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Old 28th Apr 2005, 11:36 am   #1
fishcakes@clara.co.uk
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Question AVO model 40

The serial number is 407844-344

Can anyone tell me the voltage/type of batteries that are used for the ohms ranges?
Inside the battery compartment there are 2 distinct battery areas

1. The small compartment is approx 1 1/4" X 1 1/4" x 3 3/4" deep with small knurled nuts above it.

2. The second compartment is approx 2 5/8" long 2 1/2" deep 1 3/4" wide.
It looks as if it could take 2 U2 batteries but the connections are all at one end (brass strip).
Looking inside the AVO the batteries are connected in series.

3. There is also a fudge coil inside the unit connected in series with the 2 U sized batteries, any idea what resistance it should be (mine is open circuited)?

sorry about the Imperial measurements but an AVO of this age would be in the Imperial era.

Regards
Ian
 
Old 28th Apr 2005, 12:42 pm   #2
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Default Re: AVO model 40

Your right about imperial measurements being appropriate - your Avometer was made in 1944 - the last figures of the serial number are the month(s) and year of manufacture.

The Model 40 was designed to take a 1.5V cell in a square package which has been obsolete for a long time. Most users adapt a size 'D' cell to take its place. Avo used to sell a thin SRBP (Paxolin) frame with bowed strips of phosphor bronze at each, internal, end of the frame. Wires soldered to each bow had solder tags on the free ends to connect to the screw terminals.

For the higher resistance ranges, two flat 4.5V cycle lamp batteries are used. The IEC type number is 3LR12, similar to Duracell MN1203/Varta 4012. These are available from Halfords amongst others.

Let me know if you need further information.
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Old 28th Apr 2005, 3:12 pm   #3
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Question Re: AVO model 40

Thanks for the battery info.
only one last question do you know the resistance of the coil wound on a bobbin inside the AVO in series with the high resistance batteries.
Mine was open circuit but I have rewound it with the same wire to give a resistance of approx 79Ohms.
Would this be OK?
 
Old 28th Apr 2005, 7:37 pm   #4
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Default Re: AVO model 40

According to my circuit diagram, it should be 5977 ohm, but I'll check the sums in case it's a misprint.
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Old 28th Apr 2005, 8:18 pm   #5
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Default Re: AVO model 40

By my reckoning the 790 ohms should be the right value.

With the meter's 10 ohms, that would give 800 ohms total and allow the 9 volt battery to drive over 11 mA through a terminal short circuit. The Q knob would be used to reduce the meter sensitivity (6 mA) to this value, or allow the short-circuit terminals to register full scale 6 mA with a "flat" battery voltage of 4.8 volts.

The 5977 ohms on the circuit diagram would imply a "flat" battery voltage of 36 volts. Some models, with only a 1.5volt battery, needed an external DC supply (typically 60 to 100 volts) for high resistance ranges - perhaps the circuit value is a hangover from that.

Further research needed.
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Old 28th Apr 2005, 10:35 pm   #6
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Default Re: AVO model 40

Guest, Steve,

My diagrams, for the earliest to the latest versions of the Model 40, all give a value of 1977 ohms, although in some the print is not too clear. The service manual gives a tolerance of + or - 0.5%.

You have to wonder why this exact resistance value was chosen and why such a close tolerance was specified as both will be dominated by battery variations.
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Old 28th Apr 2005, 11:15 pm   #7
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Default Re: AVO model 40

You're the expert, Peter, but 1977 ohms would require a "flat" battery voltage of almost 12 volts. I'm confused, but my "Model 40" is a single-battery 47A, while my circuit diagram is for an unspecified (but 1.5 + 2x4.5 volt battery) 47A. Empiricism is the key ! Can our Guest zero (i.e. full-scale) his meter with a (2x4.5) volt battery and his re-wound resistor ? If so, we've all got a bit of thinking to do.

It's getting late, and my mind is getting addled. I look forward to continuing this debate tomorrow.

Steve
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Old 29th Apr 2005, 11:19 am   #8
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Default Re: AVO model 40

In the cold light of dawn the sums give the following:

with 790 ohms the Q-knob can adjust over a battery range of
3.3 to 10.4 volts.

with 1977 ohms the Q-knob can adjust over a battery range of
7.9 to 26.9 volts

with 5977 ohms the Q-knob can adjust over a battery range of
23.5 to 77.8 volts.

For a nominal 9 volt battery only the 790 ohm range is realistic.

The 5977 ohms range would suit an external DC supply as required for the single-battery military version.

There may be a version which prefers the "safer" 8 to 25 volt range (Peter will know)

So many of the circuit diagrams are either poor copies, or redraws of poor copies (even of poor originals), that it's easy for an error to get copied and propagated forever. Sorting out such mysteries is half the pleasure for AVO freaks like me.
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Old 29th Apr 2005, 12:36 pm   #9
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Default Re: AVO model 40

Quote:
So many of the circuit diagrams are either poor copies, or redraws of poor copies (even of poor originals), that it's easy for an error to get copied and propagated forever. Sorting out such mysteries is half the pleasure for AVO freaks like me.

As you say Steve, misprints do arise and can be perpetuated, and not just in circuit diagrams.

If I can find a Model 40, (;-), I will measure the resistance value of this coil and let you both know. This is likely to be a busy weekend, so it may be a day or two.

Last edited by Duke_Nukem; 3rd May 2005 at 8:15 am. Reason: Fixed quote
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Old 2nd May 2005, 8:36 pm   #10
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Default Re: AVO model 40

The Model 40 100K resistance range continued:-

I have measured the resistance of the coil in question on three Model 40s, one from the 1940s and two from the 1960s. The values are 1979, 1982 and 1977 ohms, so it would seem that the circuit diagrams are correct, which takes a bit of explanation.

I've drawn out a simplified circuit diagram of the 100K range which may be of some help (See attachment). I've also calculated the battery voltage range which corresponds to a resistance value of 1977 ohms and my figures agree closely with SPCH.

I think we may have started with a false assumtion because we assumed familiar conditions in which batteries and cells are used with loads which draw between several tens of millamperes up to 1A or more. In these circumstances we think of the end condition as being when the cell voltage falls to a figure of perhaps 0.8 or 1.0V. However, in a multimeter, the cell load is of a much lower order, in this case around 4 to 10mA for full scale deflection and the cell life is closer to shelf life than normal. In this case it may be more useful to consider cell life as being limited by internal resistance.

At the end of its life, a zinc carbon cell can still give an apparently useful open circuit voltage but this is only useful if it can give the required load current. In a multimeter, this limit comes when the cell internal resistance is much higher than would normally be acceptable.

To support this hypothesis, I managed to find some old cells and batteries, either kept for their size measurements or retrieved from their final destination. The test resistance was 2171 ohms.

Nato Cell No. 12 (1.5V Nominal)

V (oc) 1.408V I(load) 0.34 mA Vout 0.76V Rint 1.88K

Exide B23 cell (1.5V nominal)

V(oc) 1.456V I(load) 0.48mA Vout 1.06V Rint 825 ohms

Vidor SP2 (D cell)

V(oc) 1.492 I(load) 0.05mA Vout 112mV Rint 27.6K

Eveready R20B (D cell)

V(oc) 1.436 I(load) 0.15mA Vout 0.36V Rint 7.17K

Eveready R20

V(oc) 1.626 I(load) 0.73mA Vout 1.60V Rint 35.6 ohms

PP9 (Zinc Carbon)

V(oc) 9,65V I(load) 4.24mA Vout 9.31V Rint 80.2ohms

This is why Avo recommended that a current "flash" test was used to check resistance range batteries and cells. For more infromation see "Energizer Battery Application Manual" on their website.

It would seem that the high resistance of the 1977 ohm coil may be to stabilise changes in battery internal impedance in a similar way to the swamp stabilising temperature variation in the resistance of the moving coil.

The value of the 'Q' adjuster is chosen to cover other applications as well as this resistance range. It is really a variable sensitivity adjuster for the movement and is used in this way with the Model 7 and Heavy Duty Avometers. This is described in British Patent No. 476683. (The label is presumably chosen to group with R for rheostat and P for potentiometer which adjust for cell impedance and voltage respectively on the lower resistance ranges).

The Q adjuster was intended for a variety of purposes although, as far as I am aware, it is only used for resistance measurement in the Model 40. For example, it can be used with the 120V (AC or DC) ranges, and external supplies between 80 and 250 volts, to measure up to 1 megohm. (early instruction books warn of the need for care if using a mains supply). It is also used with the resistance range extension unit, for values up to 1.2 or 12 ohms.

If you've followed all this, welcome to the Avo anorak club.
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Old 2nd May 2005, 9:14 pm   #11
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Default Re: AVO model 40

Well done, Peter.

Most of us think in terms of (delta)V, but at low current I*(delta)R is more significant.

It just goes to show the danger of "thinking in boxes", and I - particularly as a "classical" engineer - feel very humbled for having been guilty of just that.

One of the dangers of modern engineering is taking the technology for granted; one of the triumphs of "old" engineering, is that they rarely did. The ancients (pre-1950) "knew" what they were really dealing with; we only think we know. As somebody (?) once said, in a similar connection, "the trouble with the world today is not the things we don't know, but all the things we do know which aren't right".

Alternatively: "Many people delude themselves that they are thinking, when they are merely rearranging their prejudices".

Mea culpa.
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Old 4th May 2005, 9:41 am   #12
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Question Re: AVO model 40

Thanks for the info especially the part Diagram.
The 100,000 Ohms resistance range is now working, I made up a 1980 Ohm resistor.
So that takes care of the 1,000 & 100,000 Ohm ranges.
But this meter must have been misused ie probably mains or a high voltage applied to the Ohms range because the 10,000 Ohms range is completely open circuit.
Help again please
What value resistance am I looking for, bearing in mind that there is a plethora of wirewound components in this units, the exact position if possible.
Last but not least any chance of a circuit diagram, however sketchy it is.
Regards from a new old anorak
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Old 4th May 2005, 11:43 am   #13
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Default Re: AVO model 40

I can send you a scruffy diagram if you give me your e-mail in a PM
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Old 4th May 2005, 1:22 pm   #14
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Default Re: AVO model 40

Guest,

If you want to send me a private message, I can copy the service manual and post it to you when I know your postal address.

Steve, I didn't realise that you were short of a Model 40 manual. One will be provided.

Where wrong assumptions are concerned, I'm afraid we are all miserable sinners. Your penance will be to check my redrawn Model 40 diagram against the original to make sure it's fit to post here. (PM on the way this evening).
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