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Class D Switching Power Amplifiers and Power D/A conversion

No sound from TDA7492P Amp
No sound from TDA7492P Amp
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Old 5th February 2018, 07:17 PM   #41
FauxFrench is offline FauxFrench  France
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OK, at least we now know that the logical inputs are as they are supposed to be.

Most of the TDA7498 inputs serve both channels such that, as one channels works, they should also allow the other channel to work.
Non-regulation of a channel normally is caused by 1) a faulty chip 2) a fault in the external components on the input pins 3) an error in the feedback signal.

For TDA7498, the cause 3) is little likely because the feedback signal is internal in the TDA7498.
Cause 1) we cannot do much about, should it be the case.
So, we have to examine cause 2). For doing that I suggest that you switch off the power to the board and measure the impedance:
from pin 22 to ground and to the power supply line (+),
from pin 23 to ground and to the power supply line (+),
from pin 32 to ground and to the power supply line (+),
from pin 33 to ground and to the power supply line (+).

The reason for these measurements is that the two channels have similar external input circuits. When you cut the power, the chip inputs normally become high impedance. Then you can see any difference in impedance as an indication that the external circuits are actually not similar though they are supposed to be. Then you will get a good hint to where to look for the fault (if there are differences).
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Old 5th February 2018, 08:09 PM   #42
dutchronnie is offline dutchronnie
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Pins to ground - all 2,1 M Ohm
Pins to +3,3V - all 2,1 M Ohm
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Old 5th February 2018, 08:47 PM   #43
FauxFrench is offline FauxFrench  France
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It seems that the external input circuits have got no apparent flaws.
When we isolate the amplifier part (TDA7498) with correct logical input signals, one channel works but the other not. It seems more and more likely that the chip is faulty.

If you still have some energy left, there are a few test to do but mainly to indicate that the chip is actually faulty:

Connect a 47KOhm resistor between 3.3V and pin 28. 1KOhm on the faulty output. When you turn on the power, does the voltage on pin 28 go to 3.3V? (if yes, it indicates that the chip itself sees a fault)

No power on the board. Figure 19 of the datasheet in front of you. Concentrate on the chip-pins on the right side in FIG.19 (output pins). Do any of the chip-pins seem to have a rather low impedance to ground or power-supply "+" except for those pins that are supposed to be connected to ground or power-supply "+"?

Last edited by FauxFrench; 5th February 2018 at 08:49 PM.
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Old 6th February 2018, 04:24 PM   #44
dutchronnie is offline dutchronnie
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Thanks so far FauxFrench.

This evening i have something else, but tomorrow i do the last thing you mentioned.
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Old 6th February 2018, 07:07 PM   #45
dutchronnie is offline dutchronnie
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I have foun some time to do the measurements, i am curious too.

47kOhm between 3.3v and pin 28 - no voltage on pin 28

Between pins and ground
pin 2,3 - 95,5k ohm
pin 8,9 - 95,5k ohm
pin 10,11 - 177k ohm
pin 16,17 - 177k ohm

between pins and + all O.L.
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Old 8th February 2018, 09:48 PM   #46
FauxFrench is offline FauxFrench  France
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Sorry for having been away on travel for some days.
These measurement results unfortunately show nothing conclusive. The chip itself seems not to recognize the fault as pin 28 remains low.
The chance that the chip is defect is high. You can try to change the chip but it is a delicate job.

I am sorry that you now have two defect boards. I have bought more than 20 amplifier boards from the cheapest suppliers (in Asia) and only very recently I got one that was defect on arrival (a TDA7293 board where the low side push-pull transistor had shorted to "-").
I hope you are not too disappointed in DIY from this annoying experience.

Last edited by FauxFrench; 8th February 2018 at 09:51 PM.
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Old 9th February 2018, 07:33 AM   #47
dutchronnie is offline dutchronnie
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Thats alright FauxFrench, I appriciate your help.

No, i am not disapointed in DIY, i like it.
Not that i have 2 broken boards. but to make things i do like.

most disapointed is that it takes 3 weeks before a new board arrives.

Another question:
I have lying around another board, i use it and wait for the replacement board.
but every time i power up the board the volume is trown back to a lower level.
Is it possible to stay on the loudest volume on the board?
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Old 9th February 2018, 07:46 PM   #48
FauxFrench is offline FauxFrench  France
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My best guess is that it is made as a sequential numerical circuit such that it is started up in the same initial state every time. Then, it performs the same sequence every time, including setting the initial volume level.
It will be difficult to change that.
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