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Old 10th Jun 2017, 9:05 pm   #1
Fourlegsgood
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Default Avo VCM163. Meters back from Herts but . . . . .

The Avo VCM163 that I bought sight unseen lumbers on. The meters were sent away to Herts Meters and Robert there did wonderful work cleaning the corrosion off and recalibrating them.

I never saw the VCM163 work so I do not know what problems it might have.

Now the meters are back in the problem I am having is the first step in checking it over and calibrating it. With a variac and multimeter I have ensured that the input voltage is exactly 240v. However I cannot get the left meter to align with the ~ voltage line. The manual says to adjust RV3 so that the meter lines up with the ~ line but even at full adjustment of RV3 the meter is well short of the ~ line.

Any ideas of first things to check?
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Old 10th Jun 2017, 9:39 pm   #2
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

Some obvious things to check:

Did you put the fuse in the 240V position? What happens if you move it one step down to the 230V setting?

Have you checked the resistance of R20 & R21 (2.2 MOhm)?

Have you checked D4, D5 & D6 for leakage current and the resistance of R19 & RV3?

What current flows through the meter? You can connect a DMM in series with the meter and check what current flows at different positions of the SET AC switch and fuse.

Have you checked the relay and switch contacts for contact resistance?

I have come across some VCM163's where the resistance has increased and where some of the diodes had become leaky and poor.

/Martin
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Old 11th Jun 2017, 8:36 am   #3
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

Martin,

Thanks.

"Did you put the fuse in the 240V position? What happens if you move it one step down to the 230V setting?" I did that before posting. In the 230v position the situation improves but still doesn't get to the ~ line.

I will work through your other suggestions.

Thanks, Nick
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Old 11th Jun 2017, 1:11 pm   #4
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

"What current flows through the meter? You can connect a DMM in series with the meter and check what current flows at different positions of the SET AC switch and fuse."

It draws about 60mA. Rotating the SET AC switch anticlockwise decreases the draw by a few mA and increases for clockwise rotation. (I am talking less than 10mA difference total.) Rotating the SET AC switch gives a linear change to the meter deflection.

I have known properly working VCM163. So, before I do more testing I will swop the left meters between the two VCMs. I am sure Herts Meters will have properly checked the calibration of the refurbished meter but it will be worth checking that nothing happened on it's journey back to me.
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Old 11th Jun 2017, 2:37 pm   #5
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

Martin,

"I will swop the left meters between the two VCMs. I am sure Herts Meters will have properly checked the calibration of the refurbished meter but it will be worth checking that nothing happened on it's journey back to me."

Having swopped the meters over it is now clear that the problem lies with the refurbished meter. I will talk to Robert at Herts Meters on Monday.

Thanks for your suggestions though.

Nick
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Old 11th Jun 2017, 3:14 pm   #6
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

I was a bit unclear but I meant what current flows through the moving coil meter. You'll have to unscrew one side of the posts on the moving coil meter and hook your DMM in series with the now free lug and the moving coil meter post to see how many uA that flows.
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Old 12th Jun 2017, 2:12 pm   #7
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

Quote:
Originally Posted by Dekatron View Post
I was a bit unclear but I meant what current flows through the moving coil meter. You'll have to unscrew one side of the posts on the moving coil meter and hook your DMM in series with the now free lug and the moving coil meter post to see how many uA that flows.
Martin, The 'good' meter in my long term VCM163 flows 43 uA. The 'poor' meter flows the same in the same VCM163.

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Old 12th Jun 2017, 2:25 pm   #8
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

That is good to know, now I hope that you find the reason why it differs in the other VCM.
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Old 12th Jun 2017, 2:48 pm   #9
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

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That is good to know, now I hope that you find the reason why it differs in the other VCM.
Martin, The two meters behave the same in each of the VCMs. My 'good' meter is fine in both VCMs and it is the refurbished meter that is reading low, no matter which VCM it is in.
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Old 12th Jun 2017, 5:35 pm   #10
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

Ahh, sorry misunderstood that.

That is strange if the refurbished meter reads low - are you sure that they put the correct scale on the meter (including perspex) there is a difference in that left meter contains a temperature compensation network and the right not.
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Old 12th Jun 2017, 7:30 pm   #11
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

I have talked to Robert at Herts Meters and agreed that if I send the properly working meter and the under reading meter then he can make the under reading meter match the other one. In other words get it to be full deflection for 50uA.

The refurbished right meter seems to behave properly although so far all I have done is check that it displays the correct defection for 'Calibration'.
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Old 12th Jun 2017, 7:45 pm   #12
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

Good that Robert can check them against each other, he could then make sure that they have a correct reading on the SET AC line for both meters.

If you look at the corrected circuit diagram that I put in the document I wrote on the comparison of the different AVO valve testers you will see that the left Ia-meter has an internal resistance of 2.4k +/- 1% and that the right Gm-meter has an internal resistance of 1.5k +/- 1%.

The moving coils are identical in both meters and you can swap them (50uA and 1.5k internal resistance), but there is an NTC-resistor and a wire wound resistor in the left Ia-meter which compensates for temperature drift, the right Gm-meter does not have these components.

The meter amplifier for the Gm-meter works equally well with both meters, it doesn't matter if it is 1.5k or 2.4k that the meter amplifier sees. So there is a slight chance that the scales have been swapped. This could have happened at a previous try to restore the VCM163, before you sent them to Herts Meters.

I've seen a few Ia-meters where someone had ripped out these temperature compensating resistors and put a simple metal film resistor in their place, and in one case where they had left them out.

I just thought it would be a good idea to try to have a peek inside the Ia-meter to see if the resistors are still there, and if possible check if it has the correct internal resistance on its connections.
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Old 30th Jul 2017, 6:28 pm   #13
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Default Re: Avo VCM163. Meters back from Herts but . . . . .

Just to conclude this.

Robert at Herts remagnetised the problem meter and then recalibrated it so that it gives FSD for 50uA. Back in the AVO VCM163 it behaves perfectly and the whole machine is now behaving just as expected and with readings that are as near as possible identical to my other VCM163.

So another VCM163 lives to fight another day.

Three cheers for Herts Meters (who by the way are desperate for any terminally failed AVO meters that might be able to be salvaged to get others going that they have in their sick ward). I was lucky that my meters were electrically sound and also that the springs had not been tampered with by well intentioned amateur hands.
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