SVT 7 pro no speaker output

[lowell]

There is signal out of the phones and send but nothing from the speakon/1/4" speaker output jacks. I got the schematics from Ampeg but I do not see the power amp on any of the pages... There is 1.6vDC on the speaker outs. Not sure if that's an issue or not. Does anyone see the power amp or have any clues here?

[Enzo]

Well, don't tell them you posted it here.

All you have there is the power supply, a large SMPS, along with a preamp board, and a rear jack panel.

I am not familiar with the SVT7Pro. If no one else chimes in, call them in the morning and ask what power amp this model used. I am tempted to think that they re-used on of the others. Or if you already have the unit, look on the power amp for model name.

[Jazz P Bass]

Plug your guitar into the Power Amp In jack.
See if that works.
That will help narrow down the issue.

[MarkusBass]

... but I do not see the power amp on any of the pages... ... Does anyone see the power amp or have any clues here?

Third schematic - page 2 contains the power amps. This is class-D power amp.

Mark

[lowell]

There is nothing when plugging into the power amp in.

[lowell]

Ok I see the Mosfets now thanks Markus. I'm totally unfamiliar with this setup. It's interesting it seems that both power amp outputs are going through inductors that are thus coupled inductively, and out of phase. Also of note is the iron core in the inductor instead of your typical air core type. I'm curious what are the ltv-817m-cn opto isolators for?

Mosfet voltages seem ok but correct me if I'm wrong:

Q10
Vd: 59v
Vg: -1.6v
Vs: -3v

Q11
Vd: -3v
Vg: -58v
Vs: -60v

Q12
Vd: 59v
Vg: 2.8v
Vs: 1.6v

Q24
Vd: 1.6v
Vg: -58v
Vs: -60v

[J M Fahey]

Besides asking Ampeg itself, search all ICs used here, read their datasheets, maybe this commercial power amp is very close to some application example.
Otherwise, we are shooting in the dark.

[MarkusBass]

I'm totally unfamiliar with this setup. It's interesting it seems that both power amp outputs are going through inductors that are thus coupled inductively, and out of phase. Also of note is the iron core in the inductor instead of your typical air core type. I'm curious what are the ltv-817m-cn opto isolators for?

This is typical class-D power amp. It is quite different from valve amps and classical transistor amps. But since many manufacturers now use such power amp maybe it's time to learn it . The inductors are to get rid of high frequency switching signal on the output. And since the opto isolators are in parallel to 0.02 Ohm resistors, it is not difficult to guess that they protect the amp in case of short circuit on the output (they mute the amp).
Of course you could verify signals on the MOSFET's driver IC (carefully, with oscilloscope) but first I would check some basic stuff. Do you have signal on the power amp input? And if you do, is this signal present on U16-B and U18-B IC? This is someting you would check also with a valve amp, wouldn't you?

BTW, I wouldn't measure voltage of gates of the output transistors. There is a high frequency square signal so the information will be useless, or you may even cause MOSFET failure. Such checks should be only done with an oscilloscope.

Mark

[lowell]

Mark,
Right, now that I get the signal flow I can check for signal at those points. There is signal at the power amp input J1. There is good signal at the junction of C169/R96. At U16B pin6 the signal looks different with a weird spike on the peaks and valleys. Here's the scope pic.

Side note - Do Q18 and U23 turn U18A into a buffer when Q18 is on? That seems odd. It'd make more sense if when Q18 was on the it shut off the power amp input signal, thus was a protection circuit. Maybe that's what it is? From what I know though in an inverting opamp setup when the output is connected to the input it's a buffer right?

[MarkusBass]

Mark,
Right, now that I get the signal flow I can check for signal at those points. There is signal at the power amp input J1. There is good signal at the junction of C169/R96. At U16B pin6 the signal looks different with a weird spike on the peaks and valleys. Here's the scope pic.

Side note - Do Q18 and U23 turn U18A into a buffer when Q18 is on? That seems odd. It'd make more sense if when Q18 was on the it shut off the power amp input signal, thus was a protection circuit. Maybe that's what it is? From what I know though in an inverting opamp setup when the output is connected to the input it's a buffer right?

There is nothing odd with Q18 and U23. This is just a limiter of the input signal. The inverting amp gain is set to 150/12=12.5 and it is decreased when the signal amplitude on the output is big enough.
I would read a description of how class-D amps work: Class D Amplifier - Wikipedia, the free encyclopedia
and also the datasheet of the MOSFET driver: http://www.irf.com/product-info/data...rs20955pbf.pdf
Just in 2 words how it works: class-D amp takes an audio signal and modulates the width of the internal high frequency signal. So on the output you get a square signal but the width of the impulses changes all the time. On the ouptut of the amp there is a filter which removes the high frequency signal so that on the speaker you get again an audio signal.
I assume that you ment J13 as the power amp input (because J1 is the output on the amp). I think that U16B is the modulator for the output stage. I'm not sure what signal should be on this input but I would expect a different shape. Please take a look at U16-A. It should get a square signal from both power amps (out of phase). What signal do you get on R129 and R131 (on inputs of U16-A)?
I also wonder if the amp is not just muted. NE555(U3) can shutdown both MOSFET drivers using TEMP_MUTE and AMP_MUTE signals. You are getting a square signal so maybe it's not the case but I would check pin 2 of both IRS22955 (after reading its datasheet).

Mark

[lowell]

Please take a look at U16-A. It should get a square signal from both power amps (out of phase). What signal do you get on R129 and R131 (on inputs of U16-A)?

On the L8 side of R129 and R131 the signals are indeed pulsating signals of equal amplitude and from what I can tell (hard to see this on scope) they seem like they may be out of phase. This looks about the same on Pins2/3 of U16A.

There is 10vDC on the CSD Pin2 of the 20955Spbf. From what I read in the datasheet it's when the CSD=0v that the IC shutsdown... correct? So if there's 10vDC on the CSD then I'm guessing it's on and operating?

Thanks for the links Mark, I read them. Not sure I grasp the Class D SMPS amp. From what I read it sounds like the power amp produces a pulsating high frequency DC output (10x higher than the highest favorable spectrum of the input audio signal), then the output is filtered to reveal only the amplified input signal. I still don't quite get this.

[MarkusBass]

On the L8 side of R129 and R131 the signals are indeed pulsating signals of equal amplitude and from what I can tell (hard to see this on scope) they seem like they may be out of phase. This looks about the same on Pins2/3 of U16A.

The easiest way to tell whether the signals are out of phase, is to connect the oscilloscope directly to R129/131. If the signals are out of thase, the amplitude should be twice higher.

There is 10vDC on the CSD Pin2 of the 20955Spbf. From what I read in the datasheet it's when the CSD=0v that the IC shutsdown... correct? So if there's 10vDC on the CSD then I'm guessing it's on and operating?

Yes, it looks like the amp is working. You can tell this looking at voltages on R129/131. Or you could just connect the oscilloscope to the output of each amp. So it works but it does not get audio signal. I would search in this direction.

Thanks for the links Mark, I read them. Not sure I grasp the Class D SMPS amp. From what I read it sounds like the power amp produces a pulsating high frequency DC output (10x higher than the highest favorable spectrum of the input audio signal), then the output is filtered to reveal only the amplified input signal. I still don't quite get this.

You understand it very well. Of course, if this is "pulsating high frequency", then it is not DC any more, right?

Mark

[lowell]

Mark,
Thanks for the reply.... but where to next? I'm at a loss. It seems the power amp is functioning fine. I guess I can check connections after L8. But what about those LTVs after L8? What are those?

[MarkusBass]

Mark,
Thanks for the reply.... but where to next? I'm at a loss. It seems the power amp is functioning fine. I guess I can check connections after L8.

If you have any dummy load connected to the output (and you should have as it is required with most class-D amps), you can check with the oscilloscope whether you have this high frequency signal there (even if you don't hear it). If you don't have it, the power amp is not correctly connected to the output jack. If you have it, then you have to check the input of the power amp.
I suggest that you provide information whether signal from a signal generator plugged into the input jack is avalable on the preamp output. As Enzo says, you have to narrow the problem. Checking the preamp is one of the first steps you should make. If the preamp is OK, check whether the signal is present on the power amp input. If it is present there, check whether it modulates the high frequency signal.

But what about those LTVs after L8? What are those?

You asked about it 2 weeks ago and I answered the same day (post #8).

BTW: preamp also contains a mute circuit (a relay on the output). Isn't simply the premp muted? And you have to .... . Does the relay click when you switch the amp on?

Mark

[lowell]

I suggest that you provide information whether signal from a signal generator plugged into the input jack is avalable on the preamp output. As Enzo says, you have to narrow the problem. Checking the preamp is one of the first steps you should make. If the preamp is OK, check whether the signal is present on the power amp input. If it is present there, check whether it modulates the high frequency signal.

I stated earlier that there is signal at the power amp in. As we've been discussing I've also stated that signal reaches to U18B pin7. And I've just recently said that there is the pulsating signal at U16A pins 2 and 3.

From all this it seems that the problem may be between U18B and Pin 3 (IN) of the Mosfet driver ICs. But I'm really guessing here. I'm not sure what I should be seeing on the scope at this point in the circuit. Because of pulsating DC in Class D amps I'm not sure that I should be seeing an AC signal at this point in the circuit or if it's "hidden" amongst the pulsating DC.