Plug in Shunt for Avo mark 4 multiminor - UK Vintage Radio Repair and Restoration Discussion Forum

Luxman1050 · 2nd Apr 2020, 12:20 pm

Does anyone know what a plug in shunt for the Avo mark 4 multiminor looks like or if you can make one up. I want to measure AC mV but not able too as mA is DC only on the meter and it seems according to booklet you need a plug shunt then insert test leads into that.
Cheers

The Philpott · 2nd Apr 2020, 5:14 pm

The copy of the booklet i am looking at says only 4 external accessories were available:
One for high DC voltage, and three for DC current.

If you were able to measure AC mV with this meter by some means, i doubt the accuracy would be much good.

Dave

WME_bill · 2nd Apr 2020, 6:07 pm

Shunt for AC current on Avo Multiminor.
The varying resistance of the rectifier means that a simple shunt cannot be used for AC current measurement. See Scroggie. Radio Laboratory Handbook - try 6th Edition, p93 or earlier editions. You will have to make a transformer, rather as the Avo 7 / 8 do.
Your shunts will be for DC only.
I suspect that you could use a shunt with a moving iron meter, as that responds equally well to AC as DC. But as the impedance of the coil will vary with frequency, the calibration will only be valid at one single low frequency.
Has anyone experimented with this. I will see if I have a moving iron meter somewhere.
wme_bill

Luxman1050 · 2nd Apr 2020, 7:07 pm

Cheers guys the booklet I have is pretty limited in info just how to read the meter and a small section on using a shunt connected to the + -ve terminals then connect the test leads to the shunt for DC mV readings. Read it wrong DC only shunts 5,10, and 25A not AC.
Does not do a great deal really pretty basic but I'll use it see how readings compare with my DVM.
I have a GPO one not even looked at that yet maybe that will have a bit more usefulness.

Cheers

Stockden · 2nd Apr 2020, 9:12 pm

The 10A shunt looks like this and, though I've never seen them, I imagine that the other are similar.

Hugh

Luxman1050 · 3rd Apr 2020, 10:26 am

Ah I thought they had to plug in. Must admit I've never seen them sold with AVO meters or singly. Must be quite rare or there was not a major demand for them. I suppose measuring up to 5, 10 to 25A was not often needed not sure what equipment would give you a 25A reading TV maybe?

2nd Apr 2020, 12:20 pm	#1
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Plug in Shunt for Avo mark 4 multiminor Does anyone know what a plug in shunt for the Avo mark 4 multiminor looks like or if you can make one up. I want to measure AC mV but not able too as mA is DC only on the meter and it seems according to booklet you need a plug shunt then insert test leads into that. Cheers

2nd Apr 2020, 5:14 pm	#2
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,104	Re: Plug in Shunt for Avo mark 4 multiminor The copy of the booklet i am looking at says only 4 external accessories were available: One for high DC voltage, and three for DC current. If you were able to measure AC mV with this meter by some means, i doubt the accuracy would be much good. Dave

2nd Apr 2020, 6:07 pm	#3
WME_bill Octode Join Date: Oct 2007 Location: Exeter, Devon, UK. Posts: 1,554	Re: Plug in Shunt for Avo mark 4 multiminor Shunt for AC current on Avo Multiminor. The varying resistance of the rectifier means that a simple shunt cannot be used for AC current measurement. See Scroggie. Radio Laboratory Handbook - try 6th Edition, p93 or earlier editions. You will have to make a transformer, rather as the Avo 7 / 8 do. Your shunts will be for DC only. I suspect that you could use a shunt with a moving iron meter, as that responds equally well to AC as DC. But as the impedance of the coil will vary with frequency, the calibration will only be valid at one single low frequency. Has anyone experimented with this. I will see if I have a moving iron meter somewhere. wme_bill

2nd Apr 2020, 7:07 pm	#4
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Plug in Shunt for Avo mark 4 multiminor Cheers guys the booklet I have is pretty limited in info just how to read the meter and a small section on using a shunt connected to the + -ve terminals then connect the test leads to the shunt for DC mV readings. Read it wrong DC only shunts 5,10, and 25A not AC. Does not do a great deal really pretty basic but I'll use it see how readings compare with my DVM. I have a GPO one not even looked at that yet maybe that will have a bit more usefulness. Cheers

2nd Apr 2020, 9:12 pm	#5
Stockden Heptode Join Date: Mar 2008 Location: Forest of Dean, Gloucestershire, UK. Posts: 648	Re: Plug in Shunt for Avo mark 4 multiminor The 10A shunt looks like this and, though I've never seen them, I imagine that the other are similar. Hugh Attached Thumbnails

3rd Apr 2020, 10:26 am	#6
Luxman1050 Octode Join Date: Apr 2019 Location: Margate, Kent, UK. Posts: 1,728	Re: Plug in Shunt for Avo mark 4 multiminor Ah I thought they had to plug in. Must admit I've never seen them sold with AVO meters or singly. Must be quite rare or there was not a major demand for them. I suppose measuring up to 5, 10 to 25A was not often needed not sure what equipment would give you a 25A reading TV maybe?