SVT 7 pro no speaker output

[MarkusBass]

Lowell, do you have a signal at U18_B pin 7?
That is the preamp signal.
D57 is a clipper diode. It makes a square wave out of the preamp signal which will then ride on the PMW carrier wave.

I agree that D57 is a clipper diode. But it clips the signal only when the level would be dangerous for the output stage. Otherwise you would get distored signal on the output all the time (which is not the case with bass amp). So with normal operation of the amp it never actually clips the signal.

Mark

[lowell]

JPB,
I stated previously that I now have no signal at all at R129 and R131... not on either side of those resistors.

[lowell]

Now I think the amp is muted. I've read the 555 timer datasheet and am still trying to grasp how it and the 20955's work together to mute the amp. It's clear to me that the applied voltage on the CSD pin of the 20955 controls whether it's muted or not. It's also clear to me that the 555 timer controls the voltage applied to the CSD pin on the 20955. What I'm not clear on is what voltage and at what point will tell me that YES the amp is muted?

FYI I am getting 4.8v on the collector of Q33. I see this arrangement on the 20955 datasheet on page 11 and it's called "SHUTDOWN INPUT." I believe that this voltage is a sign as to whether or not the amp is muted, but I'm not sure how to interpret my reading.

[Jazz P Bass]

555 Mute Circuit

The way the 555 is configured, a low on the TRigger will make the OUT go high.
That is the MUTE state.(low level)
If TR is high (which is the unmuted state) Out will be low.
If OUT is low then Q33 is off (ie: NOT pulling down CSD)
CSD goes low internally (to the IC) when the ic senses a need for a mute state. (high level).
So the fact that you are seeing 4.7Vdc on the CSD pin tells me that the low level mute is NOT activated.

RANT:
My confusion on this can of worms is exactly where the triangle wave is being generated.
I just do not see it.(unless it's U16-B)
From my meager understanding of Class D, the triangle wave & the preamp signal get mixed to make the PMW wave.
That is what drives the 20955.

[lowell]

Can you please explain what this means? "a low on the Trigger will make the OUT go high." What is "a low"? Is it when there is a low DC voltage on the trigger?

In the 555 datasheet it says: "The circuit triggers on a negative-going input signal when the level reaches 1/3 VC." Does this mean that the Trigger needs a negative DC voltage? Or does it mean that the Trigger needs a negative voltge relative to some other pin on the 555 chip?

In the Ampeg schematic what exactly is happening in the mute circuitry? : Does the Trigger of the 555 see a negative voltage, then put out a high positive voltage to Q33's base which then does what to the CSD on the 20955??

[Jazz P Bass]

Sorry Lowell.
A logic low in this case is whatever it takes to keep Q33 OFF.
A logic high will turn Q33 ON.
That is the mute circuit in a nut shell.
According to the 555 datasheet TR low is anything below 1/3 of the IC + supply voltage.
Anyway if you would look at the CDS pin, it has a 10uf capacitor on it.
That allows the internal mute to reset at a preset on ramp time if the IC 20955 is in a high level mute condition.(IE: output overcurrent)
Q33 is an NPN transistor.
A low positive voltage (ie: less than .06Vdc) will keep the transistor off.
When Q33 is off, the collector, which is held high (4.7Vdc) by CSD pin, does nothing.
When a low level mute occurs (external of IC 20955 / IE: overtemp) 555 OUT goes to a high voltage.
Because overtemp pulled the TR pin down (low)
That high voltage from 555 OUT (IE: more than 0.6 Vdc) turns on Q33 base.
When Q33 base is on, the collector is now going to the emitter (IE: ground)
That condition will pull CSD pin low & shut down the 20955.

[lowell]

THANK YOU!! That helps me understand it much better. So Q33 is saturated when on, otherwise it's off. ?

[Jazz P Bass]

Yup.
And when Q33 is on, CSD pin is off (pulled low).
That is a muted state.

[Jazz P Bass]

[QUOTE=lowell;213046]
In the 555 datasheet it says: "The circuit triggers on a negative-going input signal when the level reaches 1/3 VC." Does this mean that the Trigger needs a negative DC voltage? Or does it mean that the Trigger needs a negative voltge relative to some other pin on the 555 chip?
QUOTE]

I missed this statement.
I think what confused you was the datsheet statement "a negative going input signal".
The 555 is a most wonderful chip.
The TR input can be a changing square wave signal. (or any other shape)
When the square goes from the high to the low is what they meant by "negative going".
It will not trigger on a "positive going" signal.
Pretty cool!

[lowell]

Thanks JPB... more q's to come.

On a tangent this amp has no power transformer. So... SMPS?

[lowell]

Ok so where does this negative going signal come from, the output signal? If it is 1/3 of Vcc or greater then that would be apprx +15vDC x .33 = 5v. How can a "negative going signal" be "greater than 1/3 of Vcc." Or are they referring to the top of the alternating signal on the positive peak?

[Jazz P Bass]

Yes, the power supply is an SMPS.
Now the 555 TR input is held high by R236 through R285 to +15Vdc.
Kindly forget the 1/3 VCC. That is a technical point that does not concern this circuit.
(it's kind of like a schmidt trigger, where there is a do nothing band in the middle. Hysterisis. It helps to prevent false triggering)) TR is either High or Low.
So TR is held high. Out is low. Q33 is Off. CDS is high. The amp is not in Mute.
If you follow TR to the left it goes to Q8, another NPN.
R27 makes sure Q8 is off.
If the temp sensor goes too high, it will input a logic high through R31 which will turn ON Q8.
Q8 will now pull down TR (a negative going signal), OUT goes high, Q33 turns On, CDS gets pulled low, the amp is now in Mute.

[Jazz P Bass]

If you check the datasheet of irs20955, you'll see that there should be pulse width modulated signal on pin #3 of the IC. The signal is formed by U21 - tripple inverter. ####After U21 there is a square signal. U21-B provides inverted signal for the second part of the power amp.#####
The "magic" of class D is in U16.

Lowell, if you checked the signal on inputs of U16A, why haven't you checked the signal on the output (pin #1)? And the signal on U16-B output (pin #7)?
Mark

Lowell, try to go back to this suggestion, please.

[Jazz P Bass]

JPB,
I stated previously that I now have no signal at all at R129 and R131... not on either side of those resistors.

I seem to have missed this little nugget.
R129 & R131 are the feedback from the output stage.
So if, as yet, the output section is not functioning then there will be no signal at thses resistors.

[lowell]

Its odd to me, as there WAS a PWM signal on R131 and R129 when I first started scoping the amp. Guess its possible I screwed something up while poking around.

ok so should there then be a PWM signal at U21 even without signal? I was getting 5v pulsating DC on U21A and U21C outputs last I checked. Can't remember if that was with or without signal. I will check this tomorrow and also check to see if this is also at U21B output.