Help reading 78 GL1000 color schematic

[desertrefugee]

As I said in an earlier post you need to use the highest resistance scale your meter has when testing diodes.

We were taught not to let your fingers touch the test probes when measuring high resistances (like the reverse of a diode) when I was in school and the reasons why haven't changed in the 4.5 decades since. My good analog 100,000 Ω/volt meter is still my more accurate than any of the digital ones I have (not to mention that there are some things a digital meter just doesn't do well). If I set it to the MΩ scale and grab one lead with each hand it reads about 2-3 MΩ.

Not to split hairs here, but if we're going to have this be a matter of record, then that record should be as accurate as possible. (First of all, congratulations to EdK for getting his meltdown sorted!)

As a long time electrical engineer, I will offer up a couple of final thoughts on testing diodes.

1) Any high-ohm reading will be affected by body resistance.

2) The most accurate way to test a diode is not with a pure "resistance" test, but with either a meter that has a DIODE test mode or, alternatively, measuring an actual voltage drop - either physically by forcing a voltage and recording the drop across the device or with an SMU. A simple resistance test might reveal a shorted or open component, but might miss breakdown voltages or other more subtle metrics.

3) The reverse direction of a diode will, for all intents, be an open circuit. However, when reading the forward biased direction, a lower range should be employed for accuracy.

The good news is that most diodes fail in a shorted state which is easy to detect. But, the reason I note that forward biasing should be tested in a lower range than MOhm is that it won't always be a dead short. Could be 2,3 or even 400 ohms and still be bad! A meter in high range won't show the distinction between 700 ohms and 400 accurately enough to highlight that - especially an analog one in high range.

And about modern digital instrumentation. I'm not sure in which application a digital DMM is lacking over analog magnetic meters, but I know I haven't seen an analog meter in an engineering laboratory in over 20 years. We went digital a long time ago and I can't think of any application in which functionality or accuracy was compromised. Quite the opposite, in fact.

[Sidecar Bob]

First I'll say that you are correct but in the context of a simple go-no go test to be performed by someone that doesn't understand the technical details of what they are measuring a high range works best. And I would add that the second most common failure mode of diodes is probably open and the resistance test will find that too.

Digital meters are great for measuring steady state signals or signals that change gradually but they aren't very good for measuring things that change frequently such as audio signals. Because the signal is constantly varying and the meter samples at specific intervals it simply can't hit the moving target reliably enough to give useful results.