[Heathkit] VTVM Model IM-28

[Drew P.]

Garey Barrell wrote:

[snipped]

"If you do any tube or other high impedance measurements, please do NOT fall for the internet mod to move the resistor to the switch inside the meter.  This is a typical case where some 'genius' thinks he is smarter than the Heath engineers. There is a reason Heath engineers went to the effort and expense to develop the probe design.  The resistor is there to decouple the test lead from the measurement point, and is the second most valuable feature of a VTVM over a DMM."

I have seen occasional problems when using a DMM, caused by the test lead - stray signal pickup or even instability in RF circuits.  I address that problem when using a DMM for measurements in such circuits by wrapping 1 lead of a 1.2 meg resistor around the probe tip, then probing the circuit under test with the free end of the resistor.  I mentally apply a 10% increase to the observed DMM reading.  Someday I'll put the resistor inside a real probe...

Drew