AVO 8 - resistance range repair tip - UK Vintage Radio Repair and Restoration Discussion Forum

Riccardo Grillo · 28th May 2022, 10:53 pm

Thought I'd offer the benefit of my experience.

Having owned an Avo 8 Mk.II for a long time, I have always had problems with resistance ranges, especially the Ohm/100 range not returning to zero. I have tried new batteries, cleaning the potentiometers etc and found the fault was intermittent. Wiggling the battery tended to help (but involves opening the battery case), as did pressing on the corners of the instrument.

Recently I discovered that the battery terminals were a matt finish. I cleaned them with Brasso wadding (not suggesting this is the best way to do it as Brasso is an abrasive, but these are battery contacts and not precision contacts after all) and they became a shiny finish. Possibly this is how they were supposed to all along and 60 years of oxidation had dulled them. It was no longer necessary to wiggle the battery.

I then realised over the next few months that there remained an intermittent fault, and though wiggling the battery no longer did anything, pressing on the corners of the instrument still did.

Inside the main casing of the Avometer, the battery compartment makes contact with the front of the instrument by using a number of spike to spring contacts. I suppose the spike is supposed to be enough to keep the contact clean, but, again, the metal surfaces were dull. Again Brasso (again may not be the best thing to use but as above). I had previously resisted cleaning them with anything abrasive, but anything milder did not seem to be cutting the muster. Perhaps there are better alternatives, though.

Now I have done this I have found that I have significant adjustment (leeway) available on the resistance scales and they seem more reliable and consistent.

This should be basic stuff, but worth mentioning as the Avo service manual does not suggest these contacts could ever become a problem.

fetteler · 29th May 2022, 12:09 am

These are great tips - thank you very much for posting - I will remember this if I ever encounter the same problem.

Thanks,
Steve.

The Philpott · 30th May 2022, 5:16 pm

There are a lot of things that the service manual doesn't mention!

-1- Haven't had the problem with bad connection of the cell/battery contacts unless they've actually started to turn green. I have noticed that the nickel plating does become dull if the lid has been left off the battery compartment in a dirty environment for years.
I did notice with one design of modern 'D' cell that the shape of the body wasn't suitable for avo 8 contacts- the outer casing pushed the contact clear of the -ue end, if i recall correctly. There are some rubberised pads that are supposed to be in the earlier Model 8 design, to locate the D cell positively. These can come loose and get lost.

-2- I have had issues with the spring leaf and spike arrangement that you mention, but mainly on earlier meters. The early Model 8 examples i've worked with have had nickel plated leaves which retained shine quite well....earlier Models with unplated leaves, i have always checked and polished them up whenever the meter is worked on. (Yes, i do use Brasso, but i understand Silvo is a little gentler- not tried yet..)

The other problem with 'leaf and spike' is alignment. I bought a Model 9 which had a cracked potentiometer board, which was caused by the spike riding the side of the leaf when the meter case was refitted, causing excess lateral load. Before i sussed what the problem was i broke the replacement board the same way- saved by a nice lady in Witham who then sold me a NOS pot. board! Tweaking the leaf solved this issue. My standard check for this sort of misalignment is to dab the spikes with grease and slowly trial fit the case..then note where the grease spots arrive on the leaves.

Dave

The Philpott · 30th May 2022, 8:27 pm

Specifically regarding the resistance ranges (Model 8 Mk III) i have found the most reliable way of getting the pots. to behave is running all 3 of them fully c/wise then fully anti c/wise before attempting to get ANY of the ranges to zero properly for use. This does seem to mitigate the noise encountered, especially if the two Morganite pots are worn (and most of them are..)

The manufacturer was quite modest about the accuracy of the resistance ranges (quite wise as it would have avoided complaints) but for an analogue meter i have always been very impressed.

Dave

28th May 2022, 10:53 pm	#1
Riccardo Grillo Triode Join Date: Jul 2019 Location: Colchester, Essex, UK. Posts: 24	AVO 8 - resistance range repair tip Thought I'd offer the benefit of my experience. Having owned an Avo 8 Mk.II for a long time, I have always had problems with resistance ranges, especially the Ohm/100 range not returning to zero. I have tried new batteries, cleaning the potentiometers etc and found the fault was intermittent. Wiggling the battery tended to help (but involves opening the battery case), as did pressing on the corners of the instrument. Recently I discovered that the battery terminals were a matt finish. I cleaned them with Brasso wadding (not suggesting this is the best way to do it as Brasso is an abrasive, but these are battery contacts and not precision contacts after all) and they became a shiny finish. Possibly this is how they were supposed to all along and 60 years of oxidation had dulled them. It was no longer necessary to wiggle the battery. I then realised over the next few months that there remained an intermittent fault, and though wiggling the battery no longer did anything, pressing on the corners of the instrument still did. Inside the main casing of the Avometer, the battery compartment makes contact with the front of the instrument by using a number of spike to spring contacts. I suppose the spike is supposed to be enough to keep the contact clean, but, again, the metal surfaces were dull. Again Brasso (again may not be the best thing to use but as above). I had previously resisted cleaning them with anything abrasive, but anything milder did not seem to be cutting the muster. Perhaps there are better alternatives, though. Now I have done this I have found that I have significant adjustment (leeway) available on the resistance scales and they seem more reliable and consistent. This should be basic stuff, but worth mentioning as the Avo service manual does not suggest these contacts could ever become a problem.

29th May 2022, 12:09 am	#2
fetteler Octode Join Date: Jul 2011 Location: Staffordshire Moorlands, UK. Posts: 1,475	Re: AVO 8 - resistance range repair tip These are great tips - thank you very much for posting - I will remember this if I ever encounter the same problem. Thanks, Steve. __________________ Those who lack imagination cannot imagine what is lacking...

30th May 2022, 5:16 pm	#3
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	Re: AVO 8 - resistance range repair tip There are a lot of things that the service manual doesn't mention! -1- Haven't had the problem with bad connection of the cell/battery contacts unless they've actually started to turn green. I have noticed that the nickel plating does become dull if the lid has been left off the battery compartment in a dirty environment for years. I did notice with one design of modern 'D' cell that the shape of the body wasn't suitable for avo 8 contacts- the outer casing pushed the contact clear of the -ue end, if i recall correctly. There are some rubberised pads that are supposed to be in the earlier Model 8 design, to locate the D cell positively. These can come loose and get lost. -2- I have had issues with the spring leaf and spike arrangement that you mention, but mainly on earlier meters. The early Model 8 examples i've worked with have had nickel plated leaves which retained shine quite well....earlier Models with unplated leaves, i have always checked and polished them up whenever the meter is worked on. (Yes, i do use Brasso, but i understand Silvo is a little gentler- not tried yet..) The other problem with 'leaf and spike' is alignment. I bought a Model 9 which had a cracked potentiometer board, which was caused by the spike riding the side of the leaf when the meter case was refitted, causing excess lateral load. Before i sussed what the problem was i broke the replacement board the same way- saved by a nice lady in Witham who then sold me a NOS pot. board! Tweaking the leaf solved this issue. My standard check for this sort of misalignment is to dab the spikes with grease and slowly trial fit the case..then note where the grease spots arrive on the leaves. Dave

30th May 2022, 8:27 pm	#4
The Philpott Dekatron Join Date: Aug 2016 Location: Colchester, Essex, UK. Posts: 4,106	Re: AVO 8 - resistance range repair tip Specifically regarding the resistance ranges (Model 8 Mk III) i have found the most reliable way of getting the pots. to behave is running all 3 of them fully c/wise then fully anti c/wise before attempting to get ANY of the ranges to zero properly for use. This does seem to mitigate the noise encountered, especially if the two Morganite pots are worn (and most of them are..) The manufacturer was quite modest about the accuracy of the resistance ranges (quite wise as it would have avoided complaints) but for an analogue meter i have always been very impressed. Dave