Replacement meter for Avo 160
 

Hi Folks,
Hope everyone is having a nice weekend, Question, repair or replacement for an Avo 160 meter, I am at present waiting for replies from Herts and Benchmark re this question, but am I correct in saying that the meter from an Avo 446 will do the job, if so what mods does this entail.
Thanks.
Wal

Re: Replacement meter for Avo 160
 

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.

Re: Replacement meter for Avo 160
 

Thanks Martin.
It's all a bit over my head I think I will get pro help on it, in the meantime is there a simple way to test the CT 160 meter to see if it's working, I have removed it from the tester.
Thanks.
Wal

Re: Replacement meter for Avo 160
 

Wal,

You can hook it up to a constant current source (CCS) or simply connect it in series with a digital multimeter, some resistors of the correct value and a small battery to check it. I know I wrote about it somewhere earlier in this forum but I can't seem to find it right now. Just calculate the reistors according to the current you need to have in the circuit and hook it up, you can use a potentiometer in series with the circuit to change the current somewhat to see how it behaves. Just be carefull not to overload it.

Re: Replacement meter for Avo 160
 

Wal,

As Martin told the resistance of the meter must be 3250 Ohm. You must also get FSD (full scale display) with 97.5 mV voltage between the meter terminals. This voltage is quite small, so be carefull.

Re: Replacement meter for Avo 160
 

What about using any old meter movement driven by an Op-Amp, with a resistor input of 3250 ohms, and its gain set to give FSD at 97.5mV...?

--It will then duplicate the operation of original meter.

Adapt the movement to fit the frame of the original meter-movement case, use the same original scale, and who would know the difference?

Source a regulated supply for the op-amp, and you're all sorted.

Prolly MUCH cheaper than getting an Avo 160 meter rebuilt!

Re: Replacement meter for Avo 160
 

You might find this thread of interest.

Re: Replacement meter for Avo 160
 

Hi All,
Decided to send both meters to Herts so they can check them out.
Thanks.
Wal

Re: Replacement meter for Avo 160
 

Can't recommend these guys highly enough !
They overhauled my AVO 4 meter and it came back looking like new !

Andy

Re: Replacement meter for Avo 160
 

I rather suspect that the use of a high power ferrite, rare-earth or even a neodymium or other exotic magnet will in fact produce what would for all practical purposes be a permanent repair, because these magnets lose minimal if any strength over time and once attached, the field in the meter will be stable virtually forever. I'm thinking probably centuries here, not years or decades.

This technique is not remagnetising the meter magnet per-se, which would require a somewhat different process. Attaching the ferrite magnet to the depleted meter magnet is simply infusing it with the fields from the attachment, thus the original magnet become little more than a carrier for those magnetic lines of force which then appear across the coil aperture pole pieces. Take the attachment away and the meter would almost certainly revert back to its previous insensitive state within a very short time.

As for magnet strength, I recently measured some 6-7 mm dia. 2mm thick disks (taken from Christmas novelties) at 8 to 10 kilogauss so they are not lacking in that department. What I am not clear on is how critical the magnet strength is to meter accuracy, but given the success of empirical tests carried out by others it seems that it does not make all that much difference and I presume that if it became too sensitive, shunting the movement would allow it to be set to FSD at the prescribed current.

Cheers

Billy

Re: Replacement meter for Avo 160
 

I have now been shown one CT446 with a meter that has the swamp resistor inside the meter house, so those meters do exist! But I have not seen those earlier in any of the CT446's I have repaired.

This atleast shows that they do exist, but I can't say if it is an original meter put there by AVO in the manufacturing process or if it has been fitted after some repairs to the CT446, as I do not have any production nor service information on these transistor testers.

Re: Replacement meter for Avo 160
 

This link no longer works. Is there an alternative?

Thanks.

Re: Replacement meter for Avo 160
 

I couldn't find any alternative link so I emailed Mr. Yutaka and asked, now I haven't been in contact with him for many years so I hope he answers.

Re: Replacement meter for Avo 160
 

Hi

I just corrected my MK3 meter, it showed 15% too low, after adding two (see the picture) neodymium magnets 10X3 mm (I bought different thickness, but 2*3 mm was perfect strength) above the screw on each side, it was spot on 30µA.

I removed one screw at a time, grabbed the magnet with a good grip plier and placed it above the screw hole, and then pushed it with an non magnetic screwdriver to the position I wanted. All worked fine

Br Svante

Re: Replacement meter for Avo 160
 

Hello,
My AVO VCM Mk4 showed ca. 6-8% to low Ia versus an extarnally connected
and calibreted Fluke 87V. I also repeated the Ia measuring with a calibrated
AVO8 Mk7. Both instruments indicated the same readings.
By adjusting the magnetic shunt I managed to get 30myA. at FSD.

a) The movement was tested with 30microA f.s.d. as a singel unit using
a Fluke 5101 as DC source.
b) The movement was tested for 30microA f.s.d. using the grid voltage in the
VCM for f.s.d. deflection.
Same results with a and b
Am I doing right?

Lars G