Avo Model 8 MkII

[dR-Green]

Hi all, I have acquired my father's Avo Model 8 multimeter which has been stored up the attic for nearly 30 years. I tested it (by testing the resistance of touching both terminals together) with a 1.5v battery installed but without the 15v battery because I didn't have one and hadn't got round to testing it with a power supply, it appeared to be dead, also it didn't display any voltage whatsoever from any battery or power source, so I bought another one off eBay and it arrived today, so I went and bought a 15v battery for it.

Out of curiosity I put the 15v battery in my father's Avo, /100 and x1 resistance measurement does nothing, hence my assumption that it was dead. However, with the 15v battery installed the x100 measurement works fine.

So it seems to me that the 1.5v part of the circuit is not working, or isn't getting any power from the battery. It's not totally dead, it's just half dead. Has anyone else had any experience with this sort of thing and have any ideas how to fix it? I've asked my dad who's an electronics engineer to have a look (though he's not overly enthusiastic about it) and I've got the circuit diagram and service manual, but if someone already knows what the answer might be, that would be awesome and greatly appreciated.

Thanks

[ewizard]

I don't know that particular model but I can tell you that most analog meters are not complex and that any problems they have can likely be traced to dirty or corroded contacts or a broken connection. Close examination and cleanup can do wonders. Those are nice old meters.

[dR-Green]

Thanks ewizard. Yes I was surprised at how basic the circuit really is, but still, for someone like me the circuit diagram can be intimidating and makes it look far more complex than it actually is, so I don't want to start poking around myself and causing more faults I hope it's a broken or dirty connection and nothing too drastic. I'm hopeful seeing as it's at least partially working which is a step closer than I originally thought.

[wrtner]

Hi all, I have acquired my father's Avo Model 8 multimeter which has been stored up the attic for nearly 30 years. I tested it (by testing the resistance of touching both terminals together) with a 1.5v battery installed but without the 15v battery because I didn't have one and hadn't got round to testing it with a power supply, it appeared to be dead, also it didn't display any voltage whatsoever from any battery or power source, so I bought another one off eBay and it arrived today, so I went and bought a 15v battery for it.

Out of curiosity I put the 15v battery in my father's Avo, /100 and x1 resistance measurement does nothing, hence my assumption that it was dead. However, with the 15v battery installed the x100 measurement works fine.

So it seems to me that the 1.5v part of the circuit is not working, or isn't getting any power from the battery. It's not totally dead, it's just half dead. Has anyone else had any experience with this sort of thing and have any ideas how to fix it? I've asked my dad who's an electronics engineer to have a look (though he's not overly enthusiastic about it) and I've got the circuit diagram and service manual, but if someone already knows what the answer might be, that would be awesome and greatly appreciated.

Thanks

I think there are fuses. I have one; they are excellent.
i'll have a look this evening and report back.

When you tested the battery, was the dial on "DC" ?
When you touched the probes together, was the dial on "Ohms" ?

Manuals etc to be found here:
Service Data.

[dR-Green]

Thanks for your response. That site is where I got the service manual from. The circuit diagram came from here (links down the bottom)

avo8

It's this fuse business that really confused me, which is partially why I bought another one off eBay which will make sense in a minute...

The background/cause: My father worked for some electronics company that fix all kinds of scientific instruments in all kinds of industries, so they have their own team who issue and test equipment to be used by the other teams, that sort of arrangement. So his old Avo has a sticker on the battery cover stating that it has been "Inspected" (21 Sep 1976 ).

Inside this cover it shows the polarities for inserting the batteries, and a picture of a "Fuse", which I assumed was some primitive diagram to show the arrangement of where things should go. Well a fuse was nowhere to be seen, and I spent a while trying to figure out where it's supposed to go because it doesn't make any sense, there's nowhere inside the battery compartment that I can see a fuse could go. I then read somewhere that the fuse wasn't introduced until Mk3. So I thought the guy who inspected the meter back in 1976 must have put the wrong cover back on, OR he had removed the fuse and the 15v battery at the same time because he wanted them (??), thinking that no one would notice (which they didn't until 2011). So to confirm this I bought another Mk2 off eBay. This one doesn't even have any diagrams inside the cover, and no fuses in the battery compartment either.

So to cut a long story short, I don't think there's supposed to be a fuse in the Mk2. But if you can correct me on that then please do.

[edit] I just compared the Mk2 circuit diagram to the Mk3 (I should have thought of that sooner), and I see the 1A fuse in the Mk3, but no such thing in the Mk2.

To answer your questions, yes and yes

Thanks

[dR-Green]

It has decided to start measuring DC voltage and current by itself, haven't tested AC. It still won't measure /100 and x1 resistance.

[wrtner]

It has decided to start measuring DC voltage and current by itself, haven't tested AC. It still won't measure /100 and x1 resistance.

They depend on the batteries. The 15 volt one is a BLR121. I have heard
of people using ten little silver type batteries ganged in series.

[dR-Green]

Thanks for your input, but the 15v part is not the problem Only the x100 resistance value requires the 15v battery and the meter will work perfectly fine in all other respects without it. In fact I'm currently using it with no batteries at all.