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I have a 24 volt power supply for a piece of industrial machinery that isn't working. I opened it up, applied 120 vac, and began probing different components with my voltmeter. I came to a 100 ohm 1 watt resistor that appears to be connected to an SCR, and put my meter probe on it. The power supply made a funny noise and began working! I can repeat this everytime I power down and back up, and it works even if the probe is unplugged from the meter. probing the resistor with, say a piece of wood will not work. I have replaced the resistor and there is not a bad connection. Any ideas?

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The meter is not infinitely resistive -effectless, it has effect on the circuit, and may be enough to switch the scr on.
np-pn junctions in scr fail occasionally, the voltage fluctuation due to the meter may have been just enough to trick the rectifier, the scr may need to be replaced,
Has there been any major power suge, lightning strike 'close (in lightning terms) to the unit', output short, to the power supply, slilcon can really only be tested out of circuit its often easier to replace than test

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How can i test a lite on ps 5221 06 power supply


A faulty power supply can cause a computer to function irregularly, or in some cases, not at all. Some of the potential problems that can result from a faulty power supply include the computer not powering up, the PC freezing during boot up or the PC randomly shutting itself off. Each of these cases can be a true headache for the user so having the ability to test the power supply could not only save you from experiencing too much computer downtime it could also save you the cost of purchasing an entirely new machine. Here is how to do it with a multitester.

Check the quality of the receptacle and/or power strip your computer is plugged into (this is the first test you should always perform). Set the multimeter to read voltage and test the receptacle or power strip to ensure that the voltage is reading properly. Your tester should read approximately 120 volts. If the receptacle is good, the next thing you will have to do is turn off your PC and open your computer's case. Depending on the make and model, that can be a chore in and of itself, but you can always check your documentation that came with the machine for guidance. Leave the PC plugged into the wall outlet, but keep the machine off. With the case open, locate your power supply. It is a large, silver metal box with many colored wires coming out of it. Before you put your hands into the CPU box, be sure to ground yourself by touching something metallic or by grounding yourself to the box itself using a grounding wristband that you can purchase from your local office supply or computer store. Locate a power connector that isn't being utilized. If they are all in use, just unplug the one that is plugged into the back of the hard drive or one of the CD drives. Set your multitester to read DC voltage in a range greater than 12 volts. Turn your computer on. On the power connector, look for the black and the yellow wire. Touch your tester's black probe to where the BLACK wire is connected to on the connector and the red probe to where the YELLOW wire is connected. Your multitester should read +12 volts. Keep your black probe in its current position and touch the red probe to where the RED wire is located on the connector. Your digital multimeter should now read +5 volts. If you get a different reading or no reading, then the power supply is bad and should be replaced. If you get the proper reading on your multitester, then your motherboard may be the cause of your PC's problems. THE BEST Multimeter tutorial (HD)

Apr 14, 2013 | PC Desktops

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Power supply voltage switch

Of all the thousands of individual different types, of desktop computers out there, I have never run into this one before.
(Not like I AM going to know about each, and everyone one of them, lol; but I have been around at this for years)

Looking at the Specifications on HP Support, and individual reviews on the internet; I see it uses a small external power supply.
Rated at a maximum Wattage of 65 Watts, and resembles a laptop power supply.

(I can't seem to pull up the power supply itself, to have a look at it)

If this holds true it probably does not have a switch on it, to switch from one incoming AC voltage, to another.

That is to say, 120 Volts, or 240 Volts.

If like a laptop AC adapter, it automatically sets itself to the incoming AC voltage; from your home, or place of business.
Uses the PFC technology; Power Factor Control,

Obviously the article is for a Switched-Mode Power Supply, as used in a desktop computer. (SMPS)
But the PFC information they give, also applies to a laptop type power supply. (AC adapter. Commonly known as a 'Charger')

On the label sticker for the power supply, it should state;
Input: AC @100 ~ 240V

Alternating Current at 100 to 240 Volts.

IF not, then there may be a Slide switch, or a Rocker switch.

IMHO there will be no switch.

May I ask what the problem is?

Moved from one country to another?
All you should need, is a type of adapter plug; to go onto the plug of the power supply, so you can plug it into a surge protector.

Or plug the power supply into a surge protector it fits, and use the plug adapter for the surge protector's power cord.

Post back in a Comment, Anita.


"The Pieces Included........"

Mar 17, 2013 | Compaq CQ2103WM-B (884962083048) 17 in. PC...

1 Answer

I got an acer asprine M1640 but when i start it its keeps beeping short beeps and it wont start a friend of me said it was something with the videocard when i replaced it it stayed the same it kept beeping...

Bad Power Supply

Weak voltage power rail

Do you, or your friend have access to a multimeter? An economical unit can be purchased for as little as $8 to $12. Available in a multitude of stores. An auto parts store is but one example. Analog or digital is fine.
This way you can test the 3 main voltages coming from the Power Supply.

A) 3.3 Volt power rail
B) 5 Volt power rail
C) 12 Volt power rail

(All DC voltages. In comparison, two D cell flashlight batteries store
3 Volts DC. { 1.5 Volts each )

OR; do you have a KNOWN to be good, Compatible power supply you can borrow for a test unit?
Compatible: Has to have at least the same Wattage, or more, and has to have the correct power cables, and number of power cables needed.

1) If ALL of the LED lights were on at once, they would use less than 1 Watt of power.
2) EACH fan uses 2 to 3 Watts of power.

3) A typical Processor can use 51 to 125 Watts of power. Just depends on what Processor it is.

Your Power Supply doesn't have enough power to 'b-low it's nose'.

Nothing shows on monitor? It isn't going to. No Signal. No video signal.
This is because the computer isn't sending one. Why? Because the computer isn't working.

Let's start with this. Post back in a Comment as to your results.

Jul 23, 2012 | Acer Aspire M1640 Refurbished Intel...

2 Answers

Monitor say no rgb input

Yes! I'am afraid so, make everything else is running on the computer.

Mar 11, 2012 | E-Machines T3062 PC Desktop

1 Answer

I have a EMACHINE W5243 with a MCP61SM-GM Motherboard. When I press the power button the keyboard lites flash, fans come up to full speed than drop to a slower speed, than I get beeps about every...

Suggest Don, that you check the voltage power rails in the Power Supply, first. Make sure the Power Supply is working correctly.

Power Supply voltages check out, look VERY close at the motherboard.
(Power unplugged to computer, Anti-Static Precautions taken, motherboard OUT in hand )


eMachines are budget computers. Nothing wrong with that. Problem is the manufacturer saved the consumer on the cost, by using two low quality components.

A) The Power Supply
B) The motherboard

Both of these come up for the BIOS Beep Code you stated.

Power Supply's are usually Bestec, Delta, or HiPro.
These particular generic units use low quality Electrolytic Capacitors, MOSFET's, Rectifier Bridge, less than adequate gauge of wiring, and so on.

The motherboards, (MSI, ECS, TriGem, etc), use low quality Electrolytic Capacitors.
This is generally the item/s that go bad first. They are the 'weakest link'.

Yes, Electrolytic Capacitors are the weakest link in the Power Supply, also.

In the Power Supply they are used as Filters. They filter the incoming AC electricity, (Input Stage), and the outgoing DC electricity. (Output Stage)

I really feel the problem is the Power Supply.
1) If ALL of the LED lights were on at once, they would use less than 1 Watt of power.

2) EACH fan uses 2 to 3 Watts of power.

3) A typical Processor can use 51 to 125 Watts of power. Just depends on what Processor it is. (Older Intel Pentium III's, II's, and so on, use less power than 51 Watts. Same with older AMD processors )

(The PSU { Power Supply Unit} in your personal computer is a SMPS.
Switched-Mode Power Supply)

Click on the photo to the upper right, in the link above. The letter B is on the top of an Electrolytic Capacitor. You are looking at a Top View.
The blue ring is part of a plastic sleeve, which goes around the 'can' case of the capacitor.
These large capacitors are in the Input Stage.

The letter E is near a few more Electrolytic Capacitors. These capacitors are in the Output Stage.
These capacitors are Radial Aluminum Electrolytic Capacitors.
This is a side view of an example,

The top capacitor is an Axial Electrolytic Capacitor. The leads come out of each end. The bottom capacitor is a Radial Electrolytic Capacitor. Both leads come out of the same end. The capacitors in your Power Supply, and on your motherboard, are Radial Aluminum Electrolytic Capacitors.

Your Power Supply converts AC electricity form your home, or business, and turns it into low DC electricity.

A) The 3.3 Volt power rail
B) The 5 Volt power rail
C) The 12 Volt power rail.
(In comparison two D cell flashlight batteries produce 3 Volts DC )

In the power cables coming from the Power Supply.
1) Orange wires are 3.3 Volts
2) Red wires are 5 Volts
3) Yellow wires are 12 Volts
(All DC voltage)
Black wires are Ground wires.

Now, about the voltage power rail.
All 3.3 volt wires (Orange), end in one central 3.3 Volt point, in the Power Supply.
The central point is the 3.3 Volt power rail.

Same for the 5 Volt power rail. All 5 Volt wires (Red), end in one central 5 Volt point, in the Power Supply.
Same for the 12 Volt power rail.

This means, for example, if you test one Orange wire, and it shows 3.3 Volts on the multimeter, the 3.3 Volt power rail is good.
Test one Red wire in a power cable. Shows 5 Volts, or VERY near?
The 5 Volt power rail is good.
Same thing for the 12 Volt power rail.

One method to check the 5 Volt power rail, and the 12 Volt power rail.
Use a 4-pin standard Peripheral power cable. (The connector on the cable is misnomered as a 'Molex' connector),

Most of the time there is an unused Peripheral power cable.
You can use it's Red wire, Yellow wire, and Black wire, to test for 5 Volts, and 12 Volts.

Disregard the following if you are aware;
An economical multimeter can be purchased for around $5 to $12.
Available in a multitude of stores. An auto parts store is but one example.
Analog, or Digital, is fine.

The Red lead is the Positive lead. The metal tip is the probe. I refer to both as the Positive probe lead.
The Black lead is the Negative lead.

The Positive probe lead ALWAYS goes to the power wire to be tested.
3.3 Volt, or 5 Volt, or 12 Volt.
The Negative lead ALWAYS goes to Ground. (A-N-Y Black wire, is a Ground wire)

Computer sitting on a table, or workbench, computer case open, 4-pin standard Peripheral power cable, untangled, and pulled to the outside where you can access it easily, the computer is plugged into power.

The center knob in the middle of the multimeter, is the Function Knob.
It is set to DC Voltage.
If there are only symbols, the symbol is a dotted line over a solid line.
(NOT a curved line over a solid line)

If there is more than one setting for DC Voltage, set it to the 0 to 50 Volt scale.

Turn the computer on. Holding the connector in your hand, of the 4-pin Peripheral power cable, insert the Positive probe lead's tip, into the socket hole with the Red wire.

Hold the probe lead, with the hand that is already holding the connector. With the other hand, insert the Negative probe lead's tip, into the socket hole with one of the two Black wires.

Watch the multimeter. You should be reading close to 5 Volts.
Test the 12 Volt wire using this method also. (Yellow wire, and Black wire. Black wire is Ground. Either one of them)

The 3.3 Volt power rail is tested using the Orange wire, in the 24-pin ATX main power cable.

Power cable plugged into the motherboard, as in the photo to the right.
At the end of every wire, going into that long whitish connector with 24 socket holes, is a metal terminal.

The metal terminal is pretty far down in the socket hole. The probe lead must touch the metal terminal, but cannot due to it's size.
Use a straightened out paper clip.

The paper clip's diameter is small enough to slide into the socket hole, RIGHT NEXT TO the Orange wire, and touch the metal terminal.
Then the Positive probe lead is held against it. (Or with an alligator clip attachment, it is clipped on)

Do the same for A-N-Y of the Black wires. They are all Ground wires. Pick one. Insert a straightened out paper clip. The Negative probe lead touches it.
(Paper clips inserted, THEN computer turned on. It is only 3.3 Volts DC, but I want you to feel safe)

More in a Comment.

Dec 13, 2011 | E-Machines eMachines Desktop PC

1 Answer

I have a dell computer. when i turn on the tower and monitor i get just a black screen on the monitor. i have pushed a few buttons on the bottom of the screen and it just says power save mode please use...

No, it's the Power Supply.

Power Supply has a weak voltage power rail.

The monitor goes to Power Saver mode because it is not receiving a video signal. (No Signal)
This is because the computer is not working.

Dell Support > Dimension 4700 desktop computer > Service Manual,

Click on the heading - Advanced Troubleshooting

Is the Power On LED light blinking Amber? (Yellow)

Post back in a Comment.


{You can also test the power coming out of the Power Supply.

The Power Supply in your computer is an SMPS.
Switched-Mode Power Supply,

It converts the AC electricity from your home, or business, into DC electricity.
There are three main voltages produced;

A) 3.3 Volts (3 point 3)
B) 5 Volts
C) 12 Volts
All are DC voltage

Two D cell flashlight batteries produce 3 Volts DC.
(In case you are worried about being shocked.
The 'bad' electricity is contained inside the Power Supply's case)

If you do not have a multimeter, an economical unit can be purchased for around $8 to $12. A LOT of stores carry them. An auto parts store is one example.

Orange wires are 3.3 Volts
Red wires are 5 Volts
Yellow wires are 12 Volts.

ALL 3.3 Volt wires end at the same 3.3 Volt power rail in the Power Supply.
Same with 5 Volt wires, and 12 Volt wires.

You can test ANY Orange wire, or any Red wire, or any Yellow wire.

ANY Black wire you see is a Ground wire.

The red probe lead of the multimeter is the Positive lead. The black probe lead is the Negative lead.

The Positive probe lead is touched to a positive wire.
Orange, Red, or Yellow.

The Negative probe lead is touched to ANY black wire. ALL black wires are a Ground wire.

The function knob on the multimeter is set to DC voltage. (Dotted line over a solid line, with a dotted line under the solid line. The curved S line over a solid line is for AC voltage. This = No)

If there are different scale settings, set the function knob to 0-50 Volts. (DC)

Aug 24, 2011 | Dell Dimension 4700 PC Desktop

2 Answers

When my computer starts the power light will blink and the fan starts. When the computer first had problems it froze and the fan turned on and wouldnt stop. I also smelt a burnt rubber smell. I read to...

Yes the voltmeter measures the voltage.
When you power up the computer to test the voltage on the 4 wire connectors.
Test the voltage red and balck wire should read 5 volts.
Test the voltage yellow an black wire should read 12 volts
If the burning smell is from the power supply then most likely a capacitor has blown in the switch mode power supply.
I would advise you purchase a new power supply, do not try and repair it, these mains devices can be dangerous if you don't have knowledge.

Sep 14, 2010 | HP Pavilion a705w PC Desktop

2 Answers

You press the button and it donest turn on all the time



Inside an SMPS, (Switched Mode Power Supply), are Electrolytic Capacitors.

(Radial. The bottom capacitor in the photo)

Electrolytic Capacitors slowly build up a charge, then release it all at once.
You can compare it to a large swimming pool being filled up with a garden hose, and then one wall of the pool being taken down all at once.

At the bottom, of the Radial (Style) Aluminum Electrolytic Capacitor shown in the above photo link, are two leads.
One is Positive, the other is Negative.

Touch your finger/s to the terminals on the bottom of one of these babies, and the shock can be BAD to FATAL!

If your fingers touch, and complete a circuit that one, or more capacitors are in, the shock can be BAD to FATAL!

Don't open that power supply!

Besides, a fuse goes out All At Once.
There is no work a little bit, then turn off.
It just goes Poof!
(The fusible element inside burns into two separate pieces)


"You press the power button, and it doesn't turn on all the time"

As has been suggested, I also concur that you have a bad power supply.

But you could also have a bad Power On switch, located inside that plastic Power On button.

This is an example of a Power On switch, that I have found fits a lot of computers,

I'd like you to get this switch, and install it.
From there the diagnosis will go to the Power Supply.

Also would like you to make sure the ram memory is seated tight, and the Processor fan is spinning, and looks to be spinning at the right speed.

If the fan spins a few times, and stops, then spins a few times, you know the fan has a bad bearing. Replace.

If it looks to be spinning too slowly also, you'll know it

Be sure to follow anti-static precautions before you reach inside your computer, and the computer is unplugged from power.
If you do not know what anti-static precautions are, I'll be glad to tell you. Simply state so in a Comment.
(Believe upper right of your page)

As for the Power Supply test, you need either a multimeter, or a power supply tester.

An inexpensive multimeter, that will work just fine for this application, ranges from $6 to $12. The Voltages are only 3.3 Volts, 5 Volts, and 12 Volts. (DC)
Two C cell flashlight batteries are 3 Volts. (DC)

Your computer power supply puts out three main Voltages, as stated above.
A) 3.3 Volts (3 point 3)
B) 5 Volts
C) 12 Volts

Orange insulated wires are 3.3 Volts
Red wires are 5 Volts
Yellow wires are 12 Volts

ANY Black insulated wire is a Ground wire.

This is DC Voltage.
There is a Positive, and a Negative.

The Positive (Red) probe lead of the multimeter touches a power wire. (3.3, 5, or 12)

The Negative (Black) probe lead touches ANY Ground wire. (Black)

IF, the 12 Volt power rail shows 11 to 13 Volts, the diagnosis indicates the Power Supply to be good.
(However read on)

IF the 12 Volt power rail shows 11 Volts, or less, the Power Supply is bad.

This is an example of a power supply tester,

Jun 03, 2010 | Shuttle XPC PC Desktop

1 Answer

No blue power light and doesnt turn all the way on no starting beep gateway gt5220

I would suspect a bad power supply. Before doing anything make sure power is actually getting to the computer. Check the power cord with a volt meter. You should have around 115 volts. If all is well you need to see if the power supply is putting out the correct voltage. Open the PC up and set the multimeter to DC voltage.
  1. Turn off the PC, but do not unplug it. Open the system unit. Set the multimeter to read DC volts in the next range higher than 12 volts. Locate a power connector similar to the hard drive (or CD-ROM drive connector that is unused) and turn on the PC.
  2. You can also unplug a drive connector and use it. Turn on the PC and insert the BLACK probe into the power connector on one of the BLACK wires. Touch the RED probe to the YELLOW wire on the power connector.
  3. The multimeter reading should be +12 volts. Now touch the RED probe to the RED wire and the reading should be +5 volts. If no readings or different readings occurred, you’ll have to replace the power supply. If the readings were correct, you should check the P8 or P9 connectors at the motherboard. These connectors may also be named P4 and P5. To check these connectors, perform the following:
    • Insert the BLACK probe into P8 at one of the BLACK wires. Insert the RED probe into the P8 connector at the RED wire. The reading on the multimeter should be +5 volts.
    • Check the power going to the motherboard connections by inserting the RED probe into P8 at the YELLOW wire and you should get +12 volts. Leave the BLACK wire touching the BLACK wire at the P8 connector. Check the BLUE wire and the reading should be a -12 volts.
    • Now move the BLACK probe to the BLACK wire on the P9 connector. Test the WHITE wire by inserting the RED probe and the reading should be -5 volts. Check the RED wires on the P9 connector and you should get +5 volts on each red wire. You won’t get exactly 5 or 12 volts, but the readings will be very close, such as 5.02 volts.
    • If the Power Supply is a couple of volts off in either direction, such as when the RED wire should be reading -5 volts but it reads -8 volts, or if there are no readings, replace the power supply. Curtorsey: IT Professionals

Mar 09, 2010 | Gateway GT5220 PC Desktop

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