"AVO" Auto Electrical Test Set Model 12...Help and advice needed please... - UK Vintage Radio Repair and Restoration Discussion Forum

calamero · 22nd Feb 2006, 1:40 am

Hello everyone ,nice forum you have here!

Ive been sitting with this old Avo meter for a while now and last night i decided to try it out..Its missing a "U2" battery which to be honest i havnt a clue what kind of battery that is or voltage but the compartment looked like it could take a 1 1/2 volt "D" cell so fitted that instead and fingers crossed it seems to work ok..

Ive tested the volt , current & 10x ohms ranges and they all seem to be in good working order but sadly the 1x ohms range is bit faulty..

On the 10x ohms range i can join the test leads together and adjust the pointer to show zero ohms but when i try and do the same on the 1x range the needle jumps to the far right as if theres a short circuit...using the zero adjust knob makes no impression on it at all

I thought if i opened it up and peeked inside id be able to see where the fault lies but its not as easy as that lol..and i dont want to start doing things that might make it worse...

Looking inside i dont see any burnt resitors or other components...

doing a basic resistance check inside on what i THINK is the path from battery to leads to meter ive learnt that the 1x switch switches in 25 ohms and the 10x range switches in 240 ohms..might be wrong ?

Im hoping that when one of these ranges go someone might know what actually happens and be able to help me fix it...a shematic would be great help too..

When i opened it all thats inside is resistors and little bobbin type components ..what are those ..are those old inductors or some type of resitor because when doing my simple check i noticed one of them was involved in the switching from 25 ohms to 240...might be wrong again :/

Thanks in advance

calamero

Ed_Dinning · 22nd Feb 2006, 1:39 pm

Hi Calemaro, you've guessed it pretty well, the U2 =D cell, so OK there.
The bobbins are resistors, wound with resistance wire and quite precise.
From memory there may be a resistor switched across the meter movement on the lowest ohms range as a shunt, or there are a pair of contacts that are meant to close (and don't always) to bring this resistor into circuit.
If you look at the links associated with this site you may see a copy of the circuit there.

Best regards, Ed

calamero · 22nd Feb 2006, 9:16 pm

Thanks Ed ill give it another go

What threw me off is that when i done a resistance check comparing the 1x and 10x ranges i could "see" it changing from 25 ohms to 250 ohms so switch seems to work ok..

Ill look for a shunt round the meter to see if i spot anything thanks again

ps...im looking for that circuit p;lan u said might be on a link leading from this website but cant find any headers that gives me idea where to follow from here ?

calamero

pmmunro · 22nd Feb 2006, 9:27 pm

The attached diagrams, if I can find a way to attach them, should help.

I'm sorry if the disclaimers seem a bit draconian but this is the web.

Please remember that you are working on a 1% instrument. If you need further help, please ask, or see similar threads for the Model 8.

This fault is quite common on Model 12s and is often caused by the switch contacts a and c failing to make. You could try cleaning them, and the omega shaped switch rotor with isopropyl alcohol.

If you decide to dismantle the switch, please be careful to ensure that the parts are correctly aligned before tightening the centre screw as otherwise the centre moulding is easily broken. It could be very difficult to replace.

Peter M. Munro

calamero · 23rd Feb 2006, 6:04 am

Success!!

Thanks Ed & Peter sorted

After getting plans i had another go..Not wanting to start taking things apart i did another resistance check following the circuit as i went and connection a wire across battery terminals as if battery was in circuit

Im no expert but after checking it it all looked fine to me..when 1x ohms range was switched in i was getting the 24 ohms as i should so imo as im a know it all the switch must have been working correctly and to be honest didnt fancy taking it apart but as a last resort i decided to undo the screw on top of the switch anyway and give the little omega points a clean like pete suggested and Ed mentioned..

Well i messed up big time taking that screw off as underneath theres TWO washers,i got the top one but the little copper one directly beneath it slid to the side and down inside the switch mechanism itself..i cursed starting the thing and spent the next hour or so holding it upside down shaking the meter trying to get the washer to fall out then a little ballbearing fell out instead lol...well in the end i managed to put it back together again CORECTLY and lo and behold the 1x ohms range working perfectly haha

I couldnd have done it without your helps so thanks again!!!

calamero

22nd Feb 2006, 1:40 am	#1
calamero Retired Dormant Member Join Date: Feb 2006 Location: uk Posts: 4	"AVO" Auto Electrical Test Set Model 12...Help and advice needed please... Hello everyone ,nice forum you have here! Ive been sitting with this old Avo meter for a while now and last night i decided to try it out..Its missing a "U2" battery which to be honest i havnt a clue what kind of battery that is or voltage but the compartment looked like it could take a 1 1/2 volt "D" cell so fitted that instead and fingers crossed it seems to work ok.. Ive tested the volt , current & 10x ohms ranges and they all seem to be in good working order but sadly the 1x ohms range is bit faulty.. On the 10x ohms range i can join the test leads together and adjust the pointer to show zero ohms but when i try and do the same on the 1x range the needle jumps to the far right as if theres a short circuit...using the zero adjust knob makes no impression on it at all I thought if i opened it up and peeked inside id be able to see where the fault lies but its not as easy as that lol..and i dont want to start doing things that might make it worse... Looking inside i dont see any burnt resitors or other components... doing a basic resistance check inside on what i THINK is the path from battery to leads to meter ive learnt that the 1x switch switches in 25 ohms and the 10x range switches in 240 ohms..might be wrong ? Im hoping that when one of these ranges go someone might know what actually happens and be able to help me fix it...a shematic would be great help too.. When i opened it all thats inside is resistors and little bobbin type components ..what are those ..are those old inductors or some type of resitor because when doing my simple check i noticed one of them was involved in the switching from 25 ohms to 240...might be wrong again :/ Thanks in advance calamero

22nd Feb 2006, 1:39 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: "AVO" Auto Electrical Test Set Model 12...Help and advice needed please... Hi Calemaro, you've guessed it pretty well, the U2 =D cell, so OK there. The bobbins are resistors, wound with resistance wire and quite precise. From memory there may be a resistor switched across the meter movement on the lowest ohms range as a shunt, or there are a pair of contacts that are meant to close (and don't always) to bring this resistor into circuit. If you look at the links associated with this site you may see a copy of the circuit there. Best regards, Ed

22nd Feb 2006, 9:16 pm	#3
calamero Retired Dormant Member Join Date: Feb 2006 Location: uk Posts: 4	Re: "AVO" Auto Electrical Test Set Model 12...Help and advice needed please... Thanks Ed ill give it another go What threw me off is that when i done a resistance check comparing the 1x and 10x ranges i could "see" it changing from 25 ohms to 250 ohms so switch seems to work ok.. Ill look for a shunt round the meter to see if i spot anything thanks again ps...im looking for that circuit p;lan u said might be on a link leading from this website but cant find any headers that gives me idea where to follow from here ? calamero

23rd Feb 2006, 6:04 am	#5
calamero Retired Dormant Member Join Date: Feb 2006 Location: uk Posts: 4	Re: "AVO" Auto Electrical Test Set Model 12...Help and advice needed please... Success!! Thanks Ed & Peter sorted After getting plans i had another go..Not wanting to start taking things apart i did another resistance check following the circuit as i went and connection a wire across battery terminals as if battery was in circuit Im no expert but after checking it it all looked fine to me..when 1x ohms range was switched in i was getting the 24 ohms as i should so imo as im a know it all the switch must have been working correctly and to be honest didnt fancy taking it apart but as a last resort i decided to undo the screw on top of the switch anyway and give the little omega points a clean like pete suggested and Ed mentioned.. Well i messed up big time taking that screw off as underneath theres TWO washers,i got the top one but the little copper one directly beneath it slid to the side and down inside the switch mechanism itself..i cursed starting the thing and spent the next hour or so holding it upside down shaking the meter trying to get the washer to fall out then a little ballbearing fell out instead lol...well in the end i managed to put it back together again CORECTLY and lo and behold the 1x ohms range working perfectly haha I couldnd have done it without your helps so thanks again!!! calamero