Marconi TF-2700 Universal Bridge

©Brooke Clarke, N6GCE


This is a battery powered one hand holdable universal LCR bridge.  It operates at 1 kHz.
There is a provision for External AC drive as well as external DC bias, which is why I got this instrument.  So that I can make some measurements on the PNI Magneto-Inductive (MI) sensors.


The only way to power the bridge is by means of a 9 Volt battery, not the normal type but a larger Eveready 276.

DC Bias

The ground side of the 1/4" mono phone jack for "Bias" is connected directly to the "Hi" DUT terminal.
The center of the 1/4" jack is connected to the Det + ve" terminal through a 1,000 Ohm resistor.  The limit on bias current translates into a voltage limit at the jack.

DC bias is only active when in the L, C or R function positions and in the D mode and no change with position.


With no bias applied the inductance looks like 102 mH and there is no change in inductance with magnetic bearing.
With 15 Volts (15 mA) DC bias the inductance reduces to 100 mH and changes about 1 mH with position.
Using the R int DC function switch setting results in a DE resistance reading of 120 Ohms.
Using the R function switch position ( 1 kHz gives 116 Ohms.

With 1.0 Volt DC bias the inductance is about 102.2 mH.  D = 5.2 (X1 D range) This is closer to where the sensor should be biased.
With the coil pointing up the inductance is about 104 mH.

The normal frequency of operation for this coil is closer to 59 k Hz, so the 1 kHz TF-2700 frequency may be too low for good sensitivity
Using the Ext AC input may allow much more sensitivity.  How to use it?


10 uH
1000 uF
100 uH
100 uF
1 mH
10 uF
10 mH
1 uF
100 mH
0.1 uF
10 k
1 H
10,000 pF
100 k
10 H
1000 pF
1 M
100 H
100 pF
10 M


Discrete components and point to point wiring plus a single sided PCB.
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