08-02-2021, 09:28 AM
Shunts are used for measuring current, not voltage. Here is a reasonable introduction to the subject: http://studyelectronics.blogspot.com/201...liers.html
On voltage ranges most DMMs present a 10M resistance to the outside world. Some cheap ones are only 1M. The resistors that matter around the CRT have high values, such as 1M and 2M2. A 10M meter will signifcantly alter voltages. A 1M meter will be utterly misleading. If you were using an analogue meter such as an Avo 8 you'd be using the 1000V range to measure 300+ volts. It would present 1000 x 20k = 20M at its terminals. A case where an analogue meter will load the circuit by less than a digital one.
My understanding of the 22M resistor that was added is that it's compensating for a design error. The original designer didn't imagine that current might flow into G2 of the CRT. Although the current flow may be very low, it could conceivably be in either direction. Hence a resistor is needed. I also have no idea why D10 was included. Presumably the designer had his or her reasons but I can't see them.
In any case, fitting the 22M resistor seems to make the CRT operate in a satisfactory way. So if there are no other problems with the display, just accept it. Adding the 22M certainly can't do any harm.
On voltage ranges most DMMs present a 10M resistance to the outside world. Some cheap ones are only 1M. The resistors that matter around the CRT have high values, such as 1M and 2M2. A 10M meter will signifcantly alter voltages. A 1M meter will be utterly misleading. If you were using an analogue meter such as an Avo 8 you'd be using the 1000V range to measure 300+ volts. It would present 1000 x 20k = 20M at its terminals. A case where an analogue meter will load the circuit by less than a digital one.
My understanding of the 22M resistor that was added is that it's compensating for a design error. The original designer didn't imagine that current might flow into G2 of the CRT. Although the current flow may be very low, it could conceivably be in either direction. Hence a resistor is needed. I also have no idea why D10 was included. Presumably the designer had his or her reasons but I can't see them.
In any case, fitting the 22M resistor seems to make the CRT operate in a satisfactory way. So if there are no other problems with the display, just accept it. Adding the 22M certainly can't do any harm.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
