How do you measure AC voltage in a circuit?

[rockman627]

The first time I tried to measure AC voltage in a Fender circuit, I got my multimeter, set it for AC voltage and just like I do for DC voltages, clipped the black lead to ground and put the red lead to the test point. That didn't work out to well. I haven't really measured AC voltages on a Fender circuit test point since. But when I look up how to do this, all I get is how to measure a wall outlet. So please , when Fender says you should read 550mVAC at TP16, (right now I'm looking at a Princeton 65 DSP schematic) where do you put the 2 leads?

[Helmholtz]

The first time I tried to measure AC voltage in a Fender circuit, I got my multimeter, set it for AC voltage and just like I do for DC voltages, clipped the black lead to ground and put the red lead to the test point. That didn't work out to well. I haven't really measured AC voltages on a Fender circuit test point since. But when I look up how to do this, all I get is how to measure a wall outlet. So please , when Fender says you should read 550mVAC at TP16, (right now I'm looking at a Princeton 65 DSP schematic) where do you put the 2 leads?

Exactly as you described above.
Problem is that some cheap meters don't block DCV when set for ACV, so give wrong AC readings when DC is present.
You may test by connecting a 9V battery. A decent meter should read 0V in AC mode.

You can wire a 0.1µ/600V cap in series with your meter to block DCV.
Also a cheap meter might not have enough frequency bandwidth to correctly measure frequencies above 400Hz.

[rockman627]

So, I CAN clip the black lead to ground and the red lead to the test point? The problem was the quality of my multimeter? OK, I'll try again, thanks.

[sugeda]

For mV use oscilloscope

[glebert]

Sometimes signal ground is not the same as chassis ground.

[catalin gramada]

Is not possible to measure correctly ac signals with a dmm mostly because it will show an average or at least pretty close rms volts. For ac signals, ripple voltage etc you need to measure the peak voltage so use a scope instead. For instance you measure at an AC wall outlet 120V but this is the RMS value, actually the 60cps sine is 120 × 1.41 =169.8Vpk. Or you use xVac time 1.41 to calculate dc voltage after rectification. Why? Cause the caps charging always at peak voltage, not at an average or rms one. Just some eg. why a dmm will not tell you what you want. Even with a scope measuring ac in 1mV range is not quite simple task.

[Helmholtz]

In most cases the RMS value is all you need. If its a sine wave it's easy to calculate the peak or peak-to-peak value from RMS.

The professional grade DMMs I use give correct RMS readings with mVs and frequencies up to at least 10kHz.

It is essential to know the meter specs, so read the manual.

[g1]

when Fender says you should read 550mVAC at TP16, (right now I'm looking at a Princeton 65 DSP schematic) where do you put the 2 leads?

The AC voltages on Fender schematics (usually in ovals) will only be present if you follow the instructions in the schematic notes. The notes will say what signal must be applied to the input, and how the controls must be set. They are for tracing signal through the amp.
The DC voltages (usually in square or rectangular boxes) will be present whenever amp is powered on.

[catalin gramada]

The AC voltages on Fender schematics (usually in ovals) will only be present if you follow the instructions in the schematic notes. The notes will say what signal must be applied to the input, and how the controls must be set. They are for tracing signal through the amp.
The DC voltages (usually in square or rectangular boxes) will be present whenever amp is powered on.

This is a very good point. Some service manuals clearly show in what conditions the measurements was done. Eg. use a dmm with 10 M input impedance , or so or use a scope with a x10 probe.

[Helmholtz]

Best post the schematic in question.

[mozz]

Flukes have no problem measuring audio signals. If you are using a 400hz or 1khz signal, most any meter on ac setting will read it.

[Helmholtz]

If a meter has a frequency bandwidth of 1kHz, readings may be low by 30% (= -3dB) @1kHz.
So it would be better to use a 400Hz test signal.

[catalin gramada]

I think due to the fast response time for peak recordings of 250us, 287 and 289 models may do that.

[g1]

Best post the schematic in question.

see attachment

[Helmholtz]

So the schematic expects to feed a 5mV (RMS) 1kHz sine signal to the J1 input.