triode is used in a Colpitts oscillator
to generate frequencies from 120 Kc to 130Mc on fundermentals.
The frequency of operation is set by the Freq Range switch and the tuning dial.
The calibration is accurate up to ± 2% up to 38 Mc and ± 3% to 130Mc and above.
Where higher accuracies are required, it is recommended that a heterodyne frequency meter be used,
for calibration after the generator has been "warmed up" for 2 to 3 hours.
The auxiliary scale (outer arc) can be used for such calibration.
For convenience, the 455Kc, 4.5Mc and 10.7Mc points are designated on the scales.

The other triode section is used as a mixer and modulator,
RF buffer and cathode follower to isolate the load from the RF oscillator.
The RF oscillator output is impressed directly on the grid through a small coupling capacitor.
The AF modulating voltage is applied to the grid through an RC network.
The fine and high-low output controls are in the cathode circuit.
The output cable is connected to the 2-way switch and connector.
When the crystal oscillator is in use, the mixer becomes a buffer with the output controls also effective.

AF Oscillator or Amplifier and Xtal Oscillator
A 6AR5 (V1) pentode is used for three functions.
Two audio frequencies, 400 and 1000 cps can be generated.
The Outputs can be used either to modulate the RF carrier, or externally for checking AF amplifiers.
About 4 volts from an external source is required for modulation,
and this input will be amplified to a sufficient level for the purpose.
As a crystal oscillator; xtals in the 1 to 15Mc range can be used.

The equivalent of £16/15/- in 1964 is $ 469 in 2015

Link for Manufacturers Manual