Measure coils with vintage circuit

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I like old measuring devices. From them one can get refreshing on the fundamentals of electronics. You don't get that from playing around with those cheap component testers from eBay.

This particular one is Nombrex 66 inductance meter.
It uses both a Hays bridge and Maxwell bridge which can be chosen by selector switch.

It also gives Q of coil.

I assume collpits oscillator to generate AC source for the bridge, but what exact type of transformers are being used to couple the stages. I assume impedance matching and isolation of the bridge?
Computing the values of C1 and C2 in the collpits, a primary side coil of around 2H, yes 2 henries put it in the ballpark of what the specs of the unit are, it's supposed to operate at 1.5khz.
What do you think? Which transformer gives primary of henries?
Maybe those small toko audio transformers?

I would like to replicate the circuit for curiosity.
The germanium transistors are no problem for me.

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Maybe I could change the oscillator to something more modern using op amps and a 1 to 1 isolation transformer?

[Vadim18]

Maybe I could change the oscillator to something more modern using op amps and a 1 to 1 isolation transformer?

Last year I have found two 4 and 8 Henries decoupling transformers from office telephone equipment, they are really small - approx 1x1 cm. Twin-T oscillator as variant.
Nice device.

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Here is another old project from Nuova elettronica magazine. Another bridge circuit, this one uses those 455khz oscillator coils found in old transistor radios. But those are few hundred microhenries and have internal capacitor. Interesting circuit.

[Op04]

Another one, but not really vintage

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I am going to replicate the Nombrex 66 circuit with few changes.

I will be leaving out the Tan Delta option so no need for Q/tan Delta selector switch and dual gang potentiometer.
Since I am only concerned with testing air core coils, tan Delta bridge configuration is redundant.
Tan Delta is used when testing larger iron cored inductors as would be the case with transformers and the like.

First thing, I redraw schematic for clarity, as the original schematic is a bit outdated.

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Upon closer examination of the Nombrex 66 specs, I see the bridge is sourced by a 1592 Hertz AC signal (1.592 kHz)

It is stated that this is chosen specifically because : 2πf=10,000
This was apparently done in order to make calculations simple and easy to calibrate the dial.
For example, a 1.0H inductor will have an xL of 10,000 ohms (2πfL)
Likewise, a capacitor of 0.01uF will have an xC of 10,000 ohms.

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As I stated previously, I don't need the tan Delta test, so I can eliminate the hay bridge configuration altogether and just go with the Maxwell bridge configuration.

For testing coils ranging 100uH to 1mH, this instrument should be very accurate.

My intention is to make it as a small handheld device powered by 9V battery.
Perhaps it will become a useful little gizmo.

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Of course, I prefer to use silicon transistors, so I go to ltSpice for modifications. The values of some resistors have to be altered.
Fortunately I have correct germanium transistors, so in the end I will be able to replicate the original circuit precisely as it is depicted in schematic.