sci.electronics.repair@googlegroups.com

Google Groups

Cursitor Doom <curd@notformail.com>: Jan 21 06:00PM On Thu, 21 Jan 2016 15:10:21 +0000, MJC wrote:   > Could it be an electrical discharge? Try viewing in the dark (with > precautions against contact!)...   I like your thinking so I tried it. But no sign of any arcing at all. Anyway it really does sound like more of a hissing than a rasping.

Cursitor Doom <curd@notformail.com>: Jan 21 06:11PM On Thu, 21 Jan 2016 10:30:23 -0500, Ralph Mowery wrote:   > Feel the capacitors and find which one is getting hot. An IR heat gun > may be better to keep the shock hazzard down.   I'm not really happy about prodding around this thing while it's running for that reason. I've tried feeling for hot caps after switching it off again, but at 150V I'm guessing it's not enough to heat the dud cap up enough to be able to feel the difference. I have tried using a temp sensor & DVM to check for heating but that proved inconclusive as well.

etpm@whidbey.com: Jan 21 10:49AM -0800 On Thu, 21 Jan 2016 18:00:23 -0000 (UTC), Cursitor Doom >> precautions against contact!)...   >I like your thinking so I tried it. But no sign of any arcing at all. >Anyway it really does sound like more of a hissing than a rasping. I have heard high frequency arcs that sound just like air escaping, at least to my ears. Eric

Cursitor Doom <curd@notformail.com>: Jan 21 06:58PM On Thu, 21 Jan 2016 10:49:34 -0800, etpm wrote:   > I have heard high frequency arcs that sound just like air escaping, at > least to my ears. > Eric   Interesting. What component was failing? There's a 1.5kV transformer on this board which is encased in opaque grey plastic so if it's something arcing inside that it won't show up in the dark. I thought perhaps a old AM radio next to the board might indicate something useful, but I'm guessing it would be swamped by all the spurious noise these things make even when they're running right. Spectrum analyser, perhaps? Bit drastic but I do have one!

Jon Elson <elson@pico-systems.com>: Jan 21 01:19PM -0600 Cursitor Doom wrote:     > On cranking up the supply voltage (230V here) I get to about 150V on my > variac whereupon I can hear a capacitor venting; a clear hissing sound > like air being let out of a tyre.   Well, that could just be the switching action of the power supply. Some generate pure ultrasonics, but some have lots of other frequencies in them and the part we can hear sounds like hissing. I've heard lots of SMPS make hissing sounds under some particular condition (no load, full load, intermittent current pulses to the load, etc.) Undervoltage might have the PS jumping back and forth between start-up mode and normal switching mode very fast. MANY SMPS' will make a hissing sound as the main input cap runs down, which could be exactly what you are seeing with your variac test.   If you do just put full AC to it, wear ear protectors and don't poke your face near it, just in case it IS a cap getting ready to blow.   Jon

John-Del <ohger1s@aol.com>: Jan 21 11:20AM -0800 Sometimes an smps run at low voltage (or with a defect) will not osc cleanly and make noise like hissing air. I've seen some supplies sound like that when one or more of the secondaries are feeding a short. Most modern designs shut down completely but a lot of the older ones throttled back to a safe current but otherwise would continue to run.

dplatt@coop.radagast.org (Dave Platt): Jan 21 11:41AM -0800 In article <n7r9pg$1at$3@dont-email.me>,   >Interesting. What component was failing? There's a 1.5kV transformer on >this board which is encased in opaque grey plastic so if it's something >arcing inside that it won't show up in the dark.   Try the Grand-Dad's Ear Horn approach. Get a length of 1"-diameter plastic pipe, hold one end against your ear, and move the other end around the board. This will let you "listen" to the individual components, while still maintaining a safe distance from any high voltage.   Seems to me that you could be hearing either:   - Actual venting. I'd be a bit surprised, though, if this were happening and you couldn't spot the culprit. Maybe fire it up in a really cold location, and look for a cloud of vapor?   - Switching noise from a failing cap. I've heard SMPS capacitors begin to hiss like this when they began failing due to the infamous "'lytic capacitor electolyte plague" a few years ago. They'd go high-ESR well before they'd start to leak and vent.   Checking all of the caps on that board with an ESR meter (with the power off and fully discharged, of course) might locate the culprits.   - High-voltage leakage. I had a Tek 7904 scope which was suffering from corona discharge and hiss (and an occasional "snap") due to some contamination/deterioration on the high-voltage lead from the power supply to the CRT. Cleaning didn't help. Coating the lead (from the PSU, through the cage) with a bit of high-voltage silicone putty made the problem go away.

Cursitor Doom <curd@notformail.com>: Jan 21 07:43PM On Thu, 21 Jan 2016 13:19:44 -0600, Jon Elson wrote:   I've never been in a position to work on a switcher before owing to the lack of an isolation transformer until very recently, so I was unaware of that. Well, if push comes to shove and there's no alternative, I'll have to jack up the juice to 230V and we'll find out then one way or another.

Cydrome Leader <presence@MUNGEpanix.com>: Jan 21 10:06PM > like air being let out of a tyre. Trouble is, I can't see which cap it is > out of about the dozen or so electrolytics on the board. There's no > visible steam or smoke whatsoever. I've tried using a piece of thin   You're running a switcher at way out of spec input voltage. The magnetics making awful noisy or hissing sounds is not unexpected under those conditions as it fails to startup correctly.

Cursitor Doom <curd@notformail.com>: Jan 22 11:30AM On Thu, 21 Jan 2016 22:06:02 +0000, Cydrome Leader wrote:   > You're running a switcher at way out of spec input voltage. The > magnetics making awful noisy or hissing sounds is not unexpected under > those conditions as it fails to startup correctly.   Well I've now wound it up to the correct supply volts and it's still hissing. Nothing's blown that I can see; there's just one power resistor that's running too hot to touch (because the psu is unloaded?)everything else, including the transformer is cool. Also, by use of better gauge pipe, I've been able to detect the sound is coming from the grey 1.5kV transformer in the middle of the board:   https://www.flickr.com/photos/128859641@N02/16727519128/   Could this be just normal behaviour that we don't normally hear since it's normally buried deep within the scope casing??   BTW, those burnt areas are previous problems that someone else fixed years ago and not relevant to the current issue.

Chris <cbx@noreply.com>: Jan 22 12:11PM On Fri, 22 Jan 2016 11:30:10 +0000, Cursitor Doom wrote:   > Could this be just normal behaviour that we don't normally hear since > it's normally buried deep within the scope casing??   It can be so hard to tell sometimes. Try scoping the output from the transformer (taking suitable precautions of course!) and look for any irregularities that might indicate internal arcing.

"Mark Zacharias" <mark_zacharias@sbcglobal.net>: Jan 22 06:45AM -0600 "Cursitor Doom" <curd@notformail.com> wrote in message news:n7r6bn$1at$1@dont-email.me... >> precautions against contact!)...   > I like your thinking so I tried it. But no sign of any arcing at all. > Anyway it really does sound like more of a hissing than a rasping.   I would look for anything shorted on the secondary side of the supply. Pulls the switcher frequency down into the audible range. (for those of us who can still hear anyway...)   Usually the root cause is a regulation problem (bad caps mostly) causing the monitored voltage to "appear" low, the switcher duty cycle increases, the actual DC output rails out, usually shorting a zener diode which gives up it's life to save components downstream.   Example: an 18 volt zener across a 14 volt line shorts because now that line has shot up to 35 volts.   Common problem in older consumer vcr's, but 'scopes etc often do the same thing.   My .02 worth...     Mark Z.

"Kenny" <me@privacy.net>: Jan 22 10:25AM Acer laptop with combo headphone/mic socket. The tip of a 3.5m stereo jack has detached inside the socket and it's cutting off the speakers. If I put the headphone jack with the missing tip back in the headphones work. It was either a poor quality plug or the socket gripped the plug so tightly it pulled the tip off. Problem now is getting the tip out of the socket without damaging it. This is not an easy laptop to dismantle, I recently fitted an SSD and it was awkward work. Been wondering if I put a dab of superglue on the end of the plug, push it in the socket, leave it for a while to set then pull it out will it bring the detached part out with it? An obvious pitfall is that the whole plug would then become lodged in the socket! Any ideas anyone?   Kenny Cargill

MJC <gravity@mjcoon.plus.com>: Jan 22 11:23AM In article <4sudnRvbIrfnnj_LnZ2dnUU78RGdnZ2d@brightview.com>, me@privacy.net says... > whole plug would then become lodged in the socket! > Any ideas anyone?   > Kenny Cargill   Another obvious pitfall is that the glue is unlikely to be stronger than the original assembly, which has already broken...   Mike.

"Kenny" <me@privacy.net>: Jan 22 11:55AM Didn't realise it was so common a problem, have a few YT videos to look at later.   Kenny   "MJC" wrote in message news:MPG.310c21a2d8d31ebf9896b6@news.plus.net...   In article <4sudnRvbIrfnnj_LnZ2dnUU78RGdnZ2d@brightview.com>, me@privacy.net says... > whole plug would then become lodged in the socket! > Any ideas anyone?   > Kenny Cargill   Another obvious pitfall is that the glue is unlikely to be stronger than the original assembly, which has already broken...   Mike.

N_Cook <diverse@tcp.co.uk>: Jan 22 12:09PM On 22/01/2016 10:25, Kenny wrote: > whole plug would then become lodged in the socket! > Any ideas anyone?   > Kenny Cargill   Superglue in tight spots , goes where it wants to go, not where you want it to go. Find the utube video of dismantling it , enough to get a needle or something in there to prize it out, from inside the laptop

janetnkansah1@gmail.com: Jan 21 09:32AM -0800 > l wish to know the difference between negative 48 volts and the normal 48 volts. > Also l will be glad if a could have a circuit for a regulated -ve 48 volts power supply at 3A. thanks   Thanks a lot Jeff you've actually given me a good understanding. so I can build a normal 48 volts supply and ground the positive terminal instead.

"Ralph Mowery" <rmowery28146@earthlink.net>: Jan 21 12:49PM -0500 <janetnkansah1@gmail.com> wrote in message news:1fd0baa9-59c9-42d1-83be-c64761315fc4@googlegroups.com... >> volts power supply at 3A. thanks   > Thanks a lot Jeff you've actually given me a good understanding. so I can > build a normal 48 volts supply and ground the positive terminal instead.   That is right. Just build any power supply that will meet your needs and do not ground any of the components in it. That isolates it from the chassies. The ground the positive output instead of the negative output.   While not usually as high as 48 volts the 3 terminal regulators are made for the negative as well as the positive output. The leads are sometimes in different places and most of the diodes and capacitors are just reversed. That is often the way unregulated supplies are made.

Kaz Kylheku <609-576-4089@kylheku.com>: Jan 21 09:33PM >> l wish to know the difference between negative 48 volts and the normal 48 volts. >> Also l will be glad if a could have a circuit for a regulated -ve 48 volts power supply at 3A. thanks   > Think of voltage as pressure.   ... and as you gradually ease into thinking of voltage as pressure, relax and concentrate on supressing that strange feeling that makes you want the schematic for a "regulated power supply", even though you don't know WTF that means.

M Philbrook <jamie_ka1lpa@charter.net>: Jan 21 08:10PM -0500 In article <b4ffde5b-2847-4ff5-859a-810252306777@googlegroups.com>, janetnkansah1@gmail.com says...   > l wish to know the difference between negative 48 volts and the normal 48 volts. > Also l will be glad if a could have a circuit for a regulated -ve 48 volts power supply at 3A. thanks   96 Volts   Jamie

"Ron D." <Ron.Dozier@gmail.com>: Jan 21 08:31PM -0800 It turns out that -48 V is less corrosive. that's why it's used.   the voltage is "relative". Lots of wall warts might have a coaxial power connector. Some may have center positive and others the outside positive. How you hook it up depends on the circuit.   A -48 supply may indeed have the positive side grounded, but it may not.   A +-12 V supply has a common point and when you measure to that common point you get the +12 and -12 voltage. But that common point may not yet been grounded.   An isolated 48 V supply can generally be connected either way.

Genesys <wolstech@gmail.com>: Jan 21 04:38PM -0800 On Wednesday, January 20, 2016 at 1:31:17 AM UTC-5, David Farber wrote:   > -- > David Farber > Los Osos, CA   Don't buy eBay motherboards for OEM equipment (boards for DIY PC builds have been fine for me so far). Nearly all "new" OEM motherboards in my experience are just used pulls from (then-)working units.   From ones I've bought, they always seem to have something wrong with them. My DOAs generally just didn't POST, but had one with a bad SATA controller. In other cases, they just weren't in the condition listed (typically listed as new, when "used", "seller refurbished", or in one case "as-is/for parts or repair" were more appropriate).

You received this digest because you're subscribed to updates for this group. You can change your settings on the group membership page. To unsubscribe from this group and stop receiving emails from it send an email to sci.electronics.repair+unsubscribe@googlegroups.com.