Focal Cub 3 won't start

[frankie31]

Hello everybody,

I have an issue on my Focal Cub 3 subwoofer unit : it stays desperately off.

The main fuse and the 2 MOSFET's are not blown.

Here is a picture of the main psu board :



As you can see on the picture, the 2 MOSFET's are driven by a tiny pcb board vertically soldered on the main pcb board "INDIGO 660038RA".



From left to right, the pin 5 and 8 are connected to the gates of the MOSFET's.


As I saw weird colors on the resistors, I changed all the resistors, all the diodes and all the capacitors on the tiny pcb board. The DB6 diac (hard to find !) and the MPSA92 transistor are the only survivors...

I even replaced the 2 MOSFET's with IRF740 (IRF730 originally)...

After replacing all that components, the voltages on the pin of the board are still the same...

From left to right :

120V
350V
119V
119V
119V (-> gate MOSFET)
0V
0V
0V (-> gate MOSFET)

It changed nothing.


One day, as I was doing some tests, I don't know what happened but the unit started...

I measured the following on the pins.

From left to right :

166V
342V
87V
150V
179V (+ square wave with the oscilloscope)
-20V
15V
+20V (+ square wave with the oscilloscope)


As I had to leave home, I turned the unit off and when I came back... nothing... and still nothing since that "holy" day...


I go on with my investigation...

Now, I'm wondering what is the purpose of an area of the main pcb : the 240V is directly connected thru 2 (X1 Y1) capacitors to the secondary. I guess this is a "step down circuit"... The value of the 2 capacitors is 100pF. I don't know which voltage I should have after this two capacitors... and I don't know what should be my reference point for measurement. Currently I have 0V between the GND and the OUT pins of both L79(78)15CV regulators.
I regret not to have done some measurements on the regulators the only day it worked...


Here are the top view and the bottom view of this part.





According to you what should I measure in the different areas of this part ?

Would it be interesting to change the X1, Y1 capacitors ?


It is hard to me to understand the whole operation of this unit...




Any help will be appreciated.

Thanks a lot !

[kc8adu]

Re: Focal Cub 3 won't start

conductive brown glue must be removed.and then replace startup cap.
need to see bottom to find out what smps chip it has

[CapLeaker]

Re: Focal Cub 3 won't start

Since the gate of the mosfet isn't getting anything, I am also for the start up capacitor and the PWM. If you measure around the primary of the PSU the negative lead is on the negative of the main filter capacitor. Measuring on the secondary it is GND.

[frankie31]

Re: Focal Cub 3 won't start

The start up capacitors : do you mean the two X1, Y1 (blue ones) just between the heatsink screw and the 2 red capacitors ?

What about the smps chip ? I thought the square wave was generated by the diac/capacitor association in the INDIGO tiny pcb board... Could you be more specific about this PWM chip I should looking for please ?

Thank you very much for your quick reply !

[CapLeaker]

Re: Focal Cub 3 won't start

Can we see the whole bottom of the PSU?

[budm]

Re: Focal Cub 3 won't start

Just to be clear, you at the beginning you were not getting any DCV at that white connector with grey wires that go to the preamp/auto ON/OFF control board?

'Would it be interesting to change the X1, Y1 capacitors ?' The circuit does not need X/Y caps to function.
Most sub woofer will stay in standby mode until it detects the audio signal at the inputs, and it will go back to standby 15 minutes if the signal is no longer present.

[frankie31]

Re: Focal Cub 3 won't start

I'm not at home this week so I can not take any photo of the bottom of the pcb... But I can tell you that there is no chip at the bottom ; this is a double sided pcb but the components are only located at one side.

At the white connector, there were no DC voltage...

I would like to add that there is a switch on the preamp board that allows to choose between auto On/Off mode and "always On" mode. I always switch to always ON not to have that parameter to handle with...

If the subwoofer is able to start from a signal from the source, what is the utility of this sided circuit with 2 caps ? Isn't it to get the stand-by mode ?

Many thanks.

[budm]

Re: Focal Cub 3 won't start

It looks like more than one companies use this BASH amplifier module:
http://www.teleservice-depannage.com...0.htm?start=15

http://www.diysmps.com/forums/showth...PS-DIY-Rebuild

https://community.klipsch.com/index....repair-thread/

[frankie31]

Re: Focal Cub 3 won't start

You are right, the problem is however different of mine because the units have got blown fusés or shorted transistors...


Could someone confirm me that the start-up caps are the two blue ones with glue please ?

Thank you.

[budm]

Re: Focal Cub 3 won't start

You are right, the problem is however different of mine because the units have got blown fusés or shorted transistors...


Could someone confirm me that the start-up caps are the two blue ones with glue please ?

Thank you.

Those two blue caps are safety cap, nothing to do with operation of the unit, they can be removed and the unit should still function.

[budm]

Re: Focal Cub 3 won't start

We need straight shot and well focused pictures of the whole bottom side and top side of the board to see how the circuits are wired up.

[stj]

Re: Focal Cub 3 won't start

try this!!
http://thompdale.com/bash_amplifier/bash_amp.htm

[robert]

Re: Focal Cub 3 won't start

Looks like the super-primitive self oscillating half bridge design often used in halogen lamp PSU.
No regulation, very little protection.
There is no electrolytic "start up capacitor" so stop looking for that. Check all of the high value resistors on the primary side, these can fail open. The Diac might be faulty too. Also some of the zener diodes on that vertical board might be bad if the FETs ever have failed.

[frankie31]

Re: Focal Cub 3 won't start

Thanks a lot for all your answers.

Ok, I will give you the pictures this week-end.


About the diac ; someone told me to check the diac as follows : I need to quickly short-circuit the diac (< 0.5 second) : if the supply starts... the diac is failing.

What do you think about that method ?


Thanks again !

[robert]

Re: Focal Cub 3 won't start

Shorting the Diac: yes, should work, just do it quickly. And carefully, its at mains potential after all...
I would have a good look at the large (47k as it looks) resistor above the VDR. These like to drift upwards. Unsolder to check it.

[frankie31]

Re: Focal Cub 3 won't start

Hi everybody !

Sorry for my late reply...

And the winner is... the DIAC !!! After quickly shorting the diac... the unit powers on !!

Now I'm waiting for my DIAC DB6 order... 1 month...



I still don't understand how the circuit works... Do you think it has something related to this ?

What about the "protection circuit" with X1, Y1 capacitors ? What use is it ?


Thank you very much for your help !

[budm]

Re: Focal Cub 3 won't start

X1, Y1 are safety caps, not need to function, they are used for line noise filter to pass compliance test.

[frankie31]

Re: Focal Cub 3 won't start

Ok I got it for X1, Y1 blue caps !


And what about the DIAC dimmer circuit ?

Thank you

[budm]

Re: Focal Cub 3 won't start

http://www.diysmps.com/forums/showth...PS-DIY-Rebuild
I cannot tell but the diagram can be drawn based the PCB that OP has made.

[moyo_zz]

Re: Focal Cub 3 won't start

Hello everybody, sorry I'm late, last week I bought a PSB subseries 1, it was like 15 bucks so of course it doesn't work, as I opened it someone tried to repair it so I don't know what he changed, the thing is I know there are 2 irf730 just in front of the "daughter" wich have been raplaced with p12nm50n (don't know why), and I'm not shure if the "lateral" mosfets have been replaced too, on the left (watching the board from the side of AC input) it is irf9530 and the right irf6040, again sorry for beeing so late and thanks!