Amp repair. Can see input on scope but not output which I can read

Thread Starter

Dkolko

Joined Feb 17, 2019
4
I have a PA amp (no schematic available). I have a sine wave going in (shows on scope) I can hear it over the speaker (volume and tone work) but I can not see any pattern when the scope is connected to speaker.
 

AlbertHall

Joined Jun 4, 2014
9,323
If you can hear sound from the speaker then there must be a voltage signal across it so I don't know why the 'scope can't see it.
Is the sound normal volume?
 

Thread Starter

Dkolko

Joined Feb 17, 2019
4
If you can hear sound from the speaker then there must be a voltage signal across it so I don't know why the 'scope can't see it.
Is the sound normal volume?
Yes the sound is at normal volume. I have used this setup in the past with no problems seeing the output pattern on the scope (obviously at a much higher voltage than the input signal). I even tried a different probe.
 

AlbertHall

Joined Jun 4, 2014
9,323
So this must be a measurement problem, not an amplifier problem.
When you have the amplifier producing sound through the speaker and you then connect the probe does the sound from the speaker continue unchanged?
 

Thread Starter

Dkolko

Joined Feb 17, 2019
4
So this must be a measurement problem, not an amplifier problem.
When you have the amplifier producing sound through the speaker and you then connect the probe does the sound from the speaker continue unchanged?
Yes sound is unchanged and yes it is a measurement issue. I am a newbie.
 

AlbertHall

Joined Jun 4, 2014
9,323
If you connect the probe to the 'scope calibrator do you see that signal?
Did you connect the probe tip to one speaker lead and the probe ground clip to the other speaker lead?
 

Ylli

Joined Nov 13, 2015
893
If you connect the probe to the 'scope calibrator do you see that signal?
Did you connect the probe tip to one speaker lead and the probe ground clip to the other speaker lead?
Careful there, we don't know if the output is single ended or bridged. Keep the scope probe ground on the amplifier chassis.

OP, do you have a make/model of the PA amp. Perhaps a schematic can be found.
 

LesJones

Joined Jan 8, 2017
2,511
Is it a valve (Tube.) amplifier with an output transformer and is the transformer secondary floating. (Neither side connected to ground.) ?

Les.
 

Thread Starter

Dkolko

Joined Feb 17, 2019
4
Is it a valve (Tube.) amplifier with an output transformer and is the transformer secondary floating. (Neither side connected to ground.) ?

Les.
It is a transistor amp. Even if I put the leads from the o-scope accross the output ground and 8 ohm output port (or any of the other options) I still see very little output at all. But I can hear it quite well. Heres an update on what I tried today. I connected the leads of the scope to its internal calibration point (5v) and adjusted the scale of the scope so 5v = 1 division on the scope. The input shows < 1 volt coming from the signal generator with a perfect sine wave. If I put the leads to the output it shows little or no output. I'm new so thanks again for your patience.
 

LesJones

Joined Jan 8, 2017
2,511
I would think you could probably hear 100 mW quite well. So if the speaker was 4 ohms impedance that would be the square root of P x R
so V = 0.63 volts. (RMS) = 1.78 volts peak to peak. 5 volts peak to peak is 1.77 volts RMS With that into 4 ohms it would be 0.78 watts.
Saying that you can hear it quite well does not give us any real idea of the power.

Les.
 
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