UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Comark Early 1905 Insulation Meter Checks

jimmc101 · 21st Jan 2019, 11:30 pm

The circuit is arranged such that the sum of the current through R10 and the meter is just sufficient to start to turn off TR1 and hence reduce the drive to the transformer via oscillator formed by TR2/3.

When calibrated this current is 10uA assuming that the meter resistance is much less than 2.5M (R10).

When the load across the test terminals is open circuit the meter current is of course zero and all of the 10uA must flow through R10.
On the 25v range the -ve test terminal is connected to the 'top' end of R1 giving a test voltage of 25v (10uA x 2.5M). As a load is applied and current flows through the meter the current through R10 to keep the total constant.

21st Jan 2019, 11:30 pm	#8
jimmc101 Heptode Join Date: Mar 2009 Location: Portsmouth, Hampshire, UK. Posts: 674	Re: Comark Early 1905 Insulation Meter Checks The circuit is arranged such that the sum of the current through R10 and the meter is just sufficient to start to turn off TR1 and hence reduce the drive to the transformer via oscillator formed by TR2/3. When calibrated this current is 10uA assuming that the meter resistance is much less than 2.5M (R10). When the load across the test terminals is open circuit the meter current is of course zero and all of the 10uA must flow through R10. On the 25v range the -ve test terminal is connected to the 'top' end of R1 giving a test voltage of 25v (10uA x 2.5M). As a load is applied and current flows through the meter the current through R10 to keep the total constant.