High voltage on Marconi LCR bridge

[neutronic]

Using my Marconi TF868/1 LCR-bridge I noticed (or better said felt) a rather high voltage.
When used for R measurements it gives in the 100 K range 25 Volts and in the 1Meg range 62 Volts.
But with the swich for final R balance pressed down it goes up to 250 and 300 Volts!
I don´t have the manual but does the original user´s manual warns you for this?

BTW. Now I know this bad trait.., I ¨train¨ my Electrolitics with this, before measuring the C values.

Jard N.

[WME_bill]

Marconi TF868/1. The Resistance ranges operate as a Wheatstone bridge, fed through 150k from the tap of the chain of 22K + 6.8K from HT+ (usually 280V)to Earth. So the normal voltage will be about 66V with no load, dropping on lower resistance ranges. This is increased to the full HT+ when the Press for Final Balance button is pressed. But the feed is through the 150k resistor, so the current is less than 2 ma. The manual makes no comment, except to say that the power dissipation is less than 0.7W at worst out of balance, and about 0.2W as balance is approached.
You were measuring with a high impedance voltmeter I assume, so that will give the highest no-load reading. But if you are getting a "tingle", I wonder if the series resistor is failing, as such a current flow is not noticeable unless you are wet or standing on a damp floor.
Please let us know if you find a fault, or whether it appears normal. I have not noticed it with my version of the Instrument.
I attach the circuit diagram should you not have a copy, though it is not the easiest to read with all the switching.
All the early models used this arrangement, from 1946 to 1957. The TF868B introduced in 1958 used 3.8V DC with open circuit terminals, increasing to 21V on the Rx10 setting. And then a chopper amplifier, instead of direct to meter. wme_bill.

[neutronic]

Thank you very much for your circuit diagram.

With an old fashioned analog meter (Philips PM2412A) I measure 300 volts by 12,5 mAmps. on 2 bare connectors at the top of the instrument.

So in fact this bridge is not safe.(at least to modern standards)
I´ll have to study the circuit diagram and compare it with the wiring and the components in the LCR-meter.

Best Regards
Jard N.

[neutronic]

In fact the results are as expected!
With the switch down the full power supply voltage ( 300v.) is over 22 K giving 13,5 mA.

So,when adjusting the an unknown resister into the terminals by hand and touching (accidently) the switch will give some unexpected tingling.

BTW. The max. open voltage is 75 volts DC I believe.

Jard N.