Voltage critical
Psychophysiological entity
Are there any reasonably priced DVMs that read true RMS?
tis why I keep my old AVO 8. Nowt like an AVO 8
tis why I keep my old AVO 8. Nowt like an AVO 8
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Bro Drapers kit delivered this morning,he's back up and running
different cable modem different power supply,12 volt switched buggah this time,hung onto his old modem.
Not bad service from Virgin.
different cable modem different power supply,12 volt switched buggah this time,hung onto his old modem.
Not bad service from Virgin.
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The daft blighters in the lab at work "had" to have their meters calibrated every year. They worked out that it was cheaper to buy a cheapish Chinese job every year than have the AVO calibrated.
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Rivets
I have never been aware of any version of the AVO8 that measured true RMS. I have just checked the schematic of the AVO8 Mk 5 (original and later versions) and it is simply a dumb old average-reading RMS-calibrated moving coil meter. The RMS calibration is true only for sinewaves and if I recall correctly the fudge factor is 1.11
True RMS meters belong to a later era, the first I am aware of was made by Advance (later Gould) at Hainault during the early seventies and rebranded in Germany and the USA.
In audio work during the seventies we generally used quasi-RMS circuits which read a combination of average and peak. I used the Analogue Devices True RMS IC in audio instrumentation designs from the mid-seventies onward.
I have never been aware of any version of the AVO8 that measured true RMS. I have just checked the schematic of the AVO8 Mk 5 (original and later versions) and it is simply a dumb old average-reading RMS-calibrated moving coil meter. The RMS calibration is true only for sinewaves and if I recall correctly the fudge factor is 1.11
True RMS meters belong to a later era, the first I am aware of was made by Advance (later Gould) at Hainault during the early seventies and rebranded in Germany and the USA.
In audio work during the seventies we generally used quasi-RMS circuits which read a combination of average and peak. I used the Analogue Devices True RMS IC in audio instrumentation designs from the mid-seventies onward.
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Yes fudge factor 1.11 being the ratio of
Vav = 2/pi x Vpk (for sinewaves)
Vrms = 1/sqrt2 x Vpk (for sinewaves)
Apologies to Analog Devices for spelling their name in the British sub-dialect of English!
Vav = 2/pi x Vpk (for sinewaves)
Vrms = 1/sqrt2 x Vpk (for sinewaves)
Apologies to Analog Devices for spelling their name in the British sub-dialect of English!
Psychophysiological entity
I'd assumed a perfect sinewave, but of course that is more often than not the case. I've got this distant memory of the RMS being .7 of the peak. Heck, for telly mending, we didn't have to get things more than vaguely right.
So, 240v mains X 1.42857 would be 342.857 - ish. That seems high. Perhaps it was .77, which gives an answer closer to my memory. The1950s seem a long way away sometimes.
(Not that our mains is 240 anymore.)
So, 240v mains X 1.42857 would be 342.857 - ish. That seems high. Perhaps it was .77, which gives an answer closer to my memory. The1950s seem a long way away sometimes.
(Not that our mains is 240 anymore.)
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The British 240V mains is nominally 230V rms
Vpk = sqrt2 x 230V = 325V
Vav = 2/pi x 325V = 207V
If you look at your AVO8 you will see that 207V on the dc scale (which is what it is measuring) corresponds with 230V on the ac scale (fudged to indicate rms but only for sinewaves). The fudge factor 1.11 has been used when making the ac scale.
Vpk = sqrt2 x 230V = 325V
Vav = 2/pi x 325V = 207V
If you look at your AVO8 you will see that 207V on the dc scale (which is what it is measuring) corresponds with 230V on the ac scale (fudged to indicate rms but only for sinewaves). The fudge factor 1.11 has been used when making the ac scale.
Psychophysiological entity
I recall on my AVO 8 Mk5 that the scale seemed disappointingly simplified compared to my earlier meters. It only has the 1 - 10 and 1 - 3 for V and A. plus the ohms scale. Certainly not a separate AC scale.
I've just had to look at it to make sure. (only had it 30 years) The older one I have back in the UK presumably has a separate scale. I'll find out next summer.
His kid is bigger that this little guy was then. That's the meter I've got here, so 30years??? Time flies.
No, CORRECTION. That's the older one. Much more cluttered scale. Fooled cos it looks so new.
I've just had to look at it to make sure. (only had it 30 years) The older one I have back in the UK presumably has a separate scale. I'll find out next summer.
His kid is bigger that this little guy was then. That's the meter I've got here, so 30years??? Time flies.
No, CORRECTION. That's the older one. Much more cluttered scale. Fooled cos it looks so new.
Last edited by Loose rivets; 30th Nov 2012 at 16:02.
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Rivets
There is a schematic here:
http://www.petervis.com/electronics%...%20Diagram.gif
I've made it a link so you can print it out full size.
You will see that the ac and dc circuitry has been separated, ac on the left and dc on the right. If you care to calculate your way through you will find that there is a 1.11 ratio in sensitivity.
However this is drawing attention away from my point. The AVO8 is not true RMS.
There is a schematic here:
http://www.petervis.com/electronics%...%20Diagram.gif
I've made it a link so you can print it out full size.
You will see that the ac and dc circuitry has been separated, ac on the left and dc on the right. If you care to calculate your way through you will find that there is a 1.11 ratio in sensitivity.
However this is drawing attention away from my point. The AVO8 is not true RMS.
Psychophysiological entity
There's a guy on here who is collecting records of all AVOs worldwide.
UK Vintage Radio Repair and Restoration Discussion Forum
Too much chat is discouraged on the forum because there are not many mods. They have a Face-b thread now and show pictures of AVOs I never dreamed existed.
Last time home I sold dozens of unused movements for the VCM 163 valve (tube) tester which I happened to have in storage. They were however, made in Italy, and not of the supreme quality of the AVO 8's movement. All seems so labor-intensive now and indeed, that's why the meter is no longer made.
Just making the spring was a work of dedication, a bit like hand-painting the line on the side of Rolls Royce cars.
Rapid Electronics in Colchester were I believe, selling them until the end.
UK Vintage Radio Repair and Restoration Discussion Forum
Too much chat is discouraged on the forum because there are not many mods. They have a Face-b thread now and show pictures of AVOs I never dreamed existed.
Last time home I sold dozens of unused movements for the VCM 163 valve (tube) tester which I happened to have in storage. They were however, made in Italy, and not of the supreme quality of the AVO 8's movement. All seems so labor-intensive now and indeed, that's why the meter is no longer made.
Just making the spring was a work of dedication, a bit like hand-painting the line on the side of Rolls Royce cars.
Rapid Electronics in Colchester were I believe, selling them until the end.
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I picked up a job lot of old (really old) AVOs a couple of years ago. One of them had UK mil markings (the prisoner's arrow, or W ^ D). I gave it to daughter 1's FIL who had always wanted an AVO. I couldn't see which model it was.
If I really care about waveform (sometimes I do, mostly I don't) then I fetch the oscilloscope from under the desk. Most of the time, the RMS fudge factor will do me nicely.
I was told last week that the UK mains supply now has to be a minimum of 220V (not 230, nor 240) at the meter. We just had the overhead cables to the church moved underground, and the contractors were grumbling about the heavier-duty cable they had to use because of the length of the run and the load we put on it. I think that explains why the lights used to dim when we turned the heating on.
If I really care about waveform (sometimes I do, mostly I don't) then I fetch the oscilloscope from under the desk. Most of the time, the RMS fudge factor will do me nicely.
I was told last week that the UK mains supply now has to be a minimum of 220V (not 230, nor 240) at the meter. We just had the overhead cables to the church moved underground, and the contractors were grumbling about the heavier-duty cable they had to use because of the length of the run and the load we put on it. I think that explains why the lights used to dim when we turned the heating on.
Psychophysiological entity
Hee hee . . . here, if you blink just as the A/C kicks in, you think you've gone blind. We're about the 3rd house down from the nearest transformer.
Point taken about the AVO 8 not reading true RMS, but just another aside: You'll note in the schematic the movement is 37.5 micro amps. By some means or another, they make that become 50.
When I was testing the movements prior to shipping, I set my UK AVO to 50 micro amps DC current. The movements then (mostly) agreed with the AVO.
I found one of the resistance settings provided me with 70% of 50, so I could used the battery in the meter for a quick check.
I spent days packing these unites. Shipped them to five different countries.
I seem to remember this man's name. Just look at the white highlighted bit lower down! Numbers look familiar?
I had both these movements - 72, I think, but the one with the knob seems to be hiding from me. Spent hours looking for them before heading back to Texas. Might still be there, but getting a bad feeling someone from the storage place may have stood on a box and slung them out. The knob ones could make 200 quid each.
People are restoring these - even re-manufacturing the complex switches. They really do analyze the valves in great detail.
Tube Tester AVO V.C.M. 163
Point taken about the AVO 8 not reading true RMS, but just another aside: You'll note in the schematic the movement is 37.5 micro amps. By some means or another, they make that become 50.
When I was testing the movements prior to shipping, I set my UK AVO to 50 micro amps DC current. The movements then (mostly) agreed with the AVO.
I found one of the resistance settings provided me with 70% of 50, so I could used the battery in the meter for a quick check.
I spent days packing these unites. Shipped them to five different countries.
I seem to remember this man's name. Just look at the white highlighted bit lower down! Numbers look familiar?
I had both these movements - 72, I think, but the one with the knob seems to be hiding from me. Spent hours looking for them before heading back to Texas. Might still be there, but getting a bad feeling someone from the storage place may have stood on a box and slung them out. The knob ones could make 200 quid each.
People are restoring these - even re-manufacturing the complex switches. They really do analyze the valves in great detail.
Tube Tester AVO V.C.M. 163
