Reversed polarity on Avo 7 - UK Vintage Radio Repair and Restoration Discussion Forum

0verload · 25th Jan 2014, 2:07 pm

I have an Avo model 7 which was given to me some time ago and when I came to check it out I found that on the voltage setting the polarity is reversed.
One thing I will point out is that there are no batteries in it at the moment, and I don't know whether this would be the cause of the problem.
Any help appreciated.
Thanks in advance.

threeseven · 25th Jan 2014, 2:26 pm

The battery has no effect on anything but the resistance ranges. Assuming the leads are connected correctly, have you tried pushing the polarity reverse switch to see what its effect is?

0verload · 25th Jan 2014, 5:06 pm

Many thanks, threeseven. Looks like it was pilot error. Just tried it again, and all OK. I'd possibly been at the Wood's when I tried it

Thanks, anyway.
Eric

Leon Crampin · 25th Jan 2014, 5:55 pm

The AVO 7 does not have a polarity reverse button - it's a div. 2.

If it really is wrong, then the meter is incorrectly wired internally. A check on AC will reveal whether just the movement is reversed or whether some bodger has had a major re-wiring experience and reversed the lot.

Leon.

pmmunro · 26th Jan 2014, 8:21 pm

Threeseven,

In general you are quiet corrrect in stating that the 1.5V cell and batteries in "conventional" Avometers have no part to play in voltage and current measurements. In fact it is revealing to see in advertisements, e.g. on eBay, a claim that the seller has not been able to test the meter for a lack of batteries. Either the seller has no idea how to use the meter or doesn't want to test it.

However, the Model 7 uses the 1.5V cell to apply a slight bias on alternating voltage ranges as this gives improved linearity. The effect is not great, but measurable.

PMM

threeseven · 26th Jan 2014, 9:23 pm

Interesting, thanks PMM, I wasn't aware of that. Although the lack of a battery wouldn't prevent the meter from reading in this case.

25th Jan 2014, 2:07 pm	#1
0verload Retired Dormant Member Join Date: Aug 2013 Location: Luton, Bedfordshire. Posts: 5	Reversed polarity on Avo 7 I have an Avo model 7 which was given to me some time ago and when I came to check it out I found that on the voltage setting the polarity is reversed. One thing I will point out is that there are no batteries in it at the moment, and I don't know whether this would be the cause of the problem. Any help appreciated. Thanks in advance.

25th Jan 2014, 2:26 pm	#2
threeseven Octode Join Date: May 2010 Location: Worcestershire, UK. Posts: 1,880	Re: Reversed polarity on Avo 7 The battery has no effect on anything but the resistance ranges. Assuming the leads are connected correctly, have you tried pushing the polarity reverse switch to see what its effect is? __________________ BVWS member

25th Jan 2014, 5:06 pm	#3
0verload Retired Dormant Member Join Date: Aug 2013 Location: Luton, Bedfordshire. Posts: 5	Re: Reversed polarity on Avo 7 Many thanks, threeseven. Looks like it was pilot error. Just tried it again, and all OK. I'd possibly been at the Wood's when I tried it Thanks, anyway. Eric

25th Jan 2014, 5:55 pm	#4
Leon Crampin Octode Join Date: Nov 2005 Location: Surrey, UK. Posts: 1,870	Re: Reversed polarity on Avo 7 The AVO 7 does not have a polarity reverse button - it's a div. 2. If it really is wrong, then the meter is incorrectly wired internally. A check on AC will reveal whether just the movement is reversed or whether some bodger has had a major re-wiring experience and reversed the lot. Leon.

26th Jan 2014, 8:21 pm	#5
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,812	Re: Reversed polarity on Avo 7 Threeseven, In general you are quiet corrrect in stating that the 1.5V cell and batteries in "conventional" Avometers have no part to play in voltage and current measurements. In fact it is revealing to see in advertisements, e.g. on eBay, a claim that the seller has not been able to test the meter for a lack of batteries. Either the seller has no idea how to use the meter or doesn't want to test it. However, the Model 7 uses the 1.5V cell to apply a slight bias on alternating voltage ranges as this gives improved linearity. The effect is not great, but measurable. PMM

26th Jan 2014, 9:23 pm	#6
threeseven Octode Join Date: May 2010 Location: Worcestershire, UK. Posts: 1,880	Re: Reversed polarity on Avo 7 Interesting, thanks PMM, I wasn't aware of that. Although the lack of a battery wouldn't prevent the meter from reading in this case. __________________ BVWS member