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Old 19th Sep 2022, 5:19 pm   #1
BrianAllen
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Default AVO 8X MkIII Ohms Calibration??

Can anyone tell me why when setting my Avo (left hand dial/switch set to resistance), the ohms divided by 100 and the ohms multiplied by 100 settings can't be calibrated to zero. The standard ohm range switch can be calibrated to zero though. Will I ever need to use the multiply and divide by 100 ranges?

Thank you.

Brian
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Old 19th Sep 2022, 5:38 pm   #2
G6Tanuki
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Default Re: AVO 8X MkIII Ohms Calibration??

Are the batteries in good condition? You need full voltage from the batteries to be able to properly-zero the Ohms ranges.

[Back in the 80s when AVO8s were still a thing in professional labs/workshops I remember the batteries were replaced every 6 months, and the meters sent away to a calibration-lab annually; unless your test-gear had traceable calibration-certificates you wouldn't get contracts]
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Old 19th Sep 2022, 5:49 pm   #3
BrianAllen
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Default Re: AVO 8X MkIII Ohms Calibration??

The seller said that they were new batteries but his idea of "new" could be many months!!

Interesting info regarding obtaining contracts re: calibration certificates.

Thank you.
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Old 19th Sep 2022, 6:07 pm   #4
The Philpott
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Default Re: AVO 8X MkIII Ohms Calibration??

The high range and medium range are 'tweaked' by Morganite pots which are notoriously noisy when well worn. The low range is tweaked by an (old school) wire wound pot.

The Morganites are troublesome in old age, even good ones may need to be run up to max then back down to min before using them to obtain a zero; this seems to clean the tracks up somewhat. The same procedure helps the low range pot, but also the brass shaft on the low range pot can become dulled and this can make obtaining a zero impossible.

Will you need the high and low ranges? I would say yes..for something or other.

Also make sure there's no corrosion in the battery box. Occasionally this can be invisible, but more often it's simply green!

Dave
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Old 19th Sep 2022, 6:09 pm   #5
paulsherwin
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Default Re: AVO 8X MkIII Ohms Calibration??

The low resistance ranges use a very high current. You really need an alkaline D cell.
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Old 19th Sep 2022, 6:12 pm   #6
stevehertz
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Default Re: AVO 8X MkIII Ohms Calibration??

Quote:
Originally Posted by BrianAllen View Post
The seller said that they were new batteries but his idea of "new" could be many months!!

Interesting info regarding obtaining contracts re: calibration certificates.

Thank you.
If you're unable to positively confirm that the batteries are good, then that's your start point, check them. Otherwise the poor/dirty pot scenario is the most likely culprit.
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Old 19th Sep 2022, 6:14 pm   #7
BrianAllen
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Default Re: AVO 8X MkIII Ohms Calibration??

Thank you gents; all of the above noted.

I did note that the central ohm calibration was different to the others either side of it. Thanks for explaining why they are different.
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Old 19th Sep 2022, 6:35 pm   #8
BrianAllen
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Default Re: AVO 8X MkIII Ohms Calibration??

Ok so now we have 2 values that can now be calibrated, so must have been the pot; just the divide 100 ohm to sort out now.

So much for the new batteries; the D cell although test at 1.5v the expiry date is Mar2018 but will replace anyway and the battery compartment does need a clean up.

When will I ever learn!!! Luckily only £30 spent on the AVO.
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Old 19th Sep 2022, 7:22 pm   #9
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Default Re: AVO 8X MkIII Ohms Calibration??

Worth checking the battery springs/connectors as well. I had exactly the same "ohms" problem with my Avo 8, and that's all it was. A quick clean and reseating/retensioning, and all was fine.


Mike
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Old 19th Sep 2022, 7:23 pm   #10
paulsherwin
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Default Re: AVO 8X MkIII Ohms Calibration??

They are fine instruments, but not the best way to measure resistance nowadays.
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Old 19th Sep 2022, 7:35 pm   #11
BrianAllen
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Default Re: AVO 8X MkIII Ohms Calibration??

Yes, looking a little green inside the battery compartment.

No doubt I will revert to the digital multimeter but nice to have an AVO on the bench.

Cleaned up the front and used white acrylic paint on the engraved/recessed range setting positions.
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Old 19th Sep 2022, 7:44 pm   #12
The Philpott
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Default Re: AVO 8X MkIII Ohms Calibration??

All the battery terminals are removable (but in the case of the 15 volt terminals, to get them free of the bakelite, i had to drill the rivets out and replace with 8BA set screws & nuts!)

Great results removing green verdigris from these, soak in lemon juice (30 mins?) then distilled water, dry off and wipe with vaseline.

One more thing- avoid Duracells- they often leak.

Dave
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Old 19th Sep 2022, 7:52 pm   #13
BrianAllen
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Default Re: AVO 8X MkIII Ohms Calibration??

Thanks Dave, they are very green!!
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