[Dekatron]

Thank you for the detailed explanation, way above my level of analyzing capabilities.

A description of an almost identical amplifier and oscillator circuit can be found in the AVO TT/CT/VT-537 instruction manuals, I've included the desription here both as OCR-text below but also as three pictures so you can see the component references in the text.

Now, there are of course some differences, but only small ones - the 10 Ohm measurement resistor and 1.99µF tank capacitor are missing as they aren't needed by the transistor tester, nor is there any output transformer on the oscillator as it isn't needed for the same reason. The input transformer is also connected differently, compared to the VCM163, to the emitter of the first PNP transistor - it is connected to the base via the small 1990pF capacitor in the VCM163 (I just checked that on the PCB) however I don't own a 537 so I can't check if that diagram is consistent with the actual design.

One interesting point is that AVO designates the input transformer as a current transformer in the 537 - should it be seen as one in the VCM163 too? It is also missing the trimmer so It isn't adjustable to the frequency like in the VCM163.

One other interesting point is the description of the amplitude regulation of the oscillator with the thermistors, the same design is used in the VCM163 which drive the output transformer.

----- OCR from AVO TT/CT/VT-537 Instruction manual -----

OSCILLATOR CIRCUIT

This is basically a Wien Bridge oscillator, the frequency of oscillation being determined mainly by the series elements R64 and Cl8, together with Cl9 in shunt with the effective parallel combination of R65, R66 and the input impedance of VT8. The predominant resistive factors in the parallel element are R65 and R66.

Transistors VT8 and VT9 form a low gain amplifier with negligible phase shift at the oscillator frequency of lkc/s. Transistor VT10 is an emitter follower to handle the current requirements in the output circuit. Feedback is taken from the junction of resistor R76 and thermistor TH2, the values of these components being chosen such that constancy of output is maintained for any changes other than ambient temperature changes. The operating current through thermistor TH2 is set during manufacture, by adjustment of potentiometer RV8 connected in the emitter circuit of VT8, to give a volt drop across R76 of 400 millivolts r.m.s.

The effect on the amplitude of the oscillator output due to ambient temperature changes is compensated for by the parallel connected components, thermistor TH1 and resistor R77. To ensure the correct degree of compensation the calibration network is designed such that the load on the oscillator is maintained approximately constant at 3.3k ohms.

AMPLIFIER CIRCUIT

Transistor VT11, together with current transformer T3, provide a low impedance input to the amplifier. Thus the impedance presented to the collector of the transistor under test is reduced to a minimum, the ac impedance being approximately one eighth ohm and the dc resistance approximately 60m ohms.

The signal current from the collector of the transistor under test is reduced by the current transformer T3 in the ratio 20:1 and fed into the emitter of VT11 via the coupling capacitor C24. VT11 is a high gain transistor which ensures that almost the entire signal current flows through to the collector load and this is virtually unaffected by temperature.

A voltage is developed at the base of VT12 which is determined by the magnitude of the signal current in the collector of VT11 and the effective ac impedance of VT11 collector circuit. (The effective impedance of the parallel combination R78, R84, R85 is approximately equal to 5k ohms.)

A signal voltage at low impedance is fed from the emitter follower VT12 to the input of the high stability negative feedback amplifier comprising VT13, VT14 and their associated components. The output circuit is arranged such that the ac signal current flowing in VT14 collector is rectified by MR 17 and MR 18. Its mean value is then displayed on an external ammeter which may be connected to terminations 41 and 42.

The feedback resistor R95 in shunt with R91 defines the sensitivity of the amplifier and as this is fairly high, suitable precautions have been taken to eliminate pick-up of spurious signals. Magnetic coupling between the input transformer and any stray magnetic fields is overcome by screening the input transformers with a mu-metal box. The effect of ripple in the power supply feeding the amplifier is reduced to a minimum by the de-coupling arrangements, R82 and C26, R83 and C27 and R87 and C28.

----- OCR from AVO TT/CT/VT-537 Instruction manual -----