JVC THS9 Home Cinema System SMPS No outputs or standby power

[Percyveer]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Hi Budm

"and have a few weird fluctuating voltages on the board outputs," Before the IC was replaced you DID BOT get any Voltages on the secondary side at all, right. It sounds like the new IC is startup/shutdown mode.
BTW, your meter is not capable of measuring high frequency AC of the output of the transformer winding. your meter does not have enough bandwidth to measure High frequency AC Voltage, it is in 50~100KHz range.

Correct no output at all, but the voltages are weird if you look at the drawing I uploaded in my last post.
So why is it in startup/shutdown mode, and how do we fix it?

[rievax_60]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Yes, something has changed. I always pay close attention to the secondary rail that is being monitored by the regulation loop. Working without an oscilloscope is like working blind, it is useful to see what peak voltage rails are reaching and for seeing ripple. A DMM with Peak Hold is the next best thing.
When a secondary rectifier diode is followed by a bypass capacitor, a series choke and another bypass capacitor, the condition of the first capacitor is very important. If it has high ESR, large ripple voltage will develop across it and the choke causing the voltage across the second capacitor to be much lower than the secondary + peak voltage.

[Percyveer]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Hi Rievax

Yes, something has changed. I always pay close attention to the secondary rail that is being monitored by the regulation loop. Working without an oscilloscope is like working blind, it is useful to see what peak voltage rails are reaching and for seeing ripple. A DMM with Peak Hold is the next best thing.
When a secondary rectifier diode is followed by a bypass capacitor, a series choke and another bypass capacitor, the condition of the first capacitor is very important. If it has high ESR, large ripple voltage will develop across it and the choke causing the voltage across the second capacitor to be much lower than the secondary + peak voltage.

That is some good advice, my DMM does have peak hold.
I am not convinced the switching regulator is starting with that Vin at 13v, does anybody have any ideas why it is so low, when this is up to 16v it should start.

[rievax_60]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Like most SMPS control ICs, they start at 16V then the VCC often settles down to a lower voltage.

[Percyveer]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Like most SMPS control ICs, they start at 16V then the VCC often settles down to a lower voltage.

But this never gets above 13.2v to start it, just measured it again using the touch/hold function on the Fluke as you suggested.

[Percyveer]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

I have actually found a defective component, D953.
I replaced it with an equivalent, but alas, no change in the operation, but could this point to something else defective, does it help to understand the issue?

[Percyveer]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

BINGO away to go!

Reviewed D953 and found the solder pad damaged on PCB, reworked the arrangement.

So to wrap up, IC901 and D953 were defective.

Been studying the diagram, but I do not understand how the IC901 Vin voltage was being dragged down.
Can anyone explain this to me please.

[rievax_60]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

D953 produces the rail that is monitored by the regulation loop. The condition of this diode will not prevent the IC901 VCC reaching the 16V start voltage.
If D953 was short, IC901 will be highly stressed by forward overload current. All of the transformer windings will develop little voltage. IC901 VCC will not be sustained.
With D953 open, there will be no regulation feedback. All other rails will go momentarily over voltage. IC901 is likely to sense this and shut down.
What did you replace the diode with?

[budm]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Whew!
My Post 15: BTW, you are sure that there is not bad diodes in the secondary side, or low resistance on any of the output pins?
Glad it is working now.

[Percyveer]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Hi Rievax,

D953 produces the rail that is monitored by the regulation loop. The condition of this diode will not prevent the IC901 VCC reaching the 16V start voltage.
If D953 was short, IC901 will be highly stressed by forward overload current. All of the transformer windings will develop little voltage. IC901 VCC will not be sustained.
With D953 open, there will be no regulation feedback. All other rails will go momentarily over voltage. IC901 is likely to sense this and shut down.
What did you replace the diode with?

Thank you for your detailed reply.
D953 was passing current both ways.
So the chain of events would have been, D953 breaking down (just because it gave up or mains spike?) and taking out IC901 due to stress from the forward overload current created?
I replaced with a 31DQ05.

[Percyveer]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Hi Budm

Whew!
My Post 15: BTW, you are sure that there is not bad diodes in the secondary side, or low resistance on any of the output pins?
Glad it is working now.

I did not ignore your advice......I somehow missed the largest diode on the board

Thank you and everyone who supported me in this task, I and probably others in the future will have learnt much from this.

[CapLeaker]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

Glad to read that you got it fixed.

[hitec]

Re: JVC THS9 Home Cinema System SMPS No outputs or standby power

cheek 903 otherwise the ic