11-06-2018, 10:35 AM
(11-06-2018, 08:48 AM)ppppenguin Wrote: For ordinary radio and TV mending any DMM will be more than accurate enough.
Too accurate sometimes!
Something which never seems to be explained to beginners is such things as tolerances and limitations to measurement accuracy.
For example, equipped with even the most rudimentary DMM, when taking voltage measurements on a old radio where the voltages shown on the service sheet were taken with an AVO Mk 7, many, possibly the majority, of the voltages on a set in perfect working order will be much higher than shown.
To a lesser extent there will still be errors in the voltages originally taken on an AVO Mk 8.
We might read something like "I changed R15 because its value had dropped from 1k to 908 ohms." Whilst an analogue meter cannot provide a reading of such high precision, it will give a reasonably accurate reading in this case but the novice is overlooking the note at the bottom of the component listing that says "All resistor values ±20% unless otherwise stated", so it could have been any value from 800 to 1200 ohms from the day it was fitted.
Perhaps not directly relevant to this thread but should we have somewhere for useful information like this? I'm sure I posted a diagram I created showing tolerance limits for different ranges of resistor, E6 (20%),E12 (10%), & E24 (5%) but it must be buried in the middle of another thread and I haven't a clue where to find it! (I do still have the original, though.)
