Class D Subwoofer Amplifier Repair

[geoffagain]

*edit updated attached schematic

Hi gang,

Have a BKA-130-C Buttkicker amplifier that is stuck in standby mode.

Can anyone help with methodical fault finding please?

Attached pic of board and schematic.

I have component tested the momentary buttons and the the two IRF740 mosfets which are ok.

The board looks to have got hot around the 'remote' connection - I've reflowed the four resistors R426 R427 R428 R169 and transistors Q404 and Q405 which also tested ok with a DMM.

Powers on, but standby light remains red. Pressing power momentary switch I can hear relay click but led remains red and no output.

Am I right in thinking the fault protection is controlled by the irs2153D? I'm not sure how to test.

[Harry_22]

Hi!

1. Check voltage at SW105 pin1 =+12V
2. Check voltage at (-) D403 =+12V
3. Check voltage at (+) D403. It grow slowly up to +6V and falls every time you press On (SW105) button.
4. IC403 pin 1. It changes from 0V to +12V or back every time you press On button.

PS
Seems it works if you hear relay. Sorry I missed this.

[Harry_22]

Carefully measure voltage between pin 1 and 4 of IC201A.
Next measure between pin 3 and pin 4.

Make sure the amplifier is turned on.

[geoffagain]

Thanks for the guidance.

As it happens IC201A on the schematic is listed as a L6569AD

On my board as I mentioned but didn't click its a irs2153D - still a gate driver.

I've attached here another schematic I've found that lists the irs2153D - I'll try and update the original post too.

I'll test it as described in anycase.


Thanks

[Harry_22]

Two almost identical diagrams but drawn differently.
The second one is completely unreadable. I mean it's hard to understand.

PS
I attached the first one as an example.

[geoffagain]

IC201A

Pin 1 and 4

Main power on - 0v
Pressing momentary power switch on front - 6.5V
Pressing momentary power switch again - drops slowly to 0v

Pin 3 and 4

Main power on - 0v
Pressing momentary power switch on front - 3.4mV
Pressing momentary power switch again - drops slowly to 0v

Relay is operating, front led indicator remains in 'standby'

Thanks for attaching the schematic again, I found second easier to follow but I shouldn't have assumed same or others 

[Harry_22]

Please measure voltage on R201 both sides.
Be careful!

[geoffagain]

we have on R201

with momentary power button pressed and relay activating >

-148V to ground

155V to ground

pressing power button again relay disengaging voltage drops slowly

[Harry_22]

Let me clarify.
1. R201 one side to pin 4 IC201
2. R201 second side to pin 4 IC201

One should be 320VDC next +16VDC.
Please check.

And measure R224 both sides between pin 4 IC201.

[geoffagain]

ok that makes more sense

We have

R201

325V and 7.48V

R224

6.88V and 6.86V

[Harry_22]

Great!
The diagnosis can already be made. Lack of voltage to start the IRS2153 controller.

The rectified high tension 325V passes through resistor R201 and forms a voltage drop 16V across ZD201 zener diode which powers the controller. In our case, only 7.5V is generated at the zener. The conclusion is that something is lowering the voltage.

To clarify this please first disconnect equipment from the mains.
Measure resistance between D210 anode and pin 4 of IC201.
Next measure resistance between D210 cathode and pin 4 of IC201.

[geoffagain]

Great annotation to the diagram, thank you for explaining. It's helpful to understand the circuit.


We have on the board measured at the zener ZD210

D210 anode and pin 4 of IC201 - 0hm

D210 cathode and pin 4 of IC201 - rises quickly to OL then drops and settles at 9.6Kohm

[Harry_22]

The zener diode is most likely broken.

[David_AVD]

I had a leaky Zener in a SMPS the other week. The leakage was enough to pull the rail below the Vmin.

[geoffagain]

Thanks for this,


I replaced the zener with another from a donor board - still could be broken, I need to test both of them still.

Same voltage at R201

I pulled R201 and it reads correct resistance out of circuit.

[geoffagain]

Ok so Zener in circuit ZD201 is 1N4744 15v

The one I pulled from donor is also marked IN4744. So seems for whatever they're using 15V rather than 16V on this one.

Tested ZD201 out of circuit with my bench supply and a 1k resistor -

Seems ok, holds 15v upto the 20v I can supply. Not sure on leakage current.

I'm not measuring any current or at least very minimal under 15V  with a 1k resistor in series.

Approx >

0.5mA at 15V
1mA at 16V
2mA at 17V
3mA at 18V




 

[Harry_22]

Measure the resistance  between D210 anode and pin 4 of IC201.
There are also C209 capacitor in parallel with the zener.

[geoffagain]

Cheers for your continued patience.

Resistance  between D210 anode and pin 4 of IC201 - same behaviour as before with

D210 cathode and pin 4 of IC201 - rises quickly to OL then drops low then rises and settles at 9.6Kohm - perhaps still rising very slightly over time.

I'll pull c209 and see what my ESR says.

[Harry_22]

But with Zener you had 0 Ohm. Am I right?

[geoffagain]

Sure

ZD201 anode to pin 4 is 0hm

[geoffagain]

C209 was crapped out open circuit

replaced with a good cap but no joy.

I may visit some of the other caps as this one didn't show any visual signs of failure

[Harry_22]

Anode ZD201 is connected to pin 4 of IC.
Yestrday you measured anode D210 to pin 4 and resistance was 0.

Ok let us solder ZD201 back and pull D210.
After connect the device to mains and check voltage on Zener.

[geoffagain]

Apologies

It's likely I made a mistake between ZD201 and D210

For clarity we have

D210 cathode to pin 4 - 69.5Kohm
D210 anode to pin 4 - 9.6Kohm

ZD201 cathode to pin 4 - 9.6Kohm
ZD201 cathode to pin 4 - 0ohm

Pulling D210 and checking zener voltage on ZD201

Rises to 14.93v

[Harry_22]

No need to apologize it's also my mistake.
If the resistance was zero then there would be no voltage on Zener. I could notice right away.

I forgot to warn you that in non-working SMPS the high tension remains on capacitors for a very long time.
In your case two caps are connected in series and loaded with resistors to equalize potentials. Just like in valve amps. It turns out that 195uF is discharged through 360k. Time constant R*C=70s. In a time of 3*70s the caps will be discharged by 95% to a safe value.
Please wait at least 3 minutes before touching SMPS !

Now we will work with SMPS disconnected from mains.

Let's use your 1k resistor and bench supply.
Please connect it via a 1k resistor to 1 pin Vcc of the IC. Ground to pin 4 (pls see the picture). Voltmeter between pins 1 and 4 of the IC.
Gradually increase the voltage and monitor following stages:

1. Voltage on Vcc will rise until startup point ( between 10-12V according to datasheet). The current through IC is very small and Vcc is equal to the voltage on the bench supply.
2. Startup point. You will see 1.7V difference between PS and Vcc. The controller begins to draw 1,8mA current which creates a drop across the resistor.
3. At 15,4V Vcc growth will stop as with Zener. Because IC has its own Zener inside.

[geoffagain]

Good advice on caps - if not for me then for anyone else coming across this thread.

Can't get a stable voltage when connected to my bench supply, its not happy in the slightest.

I've got current limiter set at 100mA so unless its sinking more than that I have no idea what is happening. Should be plenty of headroom for 20mA (20v @ 1kohm) plus 1.8mA from the controller.

Bench supply is probably crappy, its hardly a reference device - not clamping voltage down quickly enough or hitting current limiter or both ..etc



Resistance between pin 1 and 4 is 1.12 KOhm