Mismatched caps?

[We Rig Math Tests]

Can I replace a blown 470uF 35v cap with a 470uF 25v cap? I pulled apart a monitor earlier, and there was one bad cap. The only cap I have that matches the microfarad rating is rated ten volts lower than the bad cap I pulled. Should I just pony up and get a new cap from Radioshack?
What would using a higher microfarad rating do?
Thanks!
Matt

Edit: How did this wind up in the audio forum? Whoops. My bad.

[retiredcaps]

Re: Mismatched caps?

Can I replace a blown 470uF 35v cap with a 470uF 25v cap? I pulled apart a monitor earlier, and there was one bad cap. The only cap I have that matches the microfarad rating is rated ten volts lower than the bad cap I pulled. Should I just pony up and get a new cap from Radioshack?

You can replace a 35V cap with a 25V IF the voltage is less than 25V. If the voltage is 24V, then I would suggest that 25V is NOT suitable (too close to the rated value).

If you can verify with a multimeter that your voltage is 19V or less, then you could probably use a 25V cap.

If the cap is from the SMPS section (power supply), RadioShack does not sell low ESR (ohm) caps. A list of recommend caps and suppliers can be found in the FAQ section.

[dood]

Re: Mismatched caps?

retiredcaps has done a good job answering this, but I moved it to General Electronics anyway.

[PCBONEZ]

Re: Mismatched caps?

Radioshack caps are not low ESR.
Not suitable in many applications, and since you didn't mention the specific application...
Most 470uF 35v you will find in a monitor are related to the power and should be Low ESR caps.

The voltage marked on the side is a -limit-.
Like retiredcaps said, if that circuit's actual voltage is less then what is marked on the cap the it's safe to use it - at least so far as the voltage goes.

PROPERLY cross referencing caps is a LOT more than just uF and Volts.

.

[We Rig Math Tests]

Re: Mismatched caps?

Sorry, but I only have more questions now.
1) What setting would I use on my multimeter to test the circuit?
2) And I'd have to run power through the monitor to test it?
3) What would happen if I used a higher impedance (500uF, 1000uF)?
4) What would happen if I used the 25v cap and it was a 35v circuit? Would the cap blow or would it damage the circuit?
Thanks!

[retiredcaps]

Re: Mismatched caps?

1) What setting would I use on my multimeter to test the circuit?

DC volts on your mulitmeter.

2) And I'd have to run power through the monitor to test it?

It just needs to be plugged in. Sometimes the power board will actually list the voltages right on the PCB.

Here is a picture of one of my boards. You can clearly see

https://www.badcaps.net/forum/attach...7&d=1280167246

it says 5VDC and 19V DC in the top left hand corner.

3) What would happen if I used a higher impedance (500uF, 1000uF)?

500uF is not a cap you will likely find. 1000uF is normally found, but not recommended. Your circuit may not work at all with 1000uF.

4) What would happen if I used the 25v cap and it was a 35v circuit? Would the cap blow or would it damage the circuit?

The cap will blow and/or damage the circuit. In addition, you may be reaching for the fire extinguisher.

[PCBONEZ]

Re: Mismatched caps?

1) What setting would I use on my multimeter to test the circuit?

To the lowest scale that is higher than the highest expected value.

2) And I'd have to run power through the monitor to test it?

Yes

3) What would happen if I used a higher impedance (500uF, 1000uF)?

Impossible to say without analyzing the circuit.
Some circuits are frequency sensitive or they use the capacitance to control timing between circuits.

4) What would happen if I used the 25v cap and it was a 35v circuit? Would the cap blow or would it damage the circuit?

Possibly.
Best case: the cap was intentionally de-rated by the manufacturer and 35v is actually okay. [I wouldn't count on it.]
Worst case: the cap shorts internally becoming a wire to the circuit and burns out a whole bunch of other parts possibly even causing arcs and sparks and a flame-out.
Usual case: the cap shorts gradually [hours, days, a month] until one or two other part breaks from excessive current or it quits working because the uF went too low as the cap approached a dead short condition.
.

[PCBONEZ]

Re: Mismatched caps?

Damn I write slow....

[retiredcaps]

Re: Mismatched caps?

Damn I write slow....

But your response are *quality* responses and are often bookmarked by me.

Something about the people in Phoenix I guess!

[We Rig Math Tests]

Re: Mismatched caps?

Thanks PCBonez and retiredcaps.
I scoured my PCB for a voltage rating for about five minutes to no avail.
I plugged the monitor in, and attempted to test it, but either my multimeter is dead (likely) or I did something wrong (Also likely).
I can just put the multimeter probes on the traces/solder sockets where the cap used to be, right? If so, my multimeter is dead. I set it to DC (50v) and then DC (100v). It didn't jump even a little.
Something I noticed is that the screen flickered a reddish hue briefly and died. It did that every time I cycled the power to the monitor. I didn't have the video ribbon plugged in though, so am I wrong in guessing that the cap corresponds to the LCD backlight on the monitor?
Thanks again!

[retiredcaps]

Re: Mismatched caps?

I plugged the monitor in, and attempted to test it, but either my multimeter is dead (likely) or I did something wrong (Also likely).

Let's see what your board looks like

1) Post clear focused pictures of your boards after reading

https://www.badcaps.net/forum/showthread.php?t=1868

2) Please do not post pictures inline as they slow down the loading of pages.

3) For best picture quality and clarity take your boards to a window on a sunny day, turn flash off, and use macro mode. Take a top down view of all your boards (front and back). Make sure the photo is legible so that we can read the PCB printing clearly. A shutter speed of 1/125 or faster will produce nice clear focused pictures. Try to get a photo that is 2000x2000 resolution or as close as possible.

4) A simple multimeter test would be to get a AA or AAA battery. Put your multimeter on the lowest DC Voltage range. Put your red probe on positive terminal and black probe on your negative terminal. You should get around 1.5V DC assuming you have a good battery.

[We Rig Math Tests]

Re: Mismatched caps?

My multimeter is definitely dead.
I don't have a decent enough camera to post clear pics. I tried to take a few but they aren't legible.

[retiredcaps]

Re: Mismatched caps?

My multimeter is definitely dead.

I posted this a couple of days ago.

https://www.badcaps.net/forum/showpo...7&postcount=13

[retiredcaps]

Re: Mismatched caps?

I don't have a decent enough camera to post clear pics. I tried to take a few but they aren't legible.

Well at least post what model monitor you have and what the power board number is.

Your camera doesn't have to be super highend. You just need lots of light and a steady hand.

[We Rig Math Tests]

Re: Mismatched caps?

It's a Dell E153FPb, power board BENQ DS-1107A.
I tried to use my 1.3mp cameraphone. Believe me, the pictures aren't worth posting.

[retiredcaps]

Re: Mismatched caps?

It's a Dell E153FPb, power board BENQ DS-1107A.
I tried to use my 1.3mp cameraphone. Believe me, the pictures aren't worth posting.

1) Your board probably looks like this one.

https://www.badcaps.net/forum/attach...8&d=1252789869

2) Once you get a new multimeter, doublecheck the voltage. I'm guessing that this Benq board uses no more than 19V DC for a secondary voltage.

3) Benq power/inverter boards usually have bad solder joints on the c5707 transistors and inverter transformer pins. The standard recommendation is that you must resolder all the above mentioned pins.

4) This Dell model is well documented here. Just search the forums for it.

[We Rig Math Tests]

Re: Mismatched caps?

Thanks for the multimeter links.
Yes, that's my board exactly. I'm not going to be able to buy a new multimeter for a while (money is tight), but I wonder if someone would be kind enough to test it for me. If I knew for certain that a 25v cap would work, that would save me time and money.
As for the bad solder joints, how would I test those? They LOOK fine to me, but I'm not the expert here.
Thanks!

[starfury1]

Re: Mismatched caps?

HI Gang

By visually looking at the joints you can get an idea BUT they can look good and be bad
since its a know problem with these Boards
Then Id rework the solder joints

Normally this would be done by removing the solder with "de-solder wick" or a "solder sucker" hobbyist way since its cheap (not that pro's dont use it)
professionally with a de-solder station and then re-soldering them

I am not sure of your level of skills but if you haven't really soldered or used wick de-solder pumps before
practice on junk PCB's first!

the last thing you want to do is damage the PCB & too much heat will eventually do that (more sooner then later)

Now a problem or a question I'll leave to others is most all soldering now is done without lead "Lead Free Tin solder" which requires hotter iron I think ...me, I haven't done any for a while and I am old school tin lead 60/40 solder..

So not sure on the solder used on these PCB's... so I'll leave that open but looking Id guess 60/40

(just that this would complicate matters if your not good at soldering thats all)

seem tin joints look more like what are known as cold or bad solder joints
but are fine so theres another fun thing to now look out for
(so that sort of throws my first statement out the window)

As was said transformers... also anything that conducts large current, lives on a heat sinks (in other words gets hot and expend cools down and stress the joint) is liable to develop bad solder joints...

so bottom line... if known for it as a fault... then most defiantly rework them

This wont necessarily fix *your problem but may prevent further ones if it doesn't
(*dont know what that is exactly so cant help there)

Depending I suppose your meter may be reparable too although I dont think these days there is all that much in them..they do sometimes have internal safety protection of some kind..really depend on meter & what you did to destroy it

this actually (just noticed it was stated)
is a switch-mode Power Supply & not sure but if memory serves and inverters also for the tubes too

so one "Potentially Dangerous" pcb to be poking around on live..

A quick read as PCbonez & retiredcaps said

it depends on were the caps is in the circuit but if its part of the switcher DC line filtering then yes low ESR caps required mostly..others might work but not for very long. (few months maybe and die)
on that photo of the pcb
those 3 brownish caps bottom right facing are most likely low ESR

Post brand and series or the cap number it may well help ID what it being used for

(Note: that big one with the glue gunk is a 68 uF 450 Volt and yes it would have quite a bit of juice across it -X- no touchie that one or part of the circuit)

normally with caps you can go a value or 2 higher possibly one lower (not really recommended) capacitance wise if you dont know the circuit...

voltage wise if you know its less then 25V yes you can use a lower voltage
allow for margin of error thought like 5 V or so

you can of course go up (but other aspects do change and size too)
so normally 1 2 times is the max..again depending

Ideally best to know what its function is in the circuit and know why that specific family (type of cap) was chosen you can bet it will be the cheapest they can get away with so something better spec-ed for the job is a better replacement..
(Note: rare cases this has proven not to be the case)

Al thought a little different in layout etc all tutorial info you find here on switch modes would be useful reading in understanding its operation..there was also a thread on inverters too... DELL pcb from a monitor..somewhere forgotten now what it under maybe that post from where pic comes from

Anyway good luck hope a little of what I said helps anyway

Cheers

[We Rig Math Tests]

Re: Mismatched caps?

I got a replacement cap with the right rating, and put it in. Then I did something stupid. I had the PCB plugged in for testing and I picked it up carelessly, zapped my thumb, and now nothing will turn on or light up, not even the power indicator. I'm assuming I shorted something because without a cap in it the front LED would come on and the backlight would flicker and die. Time to give up.

[PCBONEZ]

Re: Mismatched caps?

WOW!
starfury1!
Long time no see man! - Welcome back!

Yup: 60/40 is the standard and works well but 63/37 melts at a [little] lower temp and is [a little] easier to use if you can find it.
Main thing is to use a small diameter like .020-.022" or so.
.