Zx6400 hifonics strange protect mode

[rafcio363]

I have strange failure with this ampilifier.

When its powered first time (+12 remote) he go into protect mode.(but after 1,2seconds)

But if i reconect remote +12V ,
it dont show protect again and all working good.

After 30minutes of disconnected remote,all is doing again. So i need power on radio, power off and power on and its working normally.

I get if from car and connect on table and connect power supply, other speaker and going the same way so it's not the fault system in the car.


Also its happen without connected RCA in audio.

So its problem with ampilifier.

All capacitor in converter was changed 4years ago so i thinking its other problem.

Voltage at output converter is about 31V at both side (+,-) -looks as good.

I dont have any idea what could be wrong here?

I have osciloscope.

This looking like some capacitor is bad,but i dont know where to looking..
No schematic

 

[Perry Babin]

I'd suggest checking the voltage on the KA7500 (or TLx94). Measure the voltage on all pins when in protect and when playing. That will tell you which pin is shutting it down and may help you follow the protection circuit back to the fault.

If you can't find the fault, post a good quality photo of the power supply driver board.

 

[rafcio363]

Yes,you have right i checked this today.
He is going into protect becouse on operational amplifier is too high voltage.

Should be 1.44V on both diodes which in to 16pin TL 494. (measured before diodes)
Diodes are connected to operational amplifier from second side.

1 diode is going into thermal check and that is ok

2 diode is going somewhere (i supose that is checking voltage at speaker output)

When its protect i have 11,5V on second diode - so there is way of failure.

But i need more time to check more

 

[rafcio363]

I've found that is problem from one channel output, when i disconnect one channel from protect circuit all its ok.

But i stil dont know why protect is on after first powered.

Volatage which going into protect line is equal when i powered it first time (and protect on) and second time (protect off).

2 capacitors in channel i changed.

 

[Perry Babin]

It appears that you've disconnected (J16?) the 200 series channel, not the 100 series channel with the rectangle around it. I'm assuming that the power supply is on the right side of the photo posted.

 

[rafcio363]

This trace was bad.

I found where is problem.
In one channel at output (at 2 emiter resistors 0.1 Ohm 5W) is big distortion after first power ( i've checked it with osciloscope) when i disconnect this channel output from protect checking , amplifier started normally.

I compare signal at others output and this distortion after first powering exisiting only in this one channel.
Probably i need find preamp and disconnect (or swap with other channel) from this channel to exclude preamp damage.

 

[rafcio363]

Input from preamp founded, but 30min after power to check again really slows me work

 

[rafcio363]

Disortion (protect mode) exising only when switch 3/4ch is on low pass filter.

I've crossed switch and still disortion after first powered.

So i need check operational amplifiers (4558) which generate Low pass filter.

 

[rafcio363]

I cant find source of disotrion is too small before amplfier to looking it on osciloscope.
All electrolit capacitors near all 4558 changed without succes.

 

[Perry Babin]

If you troubleshoot it with no speakers connected, you can drive a stronger signal into it which will make it easier to find.

 

[rafcio363]

Yes, i tried this - i put on input signal - Sinus 70Hz,but still too small disortion - i see it only on output operational amplifier 4558.
So i try change this 4558.

But im still wondering why it happen only in first time

I have two options :
a)Use subwoofer on 1/2 Channel with LPF filter, and 3/4 channel with stereo speakers.

b)Cut protect checking from 3channel.

But i prefer option A

 

[Perry Babin]

The signal may not be visible on the input pin of the op-amp if the circuit is feeding signal into the inverting input. When looking for the signal feeding the inverting input, you have to find the series resistor for that input and check the signal on the end of that resistor that's not connected to the op-amp.

 

[rafcio363]

Thanks for help, but i cant find what is bad.
I found all trace with LPF filter in 3/4 channel, but dont see where is bad (disortion is small only for 1s after first power- i replace all capacitors in this channels and on opamp supply).Cant compare starting signal with 1/2 channel because lpf filter is built on other opamp than 3/4 and i dont have 2channel osciloscope... 2 layers pcb and and no schema is not encouraged me to further research.

1/2 is on 4558, 3/4 is on 4558 and 169xx (i dont remember but it was dual amp with mosfet output).

When in future this channel will die,then i redo seraching.
Finally i switched 3/4 with 1/2 channel and all working good

 

[rafcio363]

Second idea is build external amplifier and look at all point of preamp ,then disortion should be visible.
But i dont have so much time to do it