Which Avometer should I purchase?

[MrBungle]

Calling all experts

The time has come for me to purchase a decent analogue / moving coil meter. I've been slowly reducing my stock of digital items over the last month and I'm down to two digital meters from the original 11. If I'm honest I'm trying to standardise on British manufactured and dead simple equipment because it's a lot easier to get parts for it. No black box ICs and ASICs - I've had enough! I've got a Heathkit VTVM (Daystrom so close enough to British ) which I'm refurbishing but would like a standard VOM to go with it. Avo comes to mind as a reputable piece of equipment to sit along side this.

Unfortunately there are a huge variety of Avometers available so I'm lost in a maze. What would be a reasonable choice for general design and repair work? Something relatively recent, maintainable, reliable and good enough for work on transistor as well as valve based circuits so quite sensitive if possible.

[G6Tanuki]

AVO8 is the classic generic AVO.

Remember though that these are very old and can suffer a lot of calibration-affecting abuse over the years. Unless the case cracks you can't tell how many times it's been dropped.

Get one that looks OK, and then pay to get it calibrated by a UKAS-accredited facility so you know the degree to which the reading-on-the-scale is not what you think it is!

[Note: a 'calibration' does not mean they adjust the meter so it reads right; it means they produce a table showing what it actually reads when fed with a number of standard currents/voltages].

A meter without calibration is no more than a guessing-tool.

[MrBungle]

Fair points all around. Thanks for the tips.

I'm intending to "self-calibrate" as the cost to pay for cal is too much for hobby use. I have a friend with an HP34401A I can borrow for an evening or two. This has traceable UKAS calibration from NPL I think so I can do comparative cal with the sources I have (variac, various DC power supplies, calibrated DDS etc). Actually once, in the distant past, I worked in the T&M department once and was responsible for doing the paperwork on such things

[The Philpott]

My experience with the Avo 8 only extends up to the 8 (Mark III)
In my limited experience this is the one i would recommend overall.

(When the 8 went over to PCB's later on there were some teething problems with reliability/fragility. See richards radios for general details)

-Parts not available but they are still sufficiently plentiful that this is not a major concern since cannibalisation will supply bits needed.
-Quite sensitive
-Magnets typically still healthy and capable of being calibrated to compensate for ageing.
-The movement is actually quite robust and more problems are encountered with deterioration of the leaf contacts behind the rotary switches. This is usually betrayed by dramatically lowered readings on voltage ranges. If there has ever been damage to the slider contacts on the rotary switch/es this can occasionally result in the cam rotor/s coming loose and spinning, thus needing to be retightened. For reasons best known to the manufacturer they are not located by a peg, so were presumably set up by eye, or with a jig!

-The resistance ranges give a very wide coverage by the standards of the day, but at the cost of leaving you 'stuck between ranges' at certain values, which reduces precision a little.
-Invariably buying one of these, you will find it does require attention before you can rely on it. Every single one i have had has required a little tweaking (or more usually replacement of inevitable corroded battery contacts)

I hope this doesn't put you off- when they are sorted and working they are a very nice piece of kit.

[MrBungle]

Thanks for the detailed write up. Much appreciated.

I'm certainly not put off. I've done some pretty extreme repairs before so I'm up for a challenge

[Andrewausfa]

I like my Mark 6 for transistor work, the 8 Mark III for everything else. If you're buying on-line be prepared for a couple of dogs before you get a good one. That good one may be covered in years of dirt and generally look fit for the dustbin of death. And vice versa.

Does anyone still actually calibrate AvoMeters?

Andrew

[theoldtrout]

There is a Welsh chap who does some of the radio rallies who sells refurbished and calibrated AVO's at sensible prices, sorry I can't remember his name but somebody here probably knows him and how to contact him.

[MrBungle]

Plan was to go to Kempton this year and grab one there so I can inspect if possible. He might be there

I can see a lot of them on eBay that are quite clearly broken in some way.

[ionburn]

I would go for an '8'. One issue with some other models is that their ohms/V is somewhat lower so give more issues with accuracy of readings (many readings given in manuals are with 20kOhm/V meter which the 8 is).

IMHO any AVO is good, the odd one may not work but are very fixable. Since I have discovered this https://www.maplin.co.uk/p/mt-2017-l...ltimeter-n60lk I haven't used my AVOs. I have three of them (the Maplin job, only two AVOs), saves getting one from another floor in the house (lazy bu**er). They are not quite like an AVO but survive a drop very well. For less than a tenner supberb value for money.

And modern batteries/cells fit.

[g8jzyian]

I would agree on the model 8 mark III
In my main and first employer we used AVO 8 s a lot and the MKIII was able to be transported to a Substation and give accurate readings. the later ones were not able to last the pace of field work. They usually failed after one year.

Ian

[Damo666]

I'm sure there's a guide to the pros and cons of the various Avometers on here, but I can't find it.

I was looking at purchasing the Avo 8, but like MerlinMaxwell, I ended up buying the MT2017 from Maplin. I also stumbled upon a NOS Simpson 260 for cheap on Gumtree so ended up buying that. Both are pretty good in my opinion.

[MrBungle]

I actually had one of those MT2017's. They don't last long from experience. I managed to kill it in about a month.

Simpson 260 was also an option as they have a slightly better battery situation (no 15v ones).

I managed to kill it in about a month.

How? so I don't do it.

[paulsherwin]

You can obviously have an Avo 8 professionally calibrated, but I wouldn't bother. You just need to confirm that it's working correctly and is reasonably accurate, which you can do with any variable voltage supply and a known good meter. If you wanted to take highly accurate precision measurements you wouldn't use an old Avo anyway nowadays.

[The Philpott]

Agree with Paul. By the time of the 8/III the individual shunts were quite stable anyway, so a basic calibration of the magnet gets you up and running. I do notice that later 8/III's have even more component resistors (as opposed to slab wound shunts) than the earlier ones. Reduces the possibility of a shorted winding giving bad readings (although they normally just go o/c rather than shorting)

Best accuracy on V at higher end of scale
*Best accuracy on A at lower end of scale
Ohms best in the centre, of course.

*Measuring high current (at very low voltage) does not produce honest readings since the inclusion of the meter in the circuit affects the current flow.

[MrBungle]

Exactly.

Originally Posted by merlinmaxwell

How? so I don't do it.

I was being a wally. Stick it in the off position, connect to circuit, power up, switch to 10A range by turning anti-clockwise, hit 50uA, bang!

Well that really wasn't the meters fault, go get another or two!

[McMurdo]

If you envisage having to repair one, the 70's ones with the flexi printed board are best avoided. The traces can vapourise with an overload and easily crack in multiple places if dismantling for an otherwise simple repair; the mylar goes brittle over time.
Some of the multiple shunts and printed multipliers are expensive and a bit fiddly if you want to make-up a substitute.

Personally I use an AVO 7 but it probably wont have the high impedance required for some microprocessor circuits (its ideal for valved stuff).

A model 8 has a much slower, greater damped movement than a 7 which can be tedious if checking lots of resistors.

I have 2 model 8's and 2 model 7's and the 8's always sit on the shelf, idle.

[mole42uk]

I rather like my EA-113 analogue AVO, it seems to do everything I need.