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Old 30th May 2018, 7:12 pm   #1
stahodad
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Default AVO 9 Mk II No AC volt or AC Current measurement.

Hi Folks.

I am working on a AVO Mk II. The AC Volts and Current are not working. The DC volts needs an adjustment, but I have adjusted the shunt and the DC current it looks OK. Resistance also looks OK. Batteries and fuses are good. I have resoldered some loose solder connections on the bridge rectifier.

If anyone has ideas, let me know.

Thanks.
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Old 31st May 2018, 7:06 pm   #2
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

Hi stahodad,

Welcome to the Forum.

If it is just the AC side of things that doesn't work, it could be a number of things
including any number of dirty or failing contacts, of which there are many in an AVO meter.

However, if all the AC ranges are faulty, but the DC side is generally working Ok, a good place to start would be to check the AC Rectifier.

I don't currently have a Schematic in front of me, but from memory, I think the Mk II is a Full bridge rectifier with Four Diodes.

Later models used Two Diodes, and Two Resisters substituted what conventionally was the other Two Diodes in a Full Wave Rectifier. Don't know if this system was used in any Mk II's, but either way, this is not a difficult check to make as a first Hit.

If the Diodes are faulty, I think the original ones might still be available, but suitable replacements are also available.
Can't remember what the originals are off hand, might be OA45's.

Do you have a Schematic, and a Service Manual?

If you do a search on this Forum, I think this has been covered before,
but let us know how you get on.

Regards


Ian
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Old 31st May 2018, 7:41 pm   #3
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

I believe the 9/II has the copper oxide rectifier, just under an inch across, with the general appearance of an anti-tank mine- is this what you have? I haven't encountered a dead one yet, but the lazy ones (ie those reading volts too low) often have evidence of corrosion on the nuts at either end.

As already said, don't overlook a problem with the finger leaf switches. While you've got it apart this is a sensible service item anyway, as being moving parts they do suffer over the decades.

Dave
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Old 31st May 2018, 9:40 pm   #4
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

What I have seen before on these, is with failed rectifers, the movement swinging rapidly due to the AC in the coil.

As others have said, check contacts.

If measuring ok on DC current, then look at the potential divider circuit for the voltage measurments.
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Old 1st Jun 2018, 3:56 pm   #5
stahodad
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

Hello there, and thank you for all of the information. I have found a schematic on this website as well as a parts list. This meter has the 4 diode bridge rectifier.

I removed the bridge rectifier to test it. I separated each pair of diodes. 2 are open and 2 are low ohms both ways. I had a rectifier from the parts bin that works, but I may have some work to improve the AC 100 volt (low by 3%) and DC 1000 (low by 4%) and 600 Volts (High by 4%0 ranges. At least we are making progress. Cheers, Chris
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Old 2nd Jun 2018, 7:00 am   #6
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

Hi Chris,

Excellent news, another AVO meter well on it's way to fight another Day.

Might I suggest, now that it is basically working, the first thing you
should Check before making any adjustments to any Ranges is that
the Meter Movement is working correctly at 37.5uA FSD.

Regarding the AC accuracy, it is possible that might be down to your choice of Rectifier.
I seem to remember it mentioned before, that the choice of Diodes is important
regarding calibration.
Somebody else on here might be able to comment on this.

But if the Ranges are all up and down a bit like yours seem to be, it sounds
like some of the Brass Contacts might need cleaning. Some of those are Fun to get at!

Ian
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Old 2nd Jun 2018, 11:34 am   #7
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

I had this 9/II apart last month- i see what you mean, it has got the 4 diodes- the soldering on mine looks as though it could be a replacement..

If you do resolve the problems do let us know as you have a head-scratching set of readings going on there.

I agree with what Ian has just typed, i am (also) wondering if you might have a slightly sticking meter movement.. Tapping the glass to settle the needle reading is mentioned in the factory literature, however as the instrument sits on 3 rubber feet a much safer method is to rock it on it's 'tripod' to confirm an honest reading.

(Incidentally, good readings cannot be obtained with the meter sitting with handle upwards under any circumstances; it must be on it's feet.)

Dave
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Old 7th Jun 2018, 1:46 pm   #8
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

Additional: I had cause to take the back of my other Model 9/II yesterday, which appears to have the original type rectifier. I believe the 4 diode type may have been a later upgrade.
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Old 18th Jun 2018, 6:14 pm   #9
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

Hello to all and thank you for the guidance on dealing with this issue.

Most of this meter was working well, so I double checked my readings and mostly my issue is with 1000 V DC range and 600 v DC range.

The switches looked in good shape, so I looked at the resistors.

On the 600 DC Volt range, R30, the 6 Meg measured 5.8 Mohm, so I added a 200 KOhm resistor in series, to get it closer to what is specified. That brought the meter needle down to where it should be.

One of the 1000 volt range resistors (R29, two 20 MOhm resistors) were off as well, measuring 20.4 Mohm, maybe due to the battery leak. So I went with a few resistors (from another old meter) in series to measure 19.6 Mohm, connected to the original 20 Meg ohm.

Now it is very close to the 1000 volts which will work ok, for the work I do. Thanks for all your help. Cheers Chris.
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Old 18th Jun 2018, 7:04 pm   #10
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

Thanks for advising the resolution. I might have a check on mine as i am surprised those figures were out of spec. to that extent. Both of mine have had to come apart for little problems that shouldn't really have been there (notwithstanding they are getting a bit elderly and have taken some knocks here and there.)
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Old 18th Jun 2018, 10:03 pm   #11
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

There's a few of my meters which read fine on current, but are out of spec on some of the voltage ranges.

I'd had in the back of my mind to look at the potential divider circuit as you have done, as that's really the only thing which it could be.
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Old 18th Jun 2018, 10:18 pm   #12
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Default Re: AVO 9 Mk II No AC volt or AC Current measurement.

Great News Chris,

Have to say, I've never heard of anybody actually
using their AVO on the 1000V Range in Anger!

For me, I haven't quite got round to designing a Test Rig
for testing these Higher Voltages.


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