I've mentioned the limitations of AVO meters before, and Rob's recent comments about the AVO 7 prompts me to raise it again.
Co-incidentally, it was a topic covered by Keith Wevill ('AKA Keith's TVs') in his excellent talk at last Sunday's Golborne Swapmeet. With his kind permission I've attached Keith's notes, which run to 32 pages. It will be evident that Keith must have spent a great deal of time and thought writing the notes, preparing his talk and sharing his expertise with us. He says that there are several items he left out due to time constraints, and that he would have liked to have shown the effect of the scope probe on the local oscillator, so maybe Mark R will be able to twist Keith's arm for a follow-up talk at a future Swapmeet? I hope so!
Keith explains the effects of various multimeters on pages 14 - 17. It's worth noting the worked example on page 16, of the effect of three different meters placed across a 1 Meg resistor at a point in a circuit at which theoretically there should be 6 Volts. Using a digital multimeter with a 10 MegOhm impedance, the result would be 5.714 Volts. A DMM with an impedance of 1 MegOhm would show 4 Volts, and a 20,000 OPV analogue meter (such as a much loved AVO 8) would show just 1.714 Volts. In Keith's worked example, an AVO 7, at only 1,000 OPV, would show even less - nearer to 1V.
I know that many forum members own Avos which they enjoy collecting and using to restore vintage radios, and since the original voltages on data sheets of valve radios will have been taken using an Avo 8 as a rule, which is 20,000 Ohms per Volt impedance - the norm for its day - the voltage readings in a working set should be similar to the datasheet. And I know that many like the idea of using vintage test gear to restore vintage radios. On valve radios, in which the voltages are quite high, the loading effect of putting a 20,000 Ohms per Volt meter across a resistor at a point at which say 100 Volts is expected would be negligible. It will have the effect of putting a 2 Megohm resistor across the circuit. (100 Volts x 20,000 Ohms), or even greater it the meter is set to a higher voltage range.
However, on low voltage solid state gear, a multimeter with an impedance of only 20,000 Ohms per Volt can become problematical. If for example, we expect 0.5V at the base of a transistor, and we plonk an Avo 8 from the base to chassis, we are effectively putting a 10k resistor into the circuit (0.5 x 20,000 OPV). Apart from altering the voltage and giving a lower than esxpected reading, it may even stop the circuit from working - an oscillator for example.
The formula for calculating the voltage in a potential divider is V2 = V1 x R2/R1+R2
There’s a useful parallel resistor calculator at this link:
http://www.sengpielaudio.com/calculator-paralresist.htm
I’m not knocking Avos, which have a loyal following, and this isn’t a case of analogue v digital – there are some applications where it’s useful to have a rising and falling needle rather than digits bobbing about in a meaningless fashion - when peaking a tuned circuit for example, using an RF probe. What I’m saying is we need to be aware of the limitations of using meters with an impedance of just 20,000 Ohms per Volt when working on modern solid state equipment.
Hope that's of interest, and more so, Keith's excellent notes, which are attached. I'm not sure if this is the right section in which to put Keith's notes - I'd suggest that they should be put where they aren't lost among other posts, but at there for ready reference. Maybe a Mod would pick up on that?
Co-incidentally, it was a topic covered by Keith Wevill ('AKA Keith's TVs') in his excellent talk at last Sunday's Golborne Swapmeet. With his kind permission I've attached Keith's notes, which run to 32 pages. It will be evident that Keith must have spent a great deal of time and thought writing the notes, preparing his talk and sharing his expertise with us. He says that there are several items he left out due to time constraints, and that he would have liked to have shown the effect of the scope probe on the local oscillator, so maybe Mark R will be able to twist Keith's arm for a follow-up talk at a future Swapmeet? I hope so!
Keith explains the effects of various multimeters on pages 14 - 17. It's worth noting the worked example on page 16, of the effect of three different meters placed across a 1 Meg resistor at a point in a circuit at which theoretically there should be 6 Volts. Using a digital multimeter with a 10 MegOhm impedance, the result would be 5.714 Volts. A DMM with an impedance of 1 MegOhm would show 4 Volts, and a 20,000 OPV analogue meter (such as a much loved AVO 8) would show just 1.714 Volts. In Keith's worked example, an AVO 7, at only 1,000 OPV, would show even less - nearer to 1V.
I know that many forum members own Avos which they enjoy collecting and using to restore vintage radios, and since the original voltages on data sheets of valve radios will have been taken using an Avo 8 as a rule, which is 20,000 Ohms per Volt impedance - the norm for its day - the voltage readings in a working set should be similar to the datasheet. And I know that many like the idea of using vintage test gear to restore vintage radios. On valve radios, in which the voltages are quite high, the loading effect of putting a 20,000 Ohms per Volt meter across a resistor at a point at which say 100 Volts is expected would be negligible. It will have the effect of putting a 2 Megohm resistor across the circuit. (100 Volts x 20,000 Ohms), or even greater it the meter is set to a higher voltage range.
However, on low voltage solid state gear, a multimeter with an impedance of only 20,000 Ohms per Volt can become problematical. If for example, we expect 0.5V at the base of a transistor, and we plonk an Avo 8 from the base to chassis, we are effectively putting a 10k resistor into the circuit (0.5 x 20,000 OPV). Apart from altering the voltage and giving a lower than esxpected reading, it may even stop the circuit from working - an oscillator for example.
The formula for calculating the voltage in a potential divider is V2 = V1 x R2/R1+R2
There’s a useful parallel resistor calculator at this link:
http://www.sengpielaudio.com/calculator-paralresist.htm
I’m not knocking Avos, which have a loyal following, and this isn’t a case of analogue v digital – there are some applications where it’s useful to have a rising and falling needle rather than digits bobbing about in a meaningless fashion - when peaking a tuned circuit for example, using an RF probe. What I’m saying is we need to be aware of the limitations of using meters with an impedance of just 20,000 Ohms per Volt when working on modern solid state equipment.
Hope that's of interest, and more so, Keith's excellent notes, which are attached. I'm not sure if this is the right section in which to put Keith's notes - I'd suggest that they should be put where they aren't lost among other posts, but at there for ready reference. Maybe a Mod would pick up on that?
Regards, David.
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
BVWS Member.
G-QRP Club Member 1339.
'I'm in my own little world, but I'm happy, and they know me here'
