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Old 13th May 2005, 7:26 pm   #1
Alf
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Default Avo "Heavy Duty" multimeter

I've got an AVO heavy duty multimeter which I'd like to know the approximate date of manufacture, the last three serial numbers are 675 but I'm sure it's older than that. There's hardly any reference to these meters anywhere.
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Old 16th May 2005, 9:00 pm   #2
pmmunro
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Default Re: Avo "Heavy Duty" multimeter

Alf,

The original Heavy Duty Avometer was designed at the request of the Great Western Railway Signal and Telecommunications Section (probably they had a simpler title at the time). It had ranges suitable for this aplication, and possibly deliberately unsuitable for other, commoner, uses. This meter has a heavy diecast aluminium front plate and case and the brass range switch escutcheon is riveted to the front panel. The movement is similar in construction to the Avominor movement It is likely that only a few, if any were delivered before the GWR was absorbed into British Railways in 1948. The example which I have is dated April 1948 and marked "Property of GWR" despite that company's having having ceased to exist from the 1st of January. Perhaps someone just couldn't believe that the GWR did not still exist.

A "civilian" version for general industrial use was available by 1949 and this had more conventional ranges and an overload cut-out. Later versions had a Bakelite front panel and a deep drawn aluminium case.

The commonest version is the Heavy Duty Mark V, from about 1956, which has a Bakelite front panel and case and the familiar Avometer red and black terminal sockets. The HD5, as it was later known, was listed in Avo price lists at least as late as April 1980, when it cost £105.50; a Model 8 Mark V was £ 98.50 in the same list. Railway versions too seem to have been made well into the 1970s.

Unless you have good reason to think otherwise, it is very likely that your meter was made in June 1975.

Let me know if you need further information.
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Old 16th May 2005, 11:46 pm   #3
Alf
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Default Re: Avo "Heavy Duty" multimeter

Thanks for the information.

What made me think it was older was the meter sensitivity at only 1kohm/volt dc, 500ohm/volt on ac and only 200ohms/volt on the 10v ac range, it seemed a bit basic to me. I had assumed it wasn't designed with the radio/tv trade in mind and now you've explained it.

Non-the-less it's an ideal little meter for general purpose use, sturdy and rugged.
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Old 17th May 2005, 1:11 pm   #4
pmmunro
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Default Re: Avo "Heavy Duty" multimeter

Alf,

Those of us who come from an electronics background tend to regard a high value of "ohms per volt" as highly desireable, which is generally true in electronics circuits which incorporate high impedances.

However, in 'power' circuits a high impedance is not always helpful. Examples are situations in which a high resistance joint or switch contact is causing a voltage drop. Testing with a high resistance meter suggests that the expected supply voltage is available when in fact it will drop severely as soon as any significant current is drawn.

Another example of a low "ohms per volt" Avometer is the Model 12, designed for auto-electrical work and produced from around 1960 to 1980.

It is not always recognised that hand-held digital multimeters are also produced in "electricians" versions which have an impedance of 1000 ohms per volt for this very reason.

When occupied with auto-electrical systems some years ago, I made up a test set (ammeter/voltmeter) in which I incorporated dial lamps for the instruments with a switch so that the loading effect of the voltmeter could be changed to test for high circuit impedance.

It should also be remembered that an analogue multimeter needs a high resistance movement for high sensitivity and this is most advantageous when measuring voltage. For current ranges however, the "trade-off" is a high resistance and consequent voltage drop. Here the low impedance meter will cause less disturbance.
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Old 17th May 2005, 9:51 pm   #5
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Default Re: Avo "Heavy Duty" multimeter

Just as a matter of interest I have a Heavy Duty model with the last three numbers 957 (September 57) and it is as described with Bakelite front panel and case and the red and black terminals.
 
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