Wal,
It depends on what movement AVO used in your CT446. The movements I have seen are not identical to the movements in the AVO VCMs, however they can usually be adapted to some degree but will not have exactly the same behaviour as an original AVO VCM movement.
If you are lucky you have a meter with 30uA FSD current and an internal resistance of less than the 3250 Ohm used in the AVO CT160. But some meters are closer to the meters used in the AVO Electronic Testmeter with a 32-34uA FSD current (this is not due to loss of magnetic flux over the years, they are specified to have this range) and have an internal resistance of the moving coil between 2800 - 3000 Ohm. These meters are not suitable without correction by one or more small magnets, and they also need a series resistor to get the correct total internal resistance.
Since AVO used a higher internal resistance of the moving coil on the CT446 (and later transistor testers too) and in the AVO Electronic Testmeter, without the swamp resistor that the AVO VCM's have inside their meter, you will need to correct the total internal resistance. Without the swamp resistor you will also have a different temperature related behaviour of the meter.
You can adjust the circuit in the AVO VCM's to accept meters with a little bit to high FSD current by changing the total resistance of the meter and the shunts outside the meter so the meter behaves like the equivalent meter that AVO uses in their VCMs. The equivalent meter has an FSD of 39.75uA and a total resistance of 2452.8 Ohm (another way of saying this is that the voltage drop across the meter at 39.75uA current through the meter should be 97.5mV). But this procedure becomes problematic when there is no swamp resistor and the internal resistance of the moving coil is high as you might not have the necessary room for adjusting then. This procedure has been described by Pamphonica here:
https://www.vintage-radio.net/forum/...ad.php?t=50786 on the forum earlier for an original AVO VCM meter that is out of tolerance. When you short out part of the swamp resistor by shunting it with a resistor you will also loose some of the temperature compensation.
How to adjust the meter FSD with magnets can be seen on my friend Mr. Yutaka Matsuzaka's website here:
http://translate.google.se/translate...n&hl=&ie=UTF-8 (I used Google Translate), beware that this procedure easily wrecks your movement if you slip when you place the magnet as it might damage the movement when it travels to fasten itself on the magnet.
Adjusting a meter with magnets and/or resistors will work, but in most cases the meter will not work exactly as the ones used in the AVO VCMs. One more thing is that many AVO meters have lost some magnetic flux over the years, and are still loosing magnetic flux, so they will need to be corrected again in the future, so the same procedure with adjusting resistors or adding magnets will have to be done again or it will have to be re-magnetized.
If you have a choice I would vote for buying a new meter movement and having it installed in the original meter housing. If you want a CT160 in original condition I think the right way is to look for an original movement or have yours repaired, but if you can accept to have a somewhat different meter behaviour you can adjust some meter movements from other AVO equipment to work quite well. If you only have access to meter movements that are not original the OPAMP solution is perhaps the best as you can then easily adjust it in the future if the meter looses more magnetic flux, and it will also work if the meter movement is replaced.