AVO meter movement sensitivity - UK Vintage Radio Repair and Restoration Discussion Forum

Paul Galpin · 5th Mar 2009, 2:58 pm

One of my (two) AVOs was reading low, by about 5%. I was scratching my head about this, looking at the beast, wondering what to do, and I spotted a bar, held by a central screw, across the poles. Memories of "magnetic shunt" came to mind, and since the bar was obviously not part of the structure, I tried loosening the screw and turning the bar. Straight away the meter, which was showing 4.8V for 5.00 in, went up to 5.4!

Logic says -
1. the number of turns on the coil is fixed by design, and anyway it can only be changed in increments of one whole turn.
2. The magnetic pole dimensions are fixed
3.The magnetic strength (flux) is fixed

So the only way to make it manufacturable is to make the magnet a bit stronger than you need, and "short-circuit" some of it out by an adjustable steel bar.

Well, it worked for me. Maybe it's not necessary on the later models, but these are 75 year-old horseshoe magnets I'm talking about.

I'd be glad to hear if anyone else has tried this, and how well it worked.
Any thoughts?

Paul Galpin

GeorgSc · 5th Mar 2009, 6:10 pm

Hello Paul,
I have seen this "magnetic shunt" on much younger
instruments.
Regards
Georg

AVO_VCM163 · 5th Mar 2009, 8:57 pm

Yes, your arguments are correct!

This is quite a usual arrangement where it is necessary to obtain both an accurate sensitivity AND resistance. Hence it is important in multi-range instruments having shunts and series components thus obviating the need for individual selection of auxiliary components in a mass production environment. Simple meters do not necessarily have this adjustment.

Unfortunately some AVO meter movements, particularly it seems those used in valve testers and some smaller multi-purpose instruments did suffer from a change in magnetic flux over the years.

Regards,
R

5th Mar 2009, 2:58 pm	#1
Paul Galpin Triode Join Date: Feb 2009 Location: Port Elizabeth, Eastern Cape, South Africa Posts: 21	AVO meter movement sensitivity One of my (two) AVOs was reading low, by about 5%. I was scratching my head about this, looking at the beast, wondering what to do, and I spotted a bar, held by a central screw, across the poles. Memories of "magnetic shunt" came to mind, and since the bar was obviously not part of the structure, I tried loosening the screw and turning the bar. Straight away the meter, which was showing 4.8V for 5.00 in, went up to 5.4! Logic says - 1. the number of turns on the coil is fixed by design, and anyway it can only be changed in increments of one whole turn. 2. The magnetic pole dimensions are fixed 3.The magnetic strength (flux) is fixed So the only way to make it manufacturable is to make the magnet a bit stronger than you need, and "short-circuit" some of it out by an adjustable steel bar. Well, it worked for me. Maybe it's not necessary on the later models, but these are 75 year-old horseshoe magnets I'm talking about. I'd be glad to hear if anyone else has tried this, and how well it worked. Any thoughts? Paul Galpin

5th Mar 2009, 6:10 pm	#2
GeorgSc Retired Dormant Member Join Date: Dec 2008 Location: Germany Palatinate Posts: 87	Re: AVO meter movement sensitivity Hello Paul, I have seen this "magnetic shunt" on much younger instruments. Regards Georg

5th Mar 2009, 8:57 pm	#3
AVO_VCM163 Retired Dormant Member Join Date: Mar 2008 Location: Hampshire, UK. Posts: 297	Re: AVO meter movement sensitivity Yes, your arguments are correct! This is quite a usual arrangement where it is necessary to obtain both an accurate sensitivity AND resistance. Hence it is important in multi-range instruments having shunts and series components thus obviating the need for individual selection of auxiliary components in a mass production environment. Simple meters do not necessarily have this adjustment. Unfortunately some AVO meter movements, particularly it seems those used in valve testers and some smaller multi-purpose instruments did suffer from a change in magnetic flux over the years. Regards, R