Please Help - My Power supply broke

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Mike
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Please Help - My Power supply broke

Post by Mike » Mon Nov 10, 2003 6:32 pm

I have a Knight KG-663 Power supply that can put out variable 0-40VDC @ variable 0-1.5A. I wanted to test a transformer without connecting it to the 120VAC line, so I connected it to my power supply. Now, my ps doesn't work anymore. It turns on and the lights light up, but the gauges shouw 0V @ 0A. My voltmeter shows -2VDC @ 0A with the voltage knob to the left, and -1.6VDC @ 0A with the voltage knob all the way toward what would be 40VDC. I have had the wires short before with no damage, but trying a transformer messed up something. What happened, and is it fixable? The fuse is fine and not blown, which makes me think that there is some internal fuse that blew. <p>Thanks for the help, Mike

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Re: Please Help - My Power supply broke

Post by bodgy » Mon Nov 10, 2003 8:09 pm

Output transistors or voltage regulator shorted?<p>Make sure that all the connecting wires are still all connected. I've an amp that shares its speaker wires with a valve amp, if the speaker wires are not swapped to the valve amp it will actually melt the solder on the MOSFET amps ouputs.<p>Colin
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Re: Please Help - My Power supply broke

Post by dyarker » Mon Nov 10, 2003 8:55 pm

Sounds like inductive kick blow something inside the power supply. If you're lucky, just an internal fuse. Less lucky, the output transistor/regulator IC.<p>Take bodgy's advice, check the board carefully. Melted solder may have run and shorted traces. Also look for black burn marks between traces and burned open traces.<p>Transformers are inductors. When you apply DC the magnetic field goes to core saturation. Any sudden interuption in current (like you pulling the transformer lead, or regulator IC over-current protection) causes the field to collapse. The voltage on the transformer winding will jump to whatever it needs to push an equal current in the opposite direction. Poof, one blown output. Automotive ignition coils use this effect to make the spark.<p>Cheers
Dale Y

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Edd
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Re: Please Help - My Power supply broke

Post by Edd » Mon Nov 10, 2003 9:12 pm

Mike:
<<I wanted to test a transformer without connecting it to the 120VAC line, so I connected it to my power supply.>>
Couldn’t quite fathom that AC transformer connect to a DC supply , unless you were possibly testing for core saturation specs.
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Sort of looks like a ~64-70’ish vintage , from Allied Radio. Probably with no chance of documentation being available. If you can pull the dust covers and give us the numbers and date codes of all discrete xstrs and IC’s we would be better able to figure what you have internally, circuitwise AND agewise. Since you are not popping a fuse, you potentially might have opened up a power pass xstr(s) emitter link internally. Typically on a shorted out unit it takes out the output pass xstr(s) and pre driver(s).
The circuitry may even be all discretes if it’s of the older vintage; if of the later date Fairchild’s ua723 would have been a popular IC regulator chip to have been incorporated or even its forerunner, the ua 105/305.<p>73's de Edd
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Re: Please Help - My Power supply broke

Post by Mike » Tue Nov 11, 2003 6:46 pm

as i said before, I have had times before where the wires have shorted before, and it didn't affect anything. I will open it up and post what i find. <p>Yes it is old. How old i don't know.<p>I just bought these new transformers that have a 120VAC input and a [email protected] max and [email protected] max secondary. i wanted to know which was the 12V and which was the 6V without connecting them to the 120V line. Should I assume that the one with the higher resistance is the 12V one and the lower resistance is the 6V one? Why does the transformer short out the ps and only have a resistance of 19ohms on the primary? does that mean it is defective and will short out the 120V line? Or cant transformers handle DC on the primary?<p>Thanks, Mike

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Re: Please Help - My Power supply broke

Post by russlk » Tue Nov 11, 2003 6:55 pm

Good grief, Mike, you need to go to the library and get a book on basic electricity, so you know the difference between resistance and inductive reactance. Since shorting the supply does not normally kill it, the stored charge in the core zapped the output when the transformer was disconnected.

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Re: Please Help - My Power supply broke

Post by Mike » Tue Nov 11, 2003 6:58 pm

Russ, how could there be a stored charge in the transformer. It was never used before. Do you mean that the 40V my power supply put into it stayed in it and when I turned it on a second time the power coming in fried the output?<p>[ November 11, 2003: Message edited by: mikea1962375 ]</p>

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Re: Please Help - My Power supply broke

Post by hp » Tue Nov 11, 2003 7:58 pm

I think what Mr. Kincaid was saying is that when you disconnected the transformer, the inductive kickback killed the output section of the powersupply. Now if the output section has a diode in parallel to the output or a diode bridge on the output then the kickback should not have killed the PS. <p>The kickback I am refuring to is the same thing (if I'm not mistaken) as the kickback when you use a relay. To prevent the kickback from damaging the driver transistor or whatever one would normally have to use a diode to supress the kickback.<p>If this information is all wrong then please tell me. I was taught all this back when I was like 10 when I compained that my science fair project was tickling me. (I was using a little 5v relay and I connected it so it would oscillate..)<p>Harrison<p>[ November 11, 2003: Message edited by: hp ]</p>

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MrAl
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Re: Please Help - My Power supply broke

Post by MrAl » Tue Nov 11, 2003 7:59 pm

Hi Mike,<p>It all happens within a tiny fraction of a second.<p>When you connected the transformer primary to
the dc power supply, the power supply caused
current to flow into the transformer which
caused it's cores magnetic field to build up
higher. Now all was fine until the transformer
was disconnected from the power supply, at which
time the power supply stopped delivering current
to the transformer, or so it tried to do. What
actually happens is once the connection is
broken the resistance shoots up and the flow of
charge is lowered, which causes the transformer
core mag field to start to collapse. This collapse
causes the voltage at the terminals of the
transformer to reverse in an attempt to keep
current flowing though the primary as it was
just before the connection was broken. Since
an air gap is very high in resistance, the
transformer doesnt find it very easy to push
upwards of 1.5 amps though the air gap at the
nominal voltage of the power supply, so it
causes the voltage to rise also. The rising
voltage maintains the current flow of about
1.5 amps even though there is very high resistance.
The voltage rises up and does one of two things:
1. blows the output of the p/s open
2. blows the output of the p/s short
If it blows short, the voltage decreases rapidly
until it reaches a low value or oscillates a
few times before reaching zero, at which time
the energy stored in the cores field is
completely dissipated.
If it blows open, the voltage continues to rise
until a condition known as 'saturation' is
reached in the core due to high voltage and
a long time period, at which time the energy
is self dissipated.<p>In any case, the voltage reaches a level that is
great enough to cause immediate distruction of
the output transistor. Depending on how it blows,
it might take out the driver transistor and/or
control circuitry with it.<p>If it's an older supply with discreet transistors
it might not be hard to fix because all of the
transistors will be either blown open or blown
short on one or more junctions.
Usually all you have to do is replace transistors.<p>Good luck with it,
Al
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bodgy
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Re: Please Help - My Power supply broke

Post by bodgy » Tue Nov 11, 2003 8:55 pm

Was it you Mike that was looking for a 12v to mains volt power supply for your car amp?<p>If so, I've found the beast you need. Octobers edition of Electronics World.<p>12v ->240v 500W power supply designed to operate a 120W mains fridge at full pelt.<p>So look in Tower or B&N whoever it is in the US is most likely to stock UK magazines.<p>your welcome to send me a consideration for my copy, but you'd probably get it cheaper in the US.<p>Colin
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jollyrgr
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Re: Please Help - My Power supply broke

Post by jollyrgr » Wed Nov 12, 2003 7:34 am

First I have a question about testing the transformer. Transformers need AC to work so why were you testing one by connecting DC to the input?<p>And, yes, anything is fixable. How did you check the fuse? Did you simply look at it and not notice a burned spot on the glass? If so, test it with an Ohm meter. I say this because more than once I have fixed computer power supplies where the fuse was bad but the break in the filament was not visible. <p>If the fuse is good check to be sure the supply is not putting out AC voltage. Connect a light bulb to the output to be sure the rectifier/regulators are not shorted and putting out straight AC.
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Mike
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Re: Please Help - My Power supply broke

Post by Mike » Wed Nov 12, 2003 6:04 pm

Now that I think about it, the voltage was still at 40V on the meter with the transformer. When I flipped the standby switch on, with the power supply still powered on, I adjusted something the I can't remember right now, but I think it was the voltage, and the next time I turned it on, I got 0V output.<p>I never knew that transformers cannot be operated off DC. I thought that it would just put out AC. <p>I know the fuse is not blown because I physically removed it from the socket and the entire thing wouldn't turn on. <p>I opened it up, and found out it was a kit that the person before me put together. There are no boards, all it is is resistors and diodes soldered together onto a bunch of strips of boards with metal connectors on it (hopefully you can understand my description). The components look fine. There are a few transistors on it. The two mounted to a heatsink both say W with a circle around it, EP 242 26 and then on the next line 6649 QS. The other 2 transistors are smaller metal can type transistors labeled "660100" and "274". They are not heatsunk.<p>Can I just set my voltmeter to AC to measure if it is AC? Or should I connect a plain 120V light bulb?<p>Thanks, Mike

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Edd
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Re: Please Help - My Power supply broke

Post by Edd » Wed Nov 12, 2003 8:03 pm

Mike:
<<I opened it up, and found out it was a kit that the person before me put together. There are no boards, all it is is resistors and diodes soldered together onto a bunch of strips of boards with metal connectors on it (hopefully you can understand my description). The components look fine. There are a few transistors on it. The two mounted to a heatsink both say W with a circle around it, EP 242 26 and then on the next line 6649 QS. The other 2 transistors are smaller metal can type transistors labeled "660100" and "274". They are not heatsunk.>><p>An Allied Radio kit product, as I mentioned. Terminal board construction…point to point wiring .. before etched PCB use. Do we interpret the unit as having only 4 xstrs in its design ? Also there might be a small copper wire wire link used as a fuse back in those days, usually near the transformers raw DC supply area, and if it opens leaves a bit of wire so that you can detect it , and usually copper balls on the wire tips where it melted.<p>The circled W would typically be Westinghouse back in those days. The EP 242 26
….Knights cryptic house number designators, and the final manuf date code of the 49th week of 1966 on the pwr xstr(s), so that would typically put it in the middle on my age guesstimate of the equipments vintage. On the 2 smaller transistors would the metal case size be the TO-5style… or yet, even smaller. ?
The last info supplied on the small transistors “274” tells us that they were RCA manufactured transistors.
Considering the units age, there is even a border line possibility that the transistors. mentioned might be of the germanium family rather than silicon. Pull 2 leads off and test out of circuit and comfirm on a DVM diode test that the junction voltage is the higher .5-.6V silicon junction range rather than the lower .3-.4 V of a germanium unit. Also are the
Power transistors in the metal To-3 casing……as that vintage of Westinhouse typically was encased in what I used to call a micro “hockey puck” with two leads coming out for the B and E and a central screw ,centered between them, being the Collector connector.
More Feedback time.<p>73's de Edd
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Mike
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Re: Please Help - My Power supply broke

Post by Mike » Thu Nov 13, 2003 4:42 am

How the power supply is built is some components ar above the metal, such as large caps, the transformer, the heatsunk chips, and the two other chips. There are 2 more transistors on the bottom, underneath the metal, in sockets. I an not right by the unit right now, so I will post more tomarrow (Friday). I will also try to get some pics<p>Thanks for the help, Mike

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