04-08-2011, 08:27 PM
(This post was last modified: 14-10-2011, 11:11 AM by Mark Hennessy.)
Your AVO and your B&K are two very different animals, and each should be reserved for its most approriate application. Your AVO has an input impedance of only 20,000 Ohms per Volt, which is fine for checking vintage radios, where the voltages being measure are generally quite high, so the loading effect of plonking such a low impedance meter across the circuit won't be too great, and where the voltages stated in the service data were, as often as not, measured using an AVO 8.
However, as I and others have mentioned before, for use on solid state equipment, where the voltages being measured are sometimes a fraction of a volt, an AVO will, at best, give very misleading readings due to the loading effect of the AVO, and at worse, stop an otherwise operational part of a circuit from working - an oscillator for example.
On solid state stuff, best leave the AVO on the shelf along with radios of the same vintage, and use your B&K, which has an input impedance of several million Ohms. You also have the advantage of an analogue meter when peaking circuits, where you can see the needle rise then fall, rather than a DVM, which though it will have a similar impedance to your B&K, the digits bob up and down and are less easy to observe.
Quite apart from its obvious superiority to an AVO, as you say, your 'B&K 375' has a separate scale which comes into view for each range selected. Very elegant, very functional and nice and clear.
It will in fact be the best piece of test gear that you are ever likely to own. Guard it with your life!
I'm not knocking AVOs, which are held in great affection by many who enjoy collecting them and using them when restoring vintage radios, and were the standard test instrument for decades - not just multimeters, but other well made and robust items of test gear, notably valve testers and signal generators, but it's important to understand their limitations, notably the low input impedance of 20,000 Ohms per Volt. (1000 OPV for the Model 7).
A DMM for most purposes is a far better bet than an AVO, and though a DMM (Like a VTVM) will read a little higher than an AVO 8 when compared to stated voltage in service data, it won't be much higher. A DMM is also much more convenient for measuring resistance than the cramped non-linear scale of an AVO. A DMM is also much more convenient for measuring resistance than the cramped non-linear scale of an AVO.
Dribbled David.
However, as I and others have mentioned before, for use on solid state equipment, where the voltages being measured are sometimes a fraction of a volt, an AVO will, at best, give very misleading readings due to the loading effect of the AVO, and at worse, stop an otherwise operational part of a circuit from working - an oscillator for example.
On solid state stuff, best leave the AVO on the shelf along with radios of the same vintage, and use your B&K, which has an input impedance of several million Ohms. You also have the advantage of an analogue meter when peaking circuits, where you can see the needle rise then fall, rather than a DVM, which though it will have a similar impedance to your B&K, the digits bob up and down and are less easy to observe.
Quite apart from its obvious superiority to an AVO, as you say, your 'B&K 375' has a separate scale which comes into view for each range selected. Very elegant, very functional and nice and clear.
It will in fact be the best piece of test gear that you are ever likely to own. Guard it with your life!
I'm not knocking AVOs, which are held in great affection by many who enjoy collecting them and using them when restoring vintage radios, and were the standard test instrument for decades - not just multimeters, but other well made and robust items of test gear, notably valve testers and signal generators, but it's important to understand their limitations, notably the low input impedance of 20,000 Ohms per Volt. (1000 OPV for the Model 7).
A DMM for most purposes is a far better bet than an AVO, and though a DMM (Like a VTVM) will read a little higher than an AVO 8 when compared to stated voltage in service data, it won't be much higher. A DMM is also much more convenient for measuring resistance than the cramped non-linear scale of an AVO. A DMM is also much more convenient for measuring resistance than the cramped non-linear scale of an AVO.
Dribbled David.
