AVO 8 fault - UK Vintage Radio Repair and Restoration Discussion Forum

Electrone · 8th Sep 2013, 3:01 pm

I have an Avo 8 with a rather baffling fault. I can be using it one day to do various voltage and resistance measurements with no problems. I then leave it undisturbed on the bench for a few days, come back to it and set it up to do a resistance or continuity measurement and it won’t work. If I set it to do a voltage measurement it will work, then it will also work on the resistance ranges. I can then use it all day with no problems.. One might think the meter movement is sticky or there is an intermittent connection somewhere but during the fault condition the movement quite free to move, tapping the case in various places doesn’t cure the fault. I’ve opened the instrument up and had a good look round but can’t see anything suspicious.. It may be temperature related as my electronics workshop is in my loft which suffers wide temperature changes.

Has any other AVO owner experienced similar problems with their instrument?

Michael D

threeseven · 8th Sep 2013, 5:28 pm

This is likely to be one of the following, intermittent test lead or leads, intermittent cut contacts or a loose contact on the meter movement.
Next time it happens and before operating the switches, try different leads. If you are confident it isn't that, operate the cutout a few times to clean the contacts. After that it's inside to check for faulty switches or meter connections.

pmmunro · 8th Sep 2013, 6:10 pm

Michael,

What mark of the Model 8 is your meter? As the internal construction varies completely, this has a significant bearing on likely faults. Threeseven's advice is good, especially on the marks 1 to 4. You could also try pressing firmly on the "reverse moving coil" button a couple of times.

PMM.

Phil G4SPZ · 8th Sep 2013, 6:14 pm

I have experienced several faults of this nature with Model 8s, and in most cases it has been caused by poor or dirty leaf switch contacts. These switches are operated by the cams behind the rotary switches and they can be difficult to get at.

Resist the temptation to spray switch cleaner all over them. I bend a pipe cleaner in the middle, dip it in meths then pull the pipe cleaner between the contact points. Look closely at the contacts and rotate the switch until they open so that the pipe cleaner will pass through. Don't use abrasive cleaners or emery cloth. The same technique works for the cut-out contacts, but these have a self-cleaning wiping action and unless the cut-out has operated to clear a heavy overload they should be OK.

threeseven · 8th Sep 2013, 6:23 pm

I was indeed assuming it was a pre Mk5 model.

MrElectronicman · 9th Sep 2013, 10:03 pm

Phil is right, I have and continue to have the same problem with my AVO 8 MK11. The leaf switches are definitely the source of the problem. I have cleaned mine as Phil says and it works for a short time and then after a period of no-use it becomes intermittent again.

The real fix I believe is to dismantle the individual leaf switch assembly and burnish the contacts and then reassemble and adjust the leaf contact pressure. BUT! that is quite a task and not for the faint-hearted.

Electrone · 9th Sep 2013, 10:25 pm

My AVO 8 is a MkII. The test leads are definitely OK. At first I thought the cutout was perhaps making intermittent contact and shorted it out withh a wire link. Considering the comments by Phil and David I think I will leave well alone and try to live with the problem until it becomes really bad then offer it for sale as "Repair or Spares" on the Forum or ebay and buy another. I didn't pay a lot for the instrument (£14 on ebay) As I said, once it is working it is OK for the rest of the day.

I have a couple of DMMs but prefer an AVO for some measurements.

Many thanks to all who offered advice on the problem.

8th Sep 2013, 3:01 pm	#1
Electrone Retired Dormant Member Join Date: Aug 2012 Location: Cromer, Norfolk, UK. Posts: 100	AVO 8 fault I have an Avo 8 with a rather baffling fault. I can be using it one day to do various voltage and resistance measurements with no problems. I then leave it undisturbed on the bench for a few days, come back to it and set it up to do a resistance or continuity measurement and it won’t work. If I set it to do a voltage measurement it will work, then it will also work on the resistance ranges. I can then use it all day with no problems.. One might think the meter movement is sticky or there is an intermittent connection somewhere but during the fault condition the movement quite free to move, tapping the case in various places doesn’t cure the fault. I’ve opened the instrument up and had a good look round but can’t see anything suspicious.. It may be temperature related as my electronics workshop is in my loft which suffers wide temperature changes. Has any other AVO owner experienced similar problems with their instrument? Michael D

8th Sep 2013, 5:28 pm	#2
threeseven Octode Join Date: May 2010 Location: Worcestershire, UK. Posts: 1,880	Re: AVO 8 fault This is likely to be one of the following, intermittent test lead or leads, intermittent cut contacts or a loose contact on the meter movement. Next time it happens and before operating the switches, try different leads. If you are confident it isn't that, operate the cutout a few times to clean the contacts. After that it's inside to check for faulty switches or meter connections. __________________ BVWS member Last edited by threeseven; 8th Sep 2013 at 5:36 pm.

8th Sep 2013, 6:10 pm	#3
pmmunro Octode Join Date: Jul 2004 Location: Dundee, UK. Posts: 1,812	Re: AVO 8 fault Michael, What mark of the Model 8 is your meter? As the internal construction varies completely, this has a significant bearing on likely faults. Threeseven's advice is good, especially on the marks 1 to 4. You could also try pressing firmly on the "reverse moving coil" button a couple of times. PMM.

8th Sep 2013, 6:14 pm	#4
Phil G4SPZ Dekatron Join Date: Apr 2005 Location: Bewdley, Worcestershire, UK. Posts: 4,748	Re: AVO 8 fault I have experienced several faults of this nature with Model 8s, and in most cases it has been caused by poor or dirty leaf switch contacts. These switches are operated by the cams behind the rotary switches and they can be difficult to get at. Resist the temptation to spray switch cleaner all over them. I bend a pipe cleaner in the middle, dip it in meths then pull the pipe cleaner between the contact points. Look closely at the contacts and rotate the switch until they open so that the pipe cleaner will pass through. Don't use abrasive cleaners or emery cloth. The same technique works for the cut-out contacts, but these have a self-cleaning wiping action and unless the cut-out has operated to clear a heavy overload they should be OK. __________________ Phil Optimist [n]: One who is not in possession of the full facts

8th Sep 2013, 6:23 pm	#5
threeseven Octode Join Date: May 2010 Location: Worcestershire, UK. Posts: 1,880	Re: AVO 8 fault I was indeed assuming it was a pre Mk5 model. __________________ BVWS member

9th Sep 2013, 10:03 pm	#6
MrElectronicman Heptode Join Date: Feb 2011 Location: Washington DC, USA Posts: 619	Re: AVO 8 fault Phil is right, I have and continue to have the same problem with my AVO 8 MK11. The leaf switches are definitely the source of the problem. I have cleaned mine as Phil says and it works for a short time and then after a period of no-use it becomes intermittent again. The real fix I believe is to dismantle the individual leaf switch assembly and burnish the contacts and then reassemble and adjust the leaf contact pressure. BUT! that is quite a task and not for the faint-hearted. __________________ David

9th Sep 2013, 10:25 pm	#7
Electrone Retired Dormant Member Join Date: Aug 2012 Location: Cromer, Norfolk, UK. Posts: 100	Re: AVO 8 fault My AVO 8 is a MkII. The test leads are definitely OK. At first I thought the cutout was perhaps making intermittent contact and shorted it out withh a wire link. Considering the comments by Phil and David I think I will leave well alone and try to live with the problem until it becomes really bad then offer it for sale as "Repair or Spares" on the Forum or ebay and buy another. I didn't pay a lot for the instrument (£14 on ebay) As I said, once it is working it is OK for the rest of the day. I have a couple of DMMs but prefer an AVO for some measurements. Many thanks to all who offered advice on the problem.