SMPS Rectifier circuit burned up a resistor

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ok, I have the exact same problem as in this thread:

RT2870 R202

but it seems the guys working the issue in that thread look into mostly audio equipment and this is really just an issue with the devices power supply.

I have some pictures of my resistor, mine is not nearly as burned up as the other guys, but the rings are still toasted and it's very hard for me to make a guess at it's value. it seems to read brown white black gold on one side, but on the other, it's brown brown black gold. obviously one side is burned worse than the other. I'll try to get some pics up tomorrow.

also, the (I'm guessing tantalum?) capacitor in the circuit doesn't seem to have micro-farad rating on it, though I'm not very familiar with reading tantalum type capacitors. all it says is "*222K 1kv 86" is 222k the farad rating? because I was thinking it was a temp rating in kelvin? wouldn't that be an insane farad rating? or is it 222k-micro-farad??? why use a number like 222k micro anythings? call me a noob, I mostly work with electrolytic and polyester type caps in crossovers and the like.

I am determined to get this stereo working again though. it can't possibly be anything beyond figuring out...

Thanks!

[budm]

Re: SMPS Rectifier circuit burned up a resistor

As you have read on that thread, the RC snubber network is for filtering the switching noise generated by the rectifier diode when the diode switching on/off. The Diode switches ON/OFF at about 50,000~80,000 time a second.
222K 1KV is 0.0022uF 10% 1000V Ceramic cap (the blue disc cap). You need to check if that cap has any resistance leakage. The power will still function without it, cheaply made PS do not even has snubber in them.

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Re: SMPS Rectifier circuit burned up a resistor

Thanks bud!

I was aware it was a switching power supply already, the right side of the board is very similar to what you see in mosfet car amplifiers.

I'm hoping only to have to replace r202 and c211. I really hope the fet is still good. if it's bad then there goes at least $5 and a fair bit of searching to get the right one since it only has a brand and model on it.

the thing seems to power on the display, but when it tries to supply power to the amplifier PCB it detects an issue and shuts itself down to prevent further damage.

I really appreciate you explaining the value of the capacitor to me as well. are you sure its a ceramic cap though? I'll most likely be ordering the parts from Mouser again for this project, I was really hoping it would just be a quick trip to radio shack for a resistor, but to get specific cap specs I usually have to order online...

if there's any other info I could get for you that may help in the solution for these SMPS boards that would be great. it seems everyone who ever had one has had it blow up with the same problem in the last two years...

I have some high res shots of my resistor that isn't nearly as burned up as the one in the other guys thread too, I'll have to host them tomorrow though, it's nearly 3am in my part of the world now and I should really get to bed.

thanks again!

[Home_Command_Center]

Re: SMPS Rectifier circuit burned up a resistor

so it occurred to me, what would happen if I just pulled out the cap and wired up the SMPS into the unit? yes, there would be hum, but it would be a good diagnotic and tell me if I need a new fet, correct?

blast, MUST PUT KEYBOARD DOWN AND SLEEP

thanks again

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Re: SMPS Rectifier circuit burned up a resistor

pics of my resistor:

[budm]

Re: SMPS Rectifier circuit burned up a resistor

OK, so be side the burnt RC snubber, you also have the problem in the AUDIO output section? if the unit has shorted circuit, the external power supply will also shutdown since the over current sensing circuit will be activated.
yes I am sure it is ceramic disc cap, very low ESR, land very very low inductance (it has to be). Can I see the pictures of the whole board. More likely to be ClassD amplifiers.

[Home_Command_Center]

Re: SMPS Rectifier circuit burned up a resistor

well the SMPS board I have is identical to the one in the other thread, or are you asking about the amplifier board? nothing was shorted on the amp board, not sure why it would have failed. the prompt for the failure came from the rear audio input board. the whole thing is strung together with a mess of ribbon wire. I can try to see where this particular circuit goes to on the molex type connector and then get a picture of the area on the board it goes to if thats what your asking for.

[budm]

Re: SMPS Rectifier circuit burned up a resistor

I thought the problem you are having is due to over current draw by the amplifiers due to suspect bad MOSFETs in the amp section (not sure which amp is bad) that causes the SMPS to shutdown. Or you are not sure if your SMPS is OK or not.

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Re: SMPS Rectifier circuit burned up a resistor

no, r202 is toasted on the SMPS PCB, just like in the other thread. the device powers on, lights up the display panel, then fades out back to just the standby indicator light being on.

if it helps, when I unplug it, the standby light stays on until i press the power button, then the capacitive hum goes away and the light goes out.

the device failed after what I will assume was a partial short on one of the rear RCA input channels.

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Re: SMPS Rectifier circuit burned up a resistor

ok, just took some more pics of the whole device. the circuit that is bad on the SMPSB is indeed the one that supplies the amplifier PCB. signal comes to the amplifier PCB via a thin film cable from the input/processor side of the device. nothing appears to show any visual signs of damage on either of those boards at the moment. if need be I can pull out the amplifier board to get a look under the heatsink and see if I see a spot of toasty thermal putty.

so I suppose the issue at this point is that there was a signal spike coming in from the input/processor board and it fried one of the fets in the amplifier board, which is in turn burning up the SMPS's circuit for powering the amplifier board?

let me go count the number of lines in that film cable...

I'll upload some pics in a bit

[budm]

Re: SMPS Rectifier circuit burned up a resistor

OK, the power supply board has two power supplies, one is an always on power supply to run the micro controller, the second one is switched on when the power switch is activated which will cause the micro controller to send the signal to turn on this power supply for the power amplifiers, it is done to save power.
Even with burnt out RC snubber resistor, the supply will still function. You can disconnect that connector and see it the unit still go into fault condition, you can also monitor the DC voltage on that connector to see if it goes away when fault occur.
Shorted RCA inout will not cause any problem since it is for receiving very small audio input signals, it will not damage anything if the input is shorted.

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Re: SMPS Rectifier circuit burned up a resistor

awesome, that's what I was thinking. so all I need to do now is take out the .0022uF cap and the snubber circuit will be deleted, so then I should then be able to reinstall the board and check to see if the device powers up and stays up. if it still powers back down then I need to remove the amplifier board and work in that department.

[budm]

Re: SMPS Rectifier circuit burned up a resistor

Just diconnect that connector on the bottom right picture, that way there will be no load for now and see if you still has the fault condition.
You do not need to remove the cap since it is in series with the resistor that you have already remove so the cap is no longer completed the circuit.

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Re: SMPS Rectifier circuit burned up a resistor

ok, powered it up with the amp power disconnected, stayed on fine. plugged in amp section, works... partially.

it has three power outputs from the power supply, and 6 channels. 4 of 6 channels are working, 2 are not.

so it would seem we have one of two things going on here, we're missing a leg on the power supply side, or a pair or fets has collapsed.

I'm leaning towards option A. what do you think?

[edit] just looked back at the diagram of the SMPSB and all three power channels come from a common source, so it has to be something on the amplifier board side.

I'll look into that in another thread I suppose, but I guess all thats left for this thread is to try and determine the value of that resistor so when I get it all back together it's as it was and not jacklegged.

Thanks Bud! you've been a great help!

[tom66]

Re: SMPS Rectifier circuit burned up a resistor

Which four are working... maybe it's only getting stereo in and not demuxing centre/sub? Assuming it has those channels.

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Re: SMPS Rectifier circuit burned up a resistor

this is the amplifier board FWIW:



Have yet to remove it though...

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Re: SMPS Rectifier circuit burned up a resistor

Well, it's not MOSFET, it's got three surface mount ICs (ST sta5178's) with something like 17 tiny legs per side. one of the ICs blew out on one side so hard it burned out more than half the legs, and left a blast mark on the board as well as the heat sink. I have no idea what sort of problem could have caused a failure this massive. in the process of trying to see if I can find a replacement amplifier board for it.

repairing surface-mount is amazingly hard. I've done some work at this scale on old Nokia cell phones back in the day. changing led colors, repairing bad parts and what not from donor phones, etc. I'd really rather not go through replacing surface mount stuff again though...

[edit] the digital amplifier ICs are available though, from shady Chinese parts houses:

http://www.alibaba.com/product-gs/63...w_STA5178.html

[budm]

Re: SMPS Rectifier circuit burned up a resistor

I have seen so many of these home theater amps come in for recycle, and also sub woofers. They just about all have blown ClassD amp IC. There is not much protection against shorted output.

[tom66]

Re: SMPS Rectifier circuit burned up a resistor

I would guess that the short could also kill the speakers attached.

SMD is easy, with the right tools -- I worked on it daily a few months ago and frankly loved it compared to through hole. Once you get over the small sizes it's much better.

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Re: SMPS Rectifier circuit burned up a resistor

yeah, I tracked down the issue, I had a 4ohm sub connected to the 8ohm output, it worked fine for about a month, but when my wife plugged in her ipod, the sudden crack of high input put the amp circuit beyond it's stability.

Right now I'm considering lifting the IC for the rear channels off the board and replacing the blown center and sub IC with it, then devising a transformer with a 1:0.7028625 winding ratio to bring the load into spec. the 1:0.7028625 winding ratio should leave me with 165 watts of the available 167 and be within spec for ohm load. I'm not sure if there is an off the shelf transformer with this ratio or how complicated it would be to build one from parts.

the end result would leave me with a 3.1 stereo system, wouldn't have rear channels, but it would still sound good for a little system in the living room.

any idea where I should start as far as determining which IC runs the rear channels though? it's a dual layer board, so I can't just hold it up to a light, and the speaker outputs are all enclosed in a little module, making it hard to trace the circuits.

also wondering if the IC may have taken some of the tiny surface mount cap along with it, though all three digital 2 channel amp circuits are mirrored, so swapping the bits around would just be an exercise with the soldering iron and some pointy tweezers.

I really must say though, this system had almost no room for lower ohm load stability, when I got it to power back up it was only on half volume. I've got an ancient sony 5.1 bookshelf system that seems to take just about anything I through at it. I wonder if this device has a 2.1 setting. it mentioned working in 3.1 from some paperwork I looked up for it, but if I could get it to just run 2.1 I could put my carver sonic holographer behind it and just use this RCA device as an amp for some front speakers and a sub...