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Old 1st May 2023, 12:34 pm   #3181
The Philpott
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Default Re: AVO Multimeter survey

Viz:-

I think this is close to the methodology recommended by a Japanese gentleman, whose name long since escapes me. I suppose that the philosophy is that getting your booster onto the concentrator makes best use of the extra flux.

Note that the magnetic shunt is blocked from moving in this meter, but it turned out that it was not an issue- calibration was achieved by shuffling the magnet around and re-checking.

Dave
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Old 1st May 2023, 1:50 pm   #3182
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Default Re: AVO Multimeter survey

Just re-checked the accuracy of the Model D in Post No.3184 on DC volts.

Inputs at intervals from 7% to 83% of FSD, most readings slightly high but by no more than 0.66% of FSD on the given range.

Worst results as regards %age of INPUT is 10v reads as 10.2v on the 30v scale, and 7.5v reads as 7.6v on the 15v scale.

These last two result correspond to around 33% and 50% of full scale, so might indicate a bit of non-linearity at these points. I was pleased enough with this though! This was in a meter which on arrival was reading short by 6 to 7% of input values.

Dave
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Old 1st May 2023, 2:36 pm   #3183
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Default Re: AVO Multimeter survey

That’s not bad at all, Dave. Considering you’ve got a single Neodymium magnet fixed on one side of the concentrator, some non-linearity of the flux within the air gap is almost to be expected.

If you wanted to be really pedantic, you could draw up a calibration graph. Mind you, how many of us use our Avos in circumstances where a 1% reading error is crucial? In my case I still use a Model 7 for vintage radio work, and readings are either “pretty close”, which is okay, or “miles out” - and you’ve found a fault.
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Old 2nd May 2023, 2:40 pm   #3184
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Default Re: AVO Multimeter survey

Taken in conjunction with Ian's pictures it does give a good idea of roughly how much boost is required for a given loss of flux. Small powerful magnets close to the action are definitely the way to go. I've seen it done with large, traditional, less powerful magnets glued at various points around the horseshoe, which looks tedious.

Model D-

Input as %age of FSD..........Error as %age of FSD
7...........................................+0.02
33.........................................+0.66
50.........................................+0.66
66.........................................+0.33
83.........................................+0.066
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Old 2nd May 2023, 2:43 pm   #3185
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Default Re: AVO Multimeter survey

Interesting, Dave. Slightly stronger flux mid-scale. Still bloomin’ good for a WWII-era meter though!
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Old 5th May 2023, 9:32 am   #3186
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Default Re: AVO Multimeter survey

I can't test the performance of the Movement I worked on yet, as it's just
too sticky to be useful.

But now that the Flux has been restored, it means it's at least now worth striping
down and cleaning it up.

Once it's all back together, I will test the Linearity with respect to the original
Scaleplate and post the results.



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Old 6th May 2023, 1:25 pm   #3187
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Default Re: AVO Multimeter survey

Thanks for the magnets, Ian! They arrived this morning. I’ve popped a note on the “Thanks Thread” too. Now to find a free hour to play…!
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Old 8th May 2023, 1:27 pm   #3188
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Default Re: AVO Multimeter survey

Quote:
Originally Posted by Phil G4SPZ View Post
It would only have made sense for Avo to do this if they could use all the standard multipliers and shunts from the Type D, so they must have adapted the 1mA movement to read 2.5mA FSD whilst exhibiting the correct overall resistance for the Type D, by inserting specially-made series and shunt resistances.
Phil,

I haven't confirmed this on the Meter yet, as I've not had a lot of time to look at it, but looking at the Schematics for the Model 7 and the Type D, it looks like the
conversion is probably quite straightforward.

The Movement resistance for the Type D is 20 Ohms @ 2.5mA FSD, and for the Model 7 the Movement resistance is 50 Ohms @ 1 mA FSD.

Conveniently, 50 Ohms is 2.5x the resistance of 20 Ohms, so all other things being equal in the Meter circuits there will be 2.5x less current available for the 1mA Movement, which
of course is 1 mA. Bingo! Job done.

So, the only component changes other than the Movement itself, would be the Series and Shunt resistances required for the different K1 and K2 sensitivities.
Unless I am missing something.

I will confirm this for sure when I get a chance to inspect the Components, but this would make the most sense.

I suspect, that the Model 7 Front Panel was probably deliberate in order to highlight a 1mA Movement being fitted.

I guess they ran short of 2.5mA Movements, as these were unique (as far as I know) to the Type D.


Quote:
Originally Posted by Phil G4SPZ View Post
Thanks for the magnets, Ian! They arrived this morning.
Excellent, glad they arrived , and didn't as you said "get stuck in the post" , well the Ferrous
parts of the Postal system that is



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Old 5th Jun 2023, 11:07 pm   #3189
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Default Re: AVO Multimeter survey

1)
I have a "Universal AvoMeter" Model 8 Mk. II

No. 139309-*-462

* The letter is either a "G" or "O" as it's been partially scratched.

It came form Rugby College (as was), thoroughly student abused over the decades, I don't think any of the current shunts are still intact!


2)
I also have a much later Model 8 Mk. V, serial No on a sticker on the side, and also in battery compartment:

GC No. 0042954
8V

(Also rescued from the old Rugby College, but survived student abuse).

This one also carries a sticker: Supplied by Universal Instruments, Leire, Leicestershire, dated: 31/01/1983

Coincidentally, I live just 10 minutes drive from the village of Leire!

Cheers, Scott.
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Old 8th Jun 2023, 5:34 pm   #3190
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Default Re: AVO Multimeter survey

Afternoon All,

Hope this is the right place to ask; I picked-up a nice AVO8 locally, everything seems to work ok, battery compartment is nice and clean; however, the front of the meter is rather dull, and many of the white engraved markings are missing their paint (?)

Please can you AVO experts advise the best course of action for a) restoring the shine, and b) touching-up the range markings.

I've given it a gentle wipe with a slightly damp cloth, and surprisingly, it wasn't that dirty; the leather case it came with (although very tatty) has done its job well; it's just a bit dull.

Many thanks!
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Old 8th Jun 2023, 6:22 pm   #3191
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Default Re: AVO Multimeter survey

Hi Max and welcome to Avo ownership!

This thread is probably the wrong place to ask. Yes, most Avo enthusiasts are here, but this thread is mainly a survey of what Avos survive. There are also likely to be several alternative suggestions and responses to the questions you ask, and there would probably be more space if you were to post a new thread in the Vintage Test Gear section.

I have restored many Avometers and have my own methods of cosmetic restoration - feel free to send me a PM and I’ll happily reply in greater detail.
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Old 7th Jul 2023, 11:52 am   #3192
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Default Re: AVO Multimeter survey

This one may already be on the database as I obtained it from another member on here a few months ago and it has been sitting on the 'round-tuit' shelf. Its an AVO Model 7 MkII with a serial number of 77500-A-1058. It has some resemblance to the Model 40 MkII as it has the two PF sockets.
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Old 7th Jul 2023, 12:59 pm   #3193
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Default Re: AVO Multimeter survey

The letter 'A' denotes a slight change in the circuitry (possibly inclusion of the PF facility?) The circuit diagram in my (Feb 1952) manual applies to meters with 'A' in the serial number.

I did notice that both of my early Model 7's are not good for measuring low DC current/s that are being pushed by low voltage/s. I also sussed that a break in the transformer windings disabled ALL AC ranges, not just AC current!

I think that the 7 at the start of the serial denotes Model 7, with it being instrument number 7500 from October 1958. Ian/Peter/Phil can confirm.

Dave
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Old 8th Jul 2023, 10:50 am   #3194
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Default Re: AVO Multimeter survey

That's a New Number to the Survey. It hasn't been reported before.

In this particular case, the leading 7 doesn't actually mean "Model 7".

This Meter just happens to fall within the 5 Digit 70k Serial Number
range which covered early 1957 until early 1959.

The Leading 7 found on the original model 7's up until 1947 did actually signify
that the Model was a 7. The Format used at that time was:
7 (Date Code) - (Serial Number).

Regarding the "A" in the middle, it has been suggested that this might have meant
an Alnico Magnet Movement installed.
Although I have no proof of this, the "A" did appear in Serial Numbers for the first time
around late 1947 when the Horseshoe Magnet Movements were being phased out and
replaced by the Alnico 1mA Movements.

I did ask some time back if anybody with a 1947 Avo Model 7 could report what type of
Movement it had, in order to try and determine if it was the "A" or the Serial No Format change
(Both happened in 1947) that signified the change of Movement.
Sadly, there was no response.


At the same time, the Model 40's started to appear with "B" in the Serial Numbers,
so it is possible the "B" meant Alnico 3mA Movements but that is pure speculation
at this time.

Regarding the PF sockets, these seem to have appeared around 1951, although
that is always subject to change if earlier PF Meters turn up.


Ian
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Old 8th Jul 2023, 5:11 pm   #3195
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Default Re: AVO Multimeter survey

It doesn't have a horseshoe magnet. I don't see Alnico written on the dial or movement anywhere but hopefully someone will recognize it from the photos. The dial has a part number: 10945 -19.A which may be a clue. Not sure whether the .A is significant or not.
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Old 8th Jul 2023, 5:43 pm   #3196
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Default Re: AVO Multimeter survey

Your Meter won't have a Horseshoe Movement, it's far too late for that.
As mentioned, the Horseshoe Movements were phased out in 1947.

The Photo shows what I would expect for a 1958 Meter.
The "A" is consistent with a Meter of that age.

Alnico refers to the Magnetic Material type, not the Manufacturer.
You won't find Alnico printed or mentioned on the Movement, unless by
some chance the "A" is indeed a reference to the new Magnet type.
(New being relative to the Time of course).

I don't have the info handy at the moment, but as I remember, Avo later
moved onto Alcomax Magnets and they weren't painted.
I think PMM knows more about this area than I do.


Ian
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Old 16th Jul 2023, 8:04 pm   #3197
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Default Re: AVO Multimeter survey

A dirty but restorable Model 2 (later model, fused, smooth panel) from the Reading McMichael rally today for £12!
Serial number 1627 (not already on register).
Can someone (Peter M?) tell me what the fuse should be? It's blown, and was rattling around inside. The circuit just shows it as "4 ohms"!
One broken connection inside, now mended. A cracked meter glass, but replaceable. Rheostat completely missing. And a lovely original "Size T" battery inside, not leaking, and still showing 1.3V! The meter coil shows continuity and the coil moves when my DMM ohms range is applied correctly. So all is promising for a restoration to cleanliness and function in due course!
all best
Jeremy
(who does not need yet another Model 2, but who's counting!)
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Old 17th Jul 2023, 12:52 pm   #3198
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Default Re: AVO Multimeter survey

Hi Jeremy, breaking current of fuse 'as low as possible' i suppose! As far as resistance goes, 4 ohms is what i have written down as the value for my very tired Model 4. The accuracy of the meter depends on it.

Is there a printed label on the inside of the battery compartment lid giving prices for replacement fuses and cells?

Dave
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Old 17th Jul 2023, 1:36 pm   #3199
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Default Re: AVO Multimeter survey

Hi Dave,
There is no label inside the battery lid. nor inside the lids of my 4 others! This new one has no instruction label either. Do you have a sample label I could apply?
After a couple of hours of careful cleaning inside and out with cotton buds, antistatic cleaner and clean cotton cloths, the dial (although very brown) is back in and dirt-free and the dial glass is clear. I decided to leave the cracked glass in. The crack is clean, and it tells part of the story of this meter. I'm also leaving the rather original-looking leads on, with early bulgin croc clips on the ends. The switch contacts needed a bit of DeOxit, applied with a toothpick.
And a final test with a short instead of the fuse still gives a reading of 11.7V for 12V input so that's not bad at all. Result.
-Jeremy
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Old 17th Jul 2023, 5:13 pm   #3200
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Default Re: AVO Multimeter survey

I seem to have forgotten to log Avo Model 2 (later, smooth, with fuse) serial number 5923.
Also, my Model 2 already logged as serial 6062 is actually 6052 on close inspection.
And my latest Model 2 serial number is an anomaly. Serial number 1627 puts it quite early on in the list of 19 Model 2's in the 3rd Edition Avo List. I wonder if an earlier movement found its way into a later fused casing?
By the way, which is the latest list issued? - I have up to #3.
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