[stuarth]

There seems to be a meter movement for the Heathkit V7-AU on a well known auction site (not me selling it) if anyone is interested.

Resistance ranges on electronic meters such as a VTVM do have advantages over those on an AVO, but linearity isn't one of them.

For either type of meter, the equivalent circuit on resistance ranges is effectively a voltage source (eg the 1.5V battery), a range resistor equal to the centre scale reading of that range, and the unknown resistor all in series. The difference is how the meter is connected - for an AVO, the meter effectively measures the voltage across the range resistor, whilst the VTVM measures the voltage across the unknown resistor.

For an AVO, FSD corresponds to zero ohms for the unknown resistor, i.e. the test leads shorted together giving the full source voltage across the range resistor, and there is a zero adjuster to set this to FSD.

For a VTVM, FSD corresponds to an infinite unknown resistor, i.e. the test leads open circuit giving the full source voltage across the unknown resistor, and again there is an adjuster to set this to FSD.

An unknown resistor equal to the range resistor puts half the source voltage across each resistor, giving half scale deflection for either type of meter.

An unknown resistor of ten times the value of the range resistor will give a reading of 9% FSD on an AVO, and 91% FSD, i.e. 9% below FSD on a VTVM. Resistors a tenth of the range resistor will do the opposite. The scales are the same shape and linearity, just mirror images of each other. Resistors 100:1 different to the range resistor will give a 1% reading or a 1% below FSD reading depending on resistor value and meter type.

Advantages of the VTVM resistance ranges are 1 - open circuit test leads show FSD, encouraging you to switch the meter off the resistance range to a safer voltage range and 2 - poor contacts in the test leads cannot be zeroed out as they can with an AVO, you have to fix the leads, this gives more reliable low resistance measurements. Also most VTVMs have a maximum resistance measurement of 1000Mohms.

Meters such as the Heathkit V7-AU are designed to have a special DC probe with a 1Meg resistor in the probe tip and a screened cable to 1/4" input jack. This is great for low capacitive loading of the circuit under test (most of the capacitance is "blocked" by the 1Meg resistor) or if there are high levels of RF flying around, but a b**** nuisance otherwise. There's a very easy mod on the Heathkit, you just need to connect another 1Meg resistor from the jack socket to the AC/Ohms 4mm socket which then becomes the "universal" input socket, leaving the jack for the special DC probe, or a diode probe, as needed.

Heathkit seemed to have several revisions of the rectifier for AC inputs before settling on the circuit for the V7-A and subsequent models. It is actually peak to peak responding, and has bias to offset the rectifier valve contact potential for better linearity. The "normal" meter scale for AC ranges show the RMS value of the input assuming a sine wave, but there is an additional p-p scale, which can be useful, for example to quickly see how close to the supply rails a transistor amp can get with music playing.

Stuart