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Power supply repair

:: EPE Chat Zone ≠:: ≠Radio Bygones Message Board :: » EPE Forum Archives 2010 - » Archive through 26 January, 2014 » Power supply repair « Previous Next »

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echase
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Username: echase

Post Number: 741
Registered: 07-2007

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Posted on Wednesday, 08 January, 2014 - 01:31 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Have a 10 year old Sony home cinema with a standard multi rail SMPS of around 100W. Lately it fails to start up when mains switch turned on or turns on for a few seconds and goes off again. I think it is temperature related as limited testing shows:

1) A cold soak makes it worse. Blasting a hot air gun at the output stage resurrects it.

2) Turning the switch on and off many times eventually gets it going, I assume because the heat thus generated in the failed component eventually is enough.

3) Worse after a long period off as presumably has cooled down.

What component would be the most likely failed/degraded item to cause this? Iíd aim for the many electrolytics, although none show any visual signs of failure and there are no dry joints. Hope itís not a transistor/IC as difficult to diagnose.
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basementboy
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Post Number: 171
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Posted on Wednesday, 08 January, 2014 - 04:06 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

If you are able to be more directional with the heat gun, you may be able to identify the component. Alternatively, you could get the board working, then use freezer spray, or maybe an ice cube in a bag.
I'd say electrolytics too. Any running hot?
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echase
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Post Number: 743
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Posted on Wednesday, 08 January, 2014 - 05:20 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Ice cube sounds good. I have always been wary of freezer sprays because of the condensation they must cause and what happens when electricity tracks along this water? There is mains nearby.

Or freeze to death doing the job outside to keep the overall unit at low temperature and then use spot heating as you suggest. It heats up too quickly inside to be able to locate the problem.

Or just remove each cap and test them.
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chippie
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Posted on Wednesday, 08 January, 2014 - 06:22 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

IME, I'd be inclined to look at the start up circuitry....
Any high value resistors connected to the switcher's base/gate?
Caps are best replaced with low ESR types as you would....
eccentric millionaire financed by 'er indoors
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gordon
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Post Number: 951
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Posted on Wednesday, 08 January, 2014 - 07:27 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Sounds like it could be one or more faulty capacitors stopping the supply from running normally. Not sure if you have seen this web based repair guide, but you might find it useful to narrow down the fault.

http://www.repairfaq.org/sam/smpsfaq.htm
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mee
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Post Number: 190
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Posted on Wednesday, 08 January, 2014 - 10:38 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Good link Gordon, thanks. First thing that occurred to me was that the problem sounded as though it might originate from a faulty bipolar (not Mosfet) device that was acting as a switch in a sensing circuit somewhere in the power supply.

A bipolar optocoupler was my second thought. They often used to be used for isolation.

Ten years isn't that old for most Sony stuff (made then at least) so good luck.

Cheers, M
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cjay
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Post Number: 30
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Posted on Wednesday, 08 January, 2014 - 11:07 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Capacitors, always. Change the lot.

Be especially suspicious of any with a four and a seven in the value.
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echase
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Post Number: 744
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Posted on Thursday, 09 January, 2014 - 11:55 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Thanks for suggestions. By working in a cold conservatory and putting a soldering iron on the tops of the caps I found one near the main switcher that immediately kicked it into life. Itís a 47u 35V and appears to couple the voltage on the feedback winding to the switcher. It measured 44u, or 47u if put in freezer. No leakage shown on DMM. But I donít have a way of measuring ESR. Can ESR go high whilst capacitance stays the same?

IC is a 5 pin STRF6267 with integrated switching transistor, possibly by Sanken. There is no datasheet available for it so probably a special for Sony but there are others by Sanken that are similar.

Other possibility is that there was sufficient transfer of heat from the cap to the many surrounding diodes to have caused a fault in one of them to disappear. Later I will put the cap back on long legs so there is less heat carry to the diodes when iron applied.

Once running the self heating is enough to keep it going.
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gordon
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Posted on Thursday, 09 January, 2014 - 05:22 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Temperature seems to have a large effect on the ESR of 'liquid' type electrolytic capacitors, but much less effect on the actual capacitance. Could explain the effects you are seeing. I think the capacitance can test correct, but the ESR can be way higher than it should be for a particular faulty capacitor. An ESR meter is the only way to be sure , or just try a new capacitor!

http://www.low-esr.com/esrtempperfcurves.asp
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echase
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Post Number: 745
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Posted on Friday, 10 January, 2014 - 01:49 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Changing the cap seems to have fixed it so fortunately it does not seem to be the diodes. The limited markings on these would make them hard to identify. It is in the start up circuit.
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joe
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Post Number: 1444
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Posted on Saturday, 11 January, 2014 - 06:34 am:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

One thing to consider is that if one of the caps is playing up, the rest may also be on their way out - they've all got the same number of run hours on them now. You could argue that you've been lucky in that no additional damage has been caused this time, but if one of the others were to fail open or short circuit, it could cause collateral damage that's harder/more expensive to rectify.

Regards,
Joe
My projects, technical info and ramblings at www.hobbyelectronics.net/
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echase
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Post Number: 747
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Posted on Saturday, 11 January, 2014 - 02:42 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Good point. The only other electrolytics on the input side are a 4.7u coupling one and the main 320V smoothing cap. Replacing the former is cheap but would failure of the latter cause collateral damage? I have not found that to be the case in the past, except perhaps the bridge recifier.

All the ones after the switching transformer, except for any in the opto feedback circuit, would not affect the input side. So I guess the worst case is one rail having too much ripple. There are too many caps on the output side to reasonably replace all and there is always the risk that the wrong ESR value is selected thus adversely affecting something.

(Message edited by echase on 11 January, 2014)
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basementboy
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Post Number: 176
Registered: 05-2012

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Posted on Saturday, 11 January, 2014 - 03:54 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

It's a tricky one - replace or not replace. As component life is measured in mean time before failure, you never know whether you are seeing representative failure with one item in a batch. You can also look at the position in a risk based model - this is business speak for 'how much will not doing something cost'
If I repair something for a friend I tend to over-replace; a business would do the same as the man hour cost of opening up and fault finding is huge compared to a few components.
There is the green view too. Don't throw away something perfectly serviceable.
FWIW echase, I'd replace identical caps, same make,shape, value, but no others - Sony aren't known for skimping.
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ian_field
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Post Number: 178
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Posted on Monday, 13 January, 2014 - 09:41 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Always worth a look at the electrolytics for any that even look the least bit tired, there's one or two that can cause the PSU to start up *ONLY ONCE*! Bulged tops, shrivelled plastic sleeve or if you lift them out look for a corrosion stain round one leg.

I'd caution against toggling the on/off switch to make it start - that can also make it start *ONCE ONLY*!

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