Alto Mixpack Express

[JP-Stereo]

I am looking at an Alto Mixpack Express that is hissing badly.
At the speaker output: 1.5vdc to 2.0vdc on a 350 hzs wave.
The protection trip after a few seconds if a speaker is connected.

It's the same generic bloc amplifier as many other model.
The (DM350W+65W limit)
This one has only the class D amp components with less powerfull rating as other i've seen.

All the voltages in the power section are correct and no noise on the input (at R53)



With these symptoms, is there something to look at first ?

I was planning to remove the Mosfet, the the diodes, etc...

[The Dude]

I wouldn't start removing parts. Do you have a scope? If so, scope the output. Do you see "hash", a sine wave oscillation, etc. That might help narrow down the cause. Also check those caps, resistor, and their connections at the output. Those should filter out some HF noise. Is there any other zobel type network not show in your schematic- like maybe on an output board?

[JP-Stereo]

Here it is:



Everything is tight on the board, but I cannot measure the caps "on board", other components are messing with the readings.

J103 and J104 are the TABs for the crimp-on connectors from the speaker, it's direct.


"I took a photo of my scope,.. Hoping this will help help."

[Helmholtz]

The scope readout is wrong regarding frequency.
Fundamental is 60Hz.

[JP-Stereo]

It is reading the noise that is riding on the 60hz fondamental.
Look like it is locking on the highest frequency, maybe there is some parameter that control that...

[JP-Stereo]

I've verified F101, F102, D131, D132, R159, R167, D2, D5, D114, D115 all looking fine.

Finally, i have a small noise at the input of U2 with the probe at 50mv, but no trace of the big AC wave.



When i try to probe around U13A, i am injecting a strong signal that kick the amp to 39v ac then the protect go on.
Without probing U13a the SD line stay quiet at near 5vdc t D100, D101.

How to verify this U13A ?

I removed C108 and the noise still there, maybe a little different but there.
So it must be from the other side of the cap or from the supply.
A kind of high pitch white noise that cut and come back, probably the protect circuit is cycling on and off...

[JP-Stereo]

The IC was easy to find so i will try to replace it next.
Now waiting for the shipping...

[g1]

The IC was easy to find so i will try to replace it next.

Do you mean the IRS driver IC? As it is obsolete (edit, not obsolete), I would be very suspicious of fakes if easy to find or cheap.

[JP-Stereo]

From Digikey !

[JP-Stereo]

The IC is replaced but no improvement.

The output are cycling on-off at around 2 seconds intervals with a loud cracking and hissing noise.
The +5/-5 rails that power U13 are solid and clean.

The SD is cycling but i cannot identify wich circuit is triggering it.
First time the SD is going up the sound come on with cracking noise for about 0.5 second and then go off for about 2 seconds.
But after the first cycle the SD line is not following the noise, the SD come up even if the sound is cut.

Any idea ?

[g1]

Have you looked at the application notes for the IRS2092 ? (attached)
pg.5 discusses the SD line, and pg.7 discusses how over-current at output is fed back to SD line.

[JP-Stereo]

Hi, i've read it carefully and the current implementation of this IC rely only on the CSD control via the SD line.
First there is a Low/High voltage windows (U14), an Over voltage protection (U4) and a DC offset protection (U3).
Each one drain the SD line to ground if a protection is triggered.
It's not clear wich one is triggering these cycling of the SD line when a speaker is connected.
Again, no cycling if no speaker...

I replace C108 without any change.
I've removed D114-115 to check if they are loading the output = they look OK.
I've removed Q104-Q107 and they look OK too.

I have a deficit of imagination on this one...

[JP-Stereo]

Correction, U14 is only interfacing with the PROTECT line, it cannot affect the SD line.
So Only U3, U4 and U2 can trigger an SD event and a PROTECT follow.



I am trying to find wich of the protection is involved.
At power up measuring the anode of D3 or (E) the signal is negative at first for about a second then it rise to a steady 5. something volts dc.
So it's not U2 that is triggering the SD line.
But at the cathode of D3 the signal is 0v at first then, when it begin to rise it's choppy and very noisy then stabilise after 5 second.
When i measure at the cathode of D101 the signal is steady.
At D100 cathode there is noise and glitch at power on.
No U12 installed on this version.
Something is triggering U3 ?