JVC THS9 Home Cinema System SMPS No outputs or standby power

[hitec]

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

cheek 903 otherwise the ic

[CapLeaker]

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

Glad to read that you got it fixed.

[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.

[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.

[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.

[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?

[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.

[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

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.

[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

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

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 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?

[budm]

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

"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.

[Percyveer]

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

Hi there Capleaker,

39uF is a very odd value, but that is what the schematic shows too... I thought that was a misprint. Put the original cap back in its place for now and change that IC. Because we need to know that the IC is working to hopefully fix the situation or troubleshoot it further.
Having too much voltage on the cap isn't good, but I am wondering "what" you actually measured. I expected the voltage to rise on the cap, when the IC is gone, but surely not a value of 76V.
Lets see if budm sees this.

I have now replaced the STR G6651, no change in any voltages on primary side, except the Vin to the IC has gone down to 13v now, and have a few weird fluctuating voltages on the board outputs, which I have marked in megenta on the uploaded drawing.
Is it the Vin voltage too low at 13v, what could be causing this?

[Percyveer]

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

The D to S zener symbol indicates the intrinsic avalanche diode that most MOSFETs have.

That was puzzling me, thank you.

[rievax_60]

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

The STR-G6615 data sheet has no description of operation.
Are you talking about this IC or generally?
I have read some well written data sheets on similar IC's, and some of them start from the drain pin and then switch over to the Vin pin when running.
Looks to me like that is what happens with this one, because in the data sheet functional block diagram, it shows a zenner across source to drain, this is the start up is'nt it, if so it only passing from source to drain (o/c with probes reversed), in which case that is the element that has failed.

Percy

Generally, mainly with the early UC3842 SMPS control ICs. The STR-G6615 needs start bias current into pin 4. I would use a 1K resistor. Remember, none of this involves applying mains voltage.
The D to S zener symbol indicates the intrinsic avalanche diode that most MOSFETs have.

[CapLeaker]

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

Maybe.

But surely it is only because the IC is out of circuit that the voltage gets to 37vdc, so with a new IC, the currently rated 25v one should be OK.

Let me rephrase my last answer then. I agree with that above.

[Percyveer]

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

I have done a similar thing with the part still in circuit by connecting a bench supply directly to the HVDC rail. NO MAINS CONNECTION MUST BE MADE, just the bench supply.
An alternate start resistor has to be temporarily added.
It is best to have an oscilloscope.

The STR-G6615 data sheet has no description of operation.
Are you talking about this IC or generally?
I have read some well written data sheets on similar IC's, and some of them start from the drain pin and then switch over to the Vin pin when running.
Looks to me like that is what happens with this one, because in the data sheet functional block diagram, it shows a zenner across source to drain, this is the start up is'nt it, if so it only passing from source to drain (o/c with probes reversed), in which case that is the element that has failed.

Percy

[Percyveer]

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

Nope.

Can you expand on that.
Is it nope I am missing something, or nope the existing 25v cap will not be OK, if so can you explain why?
Thanks Capleaker.