There’s hardly any piece of test equipment more fundamental than a volt ohm meter. Today you’re likely to have a digital one, but for most of history, these devices had real needle meters. The AVOmeter Model 8 Mark III that [Jeff Tranter] shows off had an odd banana-shaped meter. Maybe that goes with the banana plugs. You can get a closer view of this vintage piece of equipment in the video after the break.
Even the outside description of the meter is interesting. There were several unique features. For example, if the meter goes full scale a little button pops out and disconnects the probes to protect the meter. Another unusual control reversed the polarity of the leads so you didn’t have to swap them manually.
Some of the other features will be familiar to anyone who has used a good analog meter. For example, the meter movement has a mirror under the needle. This is used to make sure you are looking straight down on the needle when making readings. If you can see the reflection of the needle, then you are off to one side and will not read the precise value you are interested in.
If you only want to see the insides, [Jeff] teases you until around the six minute mark. There are no active devices and this meter is old enough to not use a printed circuit board. The AC ranges work with a transformer and germanium diodes. The rest of the circuit is mostly a bunch of resistors.
The point to point wiring always makes us wonder who built this thing sixty years ago. You can only wonder what they would think if they knew we were looking at their handiwork in the year 2020.
We see a lot of meter clocks, but it would be a shame to tear this unique meter apart for its movement. Perhaps someone should make a clock that outputs a voltage to a terminal so you could read it with your favorite meter. This instrument was probably pretty precise for its day, but we doubt it can match a modern 6.5 digit digital instrument.
The Rolls Royce of analog multimeters.
These were the “Bee’s Knees” meters when I started electronics. I would love one now.
The AVO meters worked well and were pretty tough, with an overload tripped by the meter movement (as mentioned in the video).
About 12 years ago and prior I owned a garage/workshop. One of my customers was a chap in his 80’s who still liked to work on his own cars. He was having trouble with some diagnostics and so he called in. He was using one of these, He called it a “Mega”, The back of it had been replaced with wood at some point but apparently he had it for a very long time. I used it to check his electrics and we successfully pinned down the problem with it (Faulty coil pack). It was an excellent tool tbh and I would be happy to have one in my current hobby garage.
A Mega is not the same as an Avo. Megas are for high voltage insulation testing. I grew with my Dad’s Avo and learned Ohms law fromis.
That’s just what he called it. Maybe he was referring to Megger the company that bought/owned AVO ?
I own a MK3 and a MK7 which still work after all these years… and if any one is interested ebay link
https://www.ebay.co.uk/b/AVO-Multimeters/bn_7024864182
That brought back some memories! My Dad gave me one of those when I was a teenager starting to play with electronics, back in the 70’s. I made my HiFi stereo amp -a Practical Electronics Orion- with it. Wonder whatever happed to that meter?
I have a model 8 but its a mark 1 with metal housing and the same bakelite front.
Still works well. I was shocked to hear avo stopped.
Have a Mk3 and and MRK7 , got the mark 3 for £25 as the seller said it was not working and it was only good for spare parts , put in new batteries and it works :)
had 2 of these back in the day, battered and bruised but absolutely 100% functional. they got discarded during a house move when I was a teenager, Now I want them back lol thanks for the trip down memory lane!
When I started work as a radio technician in the mid 70s the senior technician (ex-military) would insist that unless a reading had been taken with an AVO Model 7 he would not believe it.
Although AVOs were rugged, when a Fluke 8600 arrived in the department almost no one would use a AVO analogue meter unless there was no other option. The senior technician therefore purchased a AVO Digital multimeter to restore faith in AVO meters. What a joke – physically bigger than a Model 7, mains only as it had a Nixie display, and clumsy range selection.
AVOs were soon forgotten with the arrival of the handheld Fluke 8020 in the late 70s. The problem with this Fluke was that it was not very suitable for field use. The row of range selector switches on the side would bind up with dirt or moisture in industrial use. In the early 80s the Beckman HD100/110 industrial handheld meters arrived and these were found to be virtually indestructible. Beckman is now gone and the field service meter of choice is probably the Fluke 87, though it is also quite an old model.
As for megger, a megger is usually a generic reference to any insulation tester, not just those from Megger which were originally hand cranked units. My electronic “megger” is actually a Fluke.
Back in those days, I used a Beckman. I vastly preferred it to the Flukes we had in the shop. After using mine, the other techs agreed and all bought their ow Beckman units. I miss mine, it died some time in the ’90s.
Keep meaning to dig out the beast I got for a buck as non-functional. Every time I remember it though, I can’t find any 4mm plugs or leads. One of my meters uses 4mm plugs but they’re shielded and short and won’t go in it. I bought what were supposed to be a NOS set the other year, but they turned out to have set hard. (Rubber must have went sticky, welding them together, then dried hard.) The plugs and probes on those would have got rewired had they not seemed to be the sealed in type, no obvious way to get into them without destroying the casings.
Anyhoo, big meters are great for some things. They don’t chase you round the workshop when a slight kink or snag tugs the wires. You can interpret “needle gestures” to help figure out what’s going on with your component or circuit.
These were the starter meters given to twelve-year-olds when I went to secondary school and started “Integrated Science” in 1982. Older children were given digital multimeters.
These were as common as anything about 10 years ago, I still have half a dozen of different models and marks (generally Model 7 or 8). They are good meters, I like using them for current measurement as they are good for a solid 10A without issue. Main issues nowadays is the obscure battery requirements. They were often used as hammers and for everything in the forces (wheel chocks probably) where they were a standard issue. They were also scrapped by the hundreds, my uni once filled a skip with them (before my time alas). I have the official AVO AC Current transucer as well, weighs a ton. Confusion comes as Megger brand bought AVO at one point, in fact I have a fairly late AVO digital handheld meter with Mfd by Megger LTD on the back.
Simpson 260, I still have it. Reverse switch very handy for ohms tests on transistors. They were the meter of the phone company, 50 volt scale. Of course flipping it back and forth would show how many bells and hence extensions were on Ma Bell’s line in your house. Ohm! You have a pirate phone.
No mention of the input impedance. There are reasons VTVOMs, and FETVOM analog meters where developed. . Probably not a good idea to usesthis meter on modern electronic circuits. Health issue put me out a circulation for a long time. By the time I was able to get back to my toys, battery leakge ruind my Eico FETVOM beyond repair, bummer. That arc shaped meter may a result of functional design, to make a more compact instrument. When compared to a square meter that’s going to have arc shaped scales anyway. How those range switches could be confusing, fore anyone who wasn’t told how they work. Image getting that in an auction grab boxes of items?
There is another issue with input impedance, some manuals expected you to me measuring with a meter with a 20k impedance and if you tried measuring with a VTVM or FET meter the voltages could read high because of the lower loading of the circuit being tested.
Thanks for an interesting article that brought back old memories. These were indeed the “Bees Knees” back in the day. I started my career at the BBC in 1969, and we had a selection of Mk5’s, 6’s and 7’s. The BBC had their own Calibrations Lab, and about twice a year, a chap would come round with a Standard Voltage Box and check all our meters! I think I still have the Avo Minor I bought in the late 70s. The resistance trimmer is a wire-wound pot, which was so badly worn it had become completely unreliable, so I retired it and bought a cheap Chinese digital meter. Quite good enough for testing continuity and batteries. The prices on-line are not out of my reach, so I may buy one if only to keep as an ornament!
I have one of these from my grandpa! I also have his clamp on ammeter! I really should refurbish them.
” the meter movement has a mirror under the needle. This is used to make sure you are looking straight down on the needle when making readings.”
Reminds me of a joke: Why do Micronta (Radio Shack) meters have a mirror behind the needle? So you can see who’s making a bad measurement.
I did my apprenticeship as a radio and TV engineer in the mid 1960s always using AVO 8s.
When I became a fully fledged TV engineer for the Brighton Co-op I bought my own AVO 8 Mk3 that served me well.
I emigrated to Spain in the late 70s and brought my AVO 8 with me. Since then I went on to a digital meter but always remembered my old AVO 8.
Yesterday I dug it out and repaired the broken case and made up a 15V battery using five CR2032 batteries, heat shrinking them together and a small piece of 18mm. copper tube as a spacer, and of course a new 1.5V battery.
Testing it on resistance ohms x 100 is more or less OK but no readings at all on ohms or ohms – 100.
Before I start digging into it, does anyone know what is the usual suspect in this case?
I have the circuit diagram so any pointers would be really appreciated.
Many thanks,
James.