Problems with Avo Model 8 MKIV

[M0TGX Terry]

Hi everyone
Many years ago I was given an Avo 8 MkIV by a neighbour who was clearing his garage. He said it hadn't been used for a long time. I put it on my shelf and haven't touched it since I got it. Today I decided to dig it out to take some readings on a Mullard MAS82 I'm repairing for a friend. Sadly the Avo doesn't seem to want to help, and I'd be very grateful for any advice/comments/suggestions. This is where I am with it:
I fitted a new 'D' cell and set it to read Ohms. Nothing! no meter needle movement at all. I checked and cleaned the battery terminals. No improvement. I then checked the test leads - the negative one had broken its connection to its plug, so I fixed that. On touching the two test prods together, the needle now swings hard across to the right hand end of the dial (which I imagine it's supposed to do). However, trying to set the Zero point using the Set Zero knobs has no effect on either the Ohms range or the divide by 100 range. (I don't have a 15v battery so can't check the X100 range.
Since all my resistance readings are starting at a point well below zero, the Avo isn't helping me to fault find the Mullard radio (there will be more on that d****** radio in another thread, later).
I then tried the Avo on the 2.5 volts DC range, testing it on a new D cell. This cell measures 1.65v on my DMM, but the Avo needle points to 80 on the scale. as the scale is calibrated 0-100, I assume that a reading of 80 indicates a value of 2v with the range set to 2.5 volts.
Does anyone have anything to say, please (ALL comments gratefully received, up to and including "put the Avo and the bl***y Mullard set in the nearest WEE skip)
Many thanks
Terry

[Sinewave]

Sounds like it's reading a little high on voltage. There's a 0-2.5 scale below the 0-100 scale, so it's measuring around 2v.

Do you have access to a current source? It'll be worth seeing what the meter is reading when set to 10mA range and 10mA applied, that will give us good indication.

With reference to the ohms zero pots, they later in life do somehow get very dirty and it's not uncommon for them to need a good clean out in order to establish a reliable and consistent zero.

[M0TGX Terry]

Thanks for the swift response, Sinewave. Sadly I don't have a current source I could use to test the amps ranges.
I'll try some Servisol on the zero-setting pots. I didn't want to use it before seeking advice, as I didn't want to risk making a problem into a Big Problem by squirting it into the works.
Don't know how I failed to notice the 0-2.5 scale! Senility creeping up on me, I expect.

[Superscope]

Hi Terry,

First Question is, do you have a Schematic for the Model 8 Mk IV ?

It sounds like your Meter has an open circuit Shunt Chain since it is
reading high by around 30%.
You will probably find all readings on any scale will be 30% high.

That might also affect the Ohms Range, but I'm not 100% sure on that one
without looking into it..


Ian

[M0TGX Terry]

Hi Ian. I don't have a circuit diagram at present. I shall have a search round and see if I can find one on the web somwhere.
To be honest, it's not very high up my priority list as I have a couple of DMMs I can use, and I'm supposed to be getting on with repairing my friend's Mullard radio. Once that's out of tbe way I'll have a good look at the Avo.
Thanks for the information though.

[The Philpott]

Hello Terry. In my humble opinion... in the long term i would consider getting yourself an Avo 8 MkIII and abandoning the MkIV. (Prejudice or hearsay may play a part in this comment as i own the earlier version but not the later one.)

Dave

[Superscope]

Hi Terry,

Schematic attached for when / if you need it.

As Dave says, the Mk III is a better Meter in so far as it's
better built, but the Mk IV is still well worth saving.
Shouldn't be to difficult to get working.

Incidentally, if your meter has a Serial Number (on the Scaleplate or on a Sticker),
it would be great if you could report it on the Multimeter Survey Thread.


Ian

[pmmunro]

I agree with Ian that an Avometer reading 33% high probably has a break in the universal shunt, R9 to R 17. However, you should also check the range switches which, uniquely in the Model 8/9 Mk IV, are printed disks. The tracks are silver plated and can suffer from the growth of 'tin whiskers' if the meter had been stored in damp conditions. They may also be tarnished.

Be careful about spraying any switch cleaner in the interior of the meter. The mist from the spray can coalesce on the turns of the hairsprings cause the turns to stick together which seriously affects the operation of the movement. It is better to moisten a cotton bud with the switch cleaner and use that to clean the switch disks. A piece of paper moistened in the same way can also be carefully slipped between switch contacts.

I hope you will be successful in restoring your meter to good health, but even if you decide to abandon it in savour of one of the more reliable versions, please pass it on to someone willing to work on it. This version is one of the less common versions so it would be a pity if it were lost.

PMM

[M0TGX Terry]

Thanks for the schematic, Ian. And thanks to everyone else who has replied. I will have a good and careful look at it tomorrow, with a view to at least cleaning the switches and the zero set pots. Even if I don't get it sorted I shall not scrap it, especially as it appears to be one of the rarer models. Tomorrow in daylight I'll check it for a serial number and post it on the survey thread.
Once again, thanks for all the help.

[Sinewave]

Indeed the Mk4 is a rare one, there are not many about and it took me a long time to get a good one for my collection. Especially rare to find one without a cracked rear casing and with serial number intact.