Warning! Vacuum tube porn ahead.

The Vegaty ST4 signal tracer is a debug tool for HF and LF circuits. The device includes a LF oscillator to inject a signal into a circuit under test. The signal path is traced with the demodulator probe. The instruments contains a low-noise high-gain input amplifier and a double-stage audio amplifier for auditive feedback. In addition, the rectified audio signal drives an indicator tube (magic eye-ish) to give a visual indication of the received signal level. It also contains a neon bulb for testing resistors and capacitors.

The manual (scanned copy) contains the schematic but no theory of operation. However, it is very easy to understand.

Vegaty ST4 Signal Tracer

Heavy duty metal case.

Vegaty ST4 Signal Tracer

Indicator tube EM80 shows the level of the demodulated signal.

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Typical old-school construction technique: metal base supporting the large components on top and point-to-point wiring on the bottom.

The large, black transformer is the power supply transformer. The smaller, yellow one, is the audio output transformer.

Vegaty ST4 Signal Tracer

EZ80, double-diode used as a full-wave rectifier in the power supply.

Vegaty ST4 Signal Tracer

ECL82, triode and power beam tetrode, used as preamp and output stage for the audio amplifier, respectively. The tetrode anode load is the output transformer, transforming the 4 R speaker impedance to a suitable load resistance. No followers whatsoever.

Vegaty ST4 Signal Tracer

EF86, pentode. Input stage of the signal tracer. Provides a first stage of amplification, used in this location because it was designed for low noise and, being a pentode, has much higher gain than a triode (higher plate resistance).

Vegaty ST4 Signal Tracer

ECC82, double triode. Active device in the oscillator. Connected as a long-tailed pair in a positive feedback loop. 

Vegaty ST4 Signal Tracer

Double capac... condenser: 2 x 50 uF. Part of the power supply T-filter. Probably high-voltage paper type.

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Attenuator switch.

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

OA85 diode, listed as 'tube' in the manual, which it is not, obviously. It is a germanium point contact diode, used as a half-wave rectifier to create the dc to drive the indicator tube.

 

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Probe to inject the generated square wave into to circuit under test.

Vegaty ST4 Signal Tracer

Vegaty ST4 Signal Tracer

Demodulator probe. Contains another ac-coupled OA85 diode as detector, next to the upper resistor leg of the resistive voltage divider that makes up the attenuator. Useful to debug high-frequency AM and FM circuits (radios...) as this includes rudimentary demodulation.

Vegaty ST4 Signal Tracer