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SPCh · 8th Nov 2006, 10:51 am

Thanks for the inputs.

The reason the Model 40 doesn't get so much coverage is that it's not so popular in our world as the later Models 7 and 8. It was introduced at the start of WWII, and although old doesn't mean bad, it was already too insensitive for the "new" electronics.

All AVOs have to be seen in context - and that includes their range sequences. In the very early days they were targeted at telephony and telegraphy, a world of electromagnetic relays,operating with battery voltages. The only significant AC was the mains (though not everywhere) and ringing voltages and currents.

With the advent of "real" electronics, greater sensitivity was needed (though not yet achievable) and higher and lower voltage and current ranges.

The "cleverest" AVO (in my opinion) was the Model 7 introduced in August 1936. It had a sensitive (for then) movement of just 1 mA, a range sequence based on 1/10/100 (for the first time ever), and switched ranges for power - in watts and decibels - and capacitance. The latter were clever applications of the "Q-knob" (which offered variable sensitivity on both AC and DC). There was also a comprehensive range of accessories to extend its capability and measuring applications. In the hands of someone who could drive it (and in those days people thought about what they were measuring and how) the Model 7 was, and still is, a very useful instrument. The targeted applications were very clearly valve radios and amplifiers, and the power reference values (=0dB) were arbitrary (200mW in 500, 5k and 50k ohms); the days of 1mW in 600 ohms were still to come.

The Model 8 was the first (volume production) high sensitivity (50uA) AVO; it was introduced in 1951 and targeted at the "modern" radio and television repair world. The ranges were based on a 1/2.5/10 sequence, which gave wider resolution by introducing intermediate scales between 1 & 10, and between 10 & 100, but this made the decibel step between adjacent ranges messy at +8/+12/+8/+12...

The Model 8 Mk V, with an even more sensitive movement, used the range sequence 1/3/10 which was rather neater for the "intermediate" ranges, and would have cleaned up the decibel steps to a tidy +10/+10/+10, except that the decibel scale was inexplicably deleted ! (I suspect that by the 1970s few radio and television fixers understood decibels, whilst the hi-fi freaks thought they were about window-rattling volume, or used VUs).

To complete the story, there was a Heavy Duty AVO introduced for the railway signalling market, and the Model 12 for the automotive industry (so, 6 12 and 24 volt applications covered by a 9/18/36 range sequence).

I think I may have more than answered the question; hope it helps/is of interest.

8th Nov 2006, 10:51 am	#147
SPCh Rest in Peace Join Date: Nov 2004 Location: Antwerp, Belgium Posts: 345	Re: AVO Multimeter survey Thanks for the inputs. The reason the Model 40 doesn't get so much coverage is that it's not so popular in our world as the later Models 7 and 8. It was introduced at the start of WWII, and although old doesn't mean bad, it was already too insensitive for the "new" electronics. All AVOs have to be seen in context - and that includes their range sequences. In the very early days they were targeted at telephony and telegraphy, a world of electromagnetic relays,operating with battery voltages. The only significant AC was the mains (though not everywhere) and ringing voltages and currents. With the advent of "real" electronics, greater sensitivity was needed (though not yet achievable) and higher and lower voltage and current ranges. The "cleverest" AVO (in my opinion) was the Model 7 introduced in August 1936. It had a sensitive (for then) movement of just 1 mA, a range sequence based on 1/10/100 (for the first time ever), and switched ranges for power - in watts and decibels - and capacitance. The latter were clever applications of the "Q-knob" (which offered variable sensitivity on both AC and DC). There was also a comprehensive range of accessories to extend its capability and measuring applications. In the hands of someone who could drive it (and in those days people thought about what they were measuring and how) the Model 7 was, and still is, a very useful instrument. The targeted applications were very clearly valve radios and amplifiers, and the power reference values (=0dB) were arbitrary (200mW in 500, 5k and 50k ohms); the days of 1mW in 600 ohms were still to come. The Model 8 was the first (volume production) high sensitivity (50uA) AVO; it was introduced in 1951 and targeted at the "modern" radio and television repair world. The ranges were based on a 1/2.5/10 sequence, which gave wider resolution by introducing intermediate scales between 1 & 10, and between 10 & 100, but this made the decibel step between adjacent ranges messy at +8/+12/+8/+12... The Model 8 Mk V, with an even more sensitive movement, used the range sequence 1/3/10 which was rather neater for the "intermediate" ranges, and would have cleaned up the decibel steps to a tidy +10/+10/+10, except that the decibel scale was inexplicably deleted ! (I suspect that by the 1970s few radio and television fixers understood decibels, whilst the hi-fi freaks thought they were about window-rattling volume, or used VUs). To complete the story, there was a Heavy Duty AVO introduced for the railway signalling market, and the Model 12 for the automotive industry (so, 6 12 and 24 volt applications covered by a 9/18/36 range sequence). I think I may have more than answered the question; hope it helps/is of interest.