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Old 23rd Jan 2012, 5:48 pm   #1
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Default AVO VCM163 Set Up

I've just been going through the set up procedure in the Avo service manual. The Ia readings I am getting are a few percent high as some of shunt resistors on the meter have drifted a little. I cannot quite get the gm meter to hit the mark on the CAL setting; CAL requires a meter reading of 45 and the best I can do with all the pre-set variable resistors set to max is a reading of 42. The oscillator is happily pumping out 15mV of sine wave as required, but it seems as if the amplifier is not quite up to scratch.

These issues are generally consistent with comments made by Swordholder in his sticky. I am not much tempted to start doing surgery at this stage for what seem like small deviations. Anyone want to persuade me otherwise
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Old 23rd Jan 2012, 6:55 pm   #2
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Default Re: AVO VCM163 Set Up

I recommend you to start by checking the FSD current of the meters. Many of these meters have lost magnetic flux over the years and requires a higher current flowing through them to reach full deflection. A few microamperes difference make a lot of difference on the meter deflection.

If you don't have meters that work as they should, and which are within tolerance nothing else will make you get correct readings, you might end up on the calibration lines but the readings will be off nevertheless.

I also recommend you to check all resistors and all capacitors as a lot of them have been out of tolerance on those VCM163's which I have checked.
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Old 24th Jan 2012, 1:47 am   #3
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Default Re: AVO VCM163 Set Up

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Originally Posted by Dekatron View Post
I recommend you to start by checking the FSD current of the meters. Many of these meters have lost magnetic flux over the years and requires a higher current flowing through them to reach full deflection. A few microamperes difference make a lot of difference on the meter deflection.

If you don't have meters that work as they should, and which are within tolerance nothing else will make you get correct readings, you might end up on the calibration lines but the readings will be off nevertheless.

I also recommend you to check all resistors and all capacitors as a lot of them have been out of tolerance on those VCM163's which I have checked.
You are so right. The Ia meter is ~4 percent off, which I am less concerned about, but the meter reading gm needs all of 59 rather than 50uA to get it to FSD and the internal resistance is around 1.3k rather than the 1.5k spec. Given the rarity of these meters, is it possible to resolve this problem using the TL071 op amp design of Gary Tempest, either using the existing meter, or a new 1-100uA movement?
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Old 24th Jan 2012, 2:05 am   #4
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Default Re: AVO VCM163 Set Up

You can use an OPAMP solution like Gary's if you wish, Martin Scobie has done it and shows it on his webpage here: http://www.marie34.freeserve.co.uk/page15.htm .

You can also use a 100uA meter movement directly if the internal resistance is low enough for the amplifier to drive it, you will however need to adjust the feedback resistor R14, 200 Ohm, in the amplifier to be able to use the 100uA meter. Usually only Taut Band movements have low enough internal resistance as many movements usually have an internal resistance above the required 1500 Ohm. It is also well to have some headroom if you need to adjust the circuit. My friend Euan MacKenzie simulated a 100uA movement for me in his VCM163 and he had to lower R14 to about 91 Ohm while using a 1500 Ohm resistor in place of the meter and while measuring the current through the meter for FSD with 100uA.

If you want you can swap the meters but you will then have to move just the meter movement as the Ia meter is internally compensated with a resistor/NTC network which is necessary to adjust for temperature drift of this meter. The moving coil portion is identical but that resistor/NTC network makes up for the 2.4k resistance of that meter as well as the temperature compensation - this means that you might need to adjust the network to accomodate the other moving coil. If you attempt any modifications you should only adjust the metal film resistor and not the NTC nor the wirewound resistor as those are important for the temperature compensation.

Depending on what movement you have you can for some movements adjust the FSD current by adding neodynium magnets to the moving coil magnet and possibly also use a magnetic shunt if you get a too sensitive movement, but some movements have a self shielding design which will not work so well with magnet adjustment.
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Old 24th Jan 2012, 3:07 am   #5
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Default Re: AVO VCM163 Set Up

Do you know where the circuit diagram of Gary's op amp is available? Martin shows pictures of the op amp board but I cannot find the diagram on his site.

I think the meter movements on my 163 have already been swapped over. The pointer on the gm meter has been damaged at some time, suggesting that it started life measuring Ia ?

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Old 24th Jan 2012, 10:47 am   #6
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Default Re: AVO VCM163 Set Up

You can read this thread: https://www.vintage-radio.net/forum/...ad.php?t=46825 where you will see a CT160 modified with the opamp solution.

You might have to adjust the resistance in circuit with the meter to fit the 50uA / 1500 Ohm meter (1300 Ohm in your case) to protect the meter from serious overload. You can also use a pair of silicon diodes across the meter to protect it if you wish (not across the input of the opamp but across the meter).

You'll also have to change the resistor across the input to 1500 Ohm so that the amplifier in the VCM163 "sees" the correct resistance.
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Old 25th Jan 2012, 6:05 pm   #7
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Default Re: AVO VCM163 Set Up

Thanks Martin; I've now got both circuits so I can take my pick. Disconnecting the gm meter and looking at the voltage supplied to it as placed across a 1.5K resistor and measured on an DVM, I've been able to confirm that the oscillator and amplifier are working fine and that the CAL setting can be achieved. So as a temporary solution, that is going to give me more accurate test results than I had before.

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Old 25th Jan 2012, 6:15 pm   #8
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Default Re: AVO VCM163 Set Up

I am glad that it was nor more problems than the meters. Now you know that the rest of the tester probably works as it should so you can focus on getting the meters back to their original FSD.

You should however have a look at, and check, resistor R35 as in the VCM163 Sticky as it is very often damaged even though it doesn't always look like that.

/Martin
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Old 26th Jan 2012, 1:58 am   #9
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Default Re: AVO VCM163 Set Up

Something else I've just realised; the "Ia" meter is used to fine tune the 240VAC level, so when that is wrong all the other voltages are slightly wrong also. Op amps all round!

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Old 26th Jan 2012, 2:24 am   #10
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Default Re: AVO VCM163 Set Up

That's correct! That is why it is so very important that the meters have the correct FSD current and then that all other components have correct values. You can't calibrate the tester without everything being in order.

If you use a small extra transformer for the opamp solution you can hook the primary side up to the Anode/Screen volts transformer primary side as that is always fed close to 240V when the mains selector has ben set properly. This way the voltage for the opamp transformer will not vary as much as compared to if you feed it directly from the mains voltage, and it will work for 110-127V too!
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