Troubleshooting RFI from switch-mode PS

I have a Samlex 120VAC-13.5VDC at 35A switch mode power supply for my amate ur radio equipment. Working great for last 7 years. Recently it's now produ cing "swooshing" noise in my radio receiver, especially on frequencies from 1.8-10MHz. Took it apart and noticed blown tops on the 3 output filter ele ctrolytics, so I replaced those. But that did not help.

I could replace the 2 larger input filter electrolytics but they "look" OK and I wonder if maybe I should be thinking of also replacing the switcher M OSFET - I don't know where else the noise could be coming from?

Any ideas greatly appreciated.

Reply to
lagagnon
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teur radio equipment. Working great for last 7 years. Recently it's now pro ducing "swooshing" noise in my radio receiver, especially on frequencies fr om 1.8-10MHz. Took it apart and noticed blown tops on the 3 output filter e lectrolytics, so I replaced those. But that did not help.

K and I wonder if maybe I should be thinking of also replacing the switcher MOSFET - I don't know where else the noise could be coming from?

I've never seen a power mosfet do anything but work normally or become zero ohm jumpers.

Are you sure it's the supply? Put a scope to the output and see what's goi ng on. As far as the capacitors "looking" OK, remember that electrolytics need not be vented or bulged to be bad. Either put an ESR meter to *all* o f them in the supply (including the primary) or scope them (preferred metho d).

Reply to
ohger1s

teur radio equipment. Working great for last 7 years. Recently it's now pro ducing "swooshing" noise in my radio receiver, especially on frequencies fr om 1.8-10MHz. Took it apart and noticed blown tops on the 3 output filter e lectrolytics, so I replaced those. But that did not help.

K and...

The fast rectifiers on the low voltage side can fail/become leaky, and when they do, they can take out capacitors. Check those.

Reply to
whit3rd

I can't tell from here where the noise is coming from. Usually, I put a small loop antenna on my spectrum analyzer and save it around the power supply while under load. That usually locates the general area.

Much more interesting is why three capacitors would decide to bulge. Bulging is usually caused by excessive ripple current through the filter capacitors. High ripple could be coming from the diodes, or simply from undersized capacitors that can't hold a charge between switching cycles and therefore draw more current than normal.

I suggest you put a scope on the output while under load and see if there's any ripple present. Compare the value with the manufactures specs. Also, use an IR thermometer to measure the case temperature of the capacitors. If they are getting warm, then there's ripple current going through them.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

teur radio equipment. Working great for last 7 years. Recently it's now pro ducing "swooshing" noise in my radio receiver, especially on frequencies fr om 1.8-10MHz. Took it apart and noticed blown tops on the 3 output filter e lectrolytics, so I replaced those. But that did not help.

K and I wonder if maybe I should be thinking of also replacing the switcher MOSFET - I don't know where else the noise could be coming from?

a) Replace all the caps. Unless you have both X-ray eyes and an ESR meter, you have no effective way of discerning whether they are good or not. b) If it works otherwise - that is, produces the correct voltage - unlikely that the MOSFet is bad. c) It becomes a close-run thing whether the cost/time involved in repairs e xceeds the value of the device if it is more than just the caps.

Peter Wieck Melrose Park, PA

Reply to
pfjw

Is this the one or is there some reason you didn't bother disclosing the model number? Notice that the SEC-1235 is a 12V 30A device and current limits at 35A. How many amps are you drawing?

See page 10 for RFI information:

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Jeff Liebermann     jeffl@cruzio.com 
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Reply to
Jeff Liebermann

In the last 15 years, I've changed literally thousands of bulged electrolyt ics in consumer electronics for no other reason except they were just crapp y parts at the end of their limited lifetimes. Most of those that bulge or vent are higher value types, say those beyond 330uf or so. Smaller electr os fail at the same rate of their big brothers but don't generally visually vent. In the old days, capacitors didn't bulge unless they were tortured. Today it's mediocre chemistry that causes this, not circuit issues.

If you were to take apart 10 working flat screen TVs made in the last few y ears, half of them would show some sign of bulged or top vented capacitors despite running normally.

Reply to
ohger1s

I know the feeling: That's about a 5 month collection, mostly from motherboards, power supplies, and LCD monitors. I've cut back on the component level repairs in the last year, so this years collection is about 1/3 the size. I have 4 different ESR testers and use them often.

In the bad old daze, the electrolytics were running at 60Hz, not

60KHz. The power (heat) dissipated in the electrolytics is a function of frequency.

Ok, time to vent...

I blame CAD (computer aided design) for the problem. CAD has allowed designers to accurately predict the life of some components including electrolytics. If the operating parameters are known (voltage, temp, ripple current, starting ESR, etc), the lifetime can be calculated. Having a component last longer than the warranty period is considered wasteful by many manufacturers. Therefore, they select the cheapest possible part that will work up to when the warranty expires but no longer. The result is a product where every component blows up at the same time, just after the warranty expires. The 10 flat screens in your example will not show one bad component. Instead, there will be dozens, all different brands and values. This is not due to crappy manufacturing, but rather due to careful selection for the MINIMUM lifetime of the component.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

I would refine this somewhat. When this happens, the average consumer will change brands as most (well, not the typical Trump supporter) recognize tha t something failing just after the warranty period is the manufacturer want ing to maintain the demand stream. I have found that such electronics - bas ed on average hours used and other factors - tent to last about 25% beyond the warranty period - that is, if they last the first six weeks. Just long enough for the consumer to forget the warranty period.

There are exceptions: We have a Sony CRT-type TV purchased in Saudi in 2002 that has probably 13,000 hours on it that has survived two moves, two ocea n voyages, kids, cats, dogs and such without a hitch. The Panasonic Plasma with about 6,000 hours on it is doing fine (so far) as well. Frigidaire re frigerators have about 5 years + one month before they crap out. GE units o f US origin (in our experience) are pretty bullet proof. Bosch appliances, also, are pretty bullet-proof. It's all according to a mix of decent manufa cture and pure blind luck.

Peter Wieck Melrose Park, PA

Reply to
pfjw

That's an "honorable" American tradition that dates back at least a century. The story goes that Henry Ford has his engineers buy and disassemble scrapped Ford cars, to see which parts had failed and which ones were still in good shape. He pointed out that the company could often save money by redesigning (more cheaply) parts which rarely failed... so the new versions would wear out and fail at about the same time as everything else.

I remember one of the Lord Darcy stories by Randall Garrett, which involved a (magic-based) process somewhat like this for the human body. The spells would draw upon the health of long-lived parts of the body, to support those which were more prone to obvious aging.

The recipient of this treatment might have the apparent health and vigor of one in his thirties, and sustain this for several decades (e.g. up until the age of 60 or so). At that point, all of the body systems would collapse simultaneously, and the patient would die of old age in the space of an hour or two.

Reply to
Dave Platt

Live fast. Die young. Leave a good looking corpse.

Peter Wieck Melrose Park, PA

Reply to
pfjw

** The heat loss in an electro is related to ripple current and ESR at the frequency in use. ESR is much HIGHER at 60Hz or 120Hz than it is at 60kHz - so losses are LOWER at high frequencies.

Also, ESR goes down with increasing temperature - by up 5 times at the max rated temp of the cap.

Running in a hot environment is the main cause of short lifetimes, eg with nearby heatsinks and no fan.

..... Phil

Reply to
Phil Allison

They've been using high frequency smps since the late 70s (Sony was the fir st that I can recall (and boy were they *squirrely*). And while we replace d capacitors on the wholesale level even back then, they didn't bulge. The vented cap syndrome (VCS?) has really only been an issue since about 2004 o r so. Around 2008 was the real big explosion of top bulged caps, when we'd see TVs in warranty with bulged/vented caps. There's been a big slowdown in bulged caps in recent years.

Reply to
ohger1s

Sounds like this might be the "capacitor plague" problem? Bad water-based electrolyte?

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Reply to
Dave Platt

** Transceivers tend to draw a LOT LESS current while in receive mode - which is the only time PSU generated RF noise might be a problem. SMPSs often send a fair amount of RFI back up the AC power cable, even when running at low power outputs which makes them troublesome when used with radio receivers - especially in the lower part of the HF band.

The OP's supply may have deteriorated components ( plastic film caps most likely ) in the filter fitted to the incoming AC line. Only way to know for sure is to get another, similar supply and try it.

If the OP has a linear 12V supply that can run the transceiver in receive mode, he should try that too.

..... Phil

Reply to
Phil Allison

I will respond to some of the posts above, without discussing the side topic of poorly manufactured capacitors etc.

1) the PS is rated at 35A, but I rarely draw more than 18A - and that is not a continuous draw. 2) I run the PS in a cool room, and also as a result of point 1 above the cooling fan has never come one (but has been tested to work). In other words the PS runs very cool 3) nothing has changed for 7 years until recently so this is definitely recent noise coming from the PS as I can switch to a deep cycle battery instantly and the RX noise disappears.

I have ordered the two 1000uF, 200V input caps and should replace them in about 3 days and will let you all know the results.

Reply to
lagagnon

Sorry for the topic drift, but my intuition tells me that the noise is somehow related to the bulging caps.

That's nice. Now, for the 2nd time, will you kindly disclose the model number of the power supply. Is it this one?

I didn't suggest that the power supply was overheating. I suggested that the capacitors might be overheating from too much ripple current. I also suggested that you use an IR thermometer to measure the case temperature of the capacitors.

Nothing has changed is what all my customers say. Nothing means nothing that is obvious to them. In most cases, that's a polite way for them to suggest that I not pry too deeply into what the customer has been doing with the device. If I do pry, I often find questionable practices. I had one like that 2 days ago. Nothing had changed in years before the computer crapped out. Too bad the fan and most of the insiders were packed with cat hair.

I rarely have any problems with the input caps. It's the lower voltage caps that blow.

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Jeff Liebermann     jeffl@cruzio.com 
150 Felker St #D    http://www.LearnByDestroying.com 
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Reply to
Jeff Liebermann

Not common, but not unheard of. I've replaced a bunch of 450V PFC capacito rs in smps that don't show signs of venting, but they're dead and when remo ved, feel like half the weight of what they should be. They also "rattle" inside as the assy inside has shrunk.

Reply to
ohger1s

Yes, that's the one. Does that make a difference to your assessment?

OK, I understand, and I wonder whether the output caps which I've replaced have blown because the input caps are not as effective at removing ripple?

How is that relevant to my post and my situation?

See my suggestion above....

Reply to
lagagnon

The input filter capacitors serve no RF function, so won't solve anything related to RF noise.

Reply to
whit3rd

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