I ordered replacement caps from.digikey but when.soldered on they overheated. The specifications were same as the ones being replaced. 470uf, 105 c , 35V.. the caps that came in looked a little thinner and taller than.original ones. So apparently something not right. The receiver I'm trying to repair is sonicview 360 e. The.caps are I'm.trying to replace are.responsible for providing power to the lnb and they are mounted on the power supply board. Any help in.identifying which caps to order.is greatly appreciated. No idea what the ripple.effect should be as there are many options. ):
blown caps for fta receiver
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Did you observe the polarity of the leads and install the new caps with the same polarity, + to + and - to -?
Ripple is not important in this application. Just select with the lowest spec.
Ripple is not important in this application. Just select with the lowest spec.
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electrolytic caps never overheat unless installed backwards or the voltage rating is too low.
They should be the same size physically or even larger.
Maybe the voltage rating of the caps were wrong to begin with.
They should be the same size physically or even larger.
Maybe the voltage rating of the caps were wrong to begin with.
Fhorta,
Did they overheat when you performed the soldering or did they overheat when the unit was powered up?
If they failed when power was applied, then you might have installed them with reverse polarity as Brian has mentioned.
Also, I have seen electrolytic capacitors which have the polarity indication reversed. This would be rare,
but be aware that it is possible. Always inspect the lead length and compare it to the polarity signature on the case.
The longer lead should normally always be the + lead.
If they overheated when you were soldering them in, then your soldering iron is too hot or unregulated or you are applying
the heat for too long of a period. Roughly two or three seconds of the soldering tip in contact with the lead and the circuit trace
should be the maximum time and provide sufficient heat to melt and flow the solder. A good idea is to leave the leads long,
install a spacer underneath the cap to raise the cap off the circuit board just about 1-2 mm and solder the leads to the board,
then trim the leads. This allows the leads themselves to become a heat sink so that the heat energy doesn't conduct into the
actual guts of the cap as much.
RADAR
Did they overheat when you performed the soldering or did they overheat when the unit was powered up?
If they failed when power was applied, then you might have installed them with reverse polarity as Brian has mentioned.
Also, I have seen electrolytic capacitors which have the polarity indication reversed. This would be rare,
but be aware that it is possible. Always inspect the lead length and compare it to the polarity signature on the case.
The longer lead should normally always be the + lead.
If they overheated when you were soldering them in, then your soldering iron is too hot or unregulated or you are applying
the heat for too long of a period. Roughly two or three seconds of the soldering tip in contact with the lead and the circuit trace
should be the maximum time and provide sufficient heat to melt and flow the solder. A good idea is to leave the leads long,
install a spacer underneath the cap to raise the cap off the circuit board just about 1-2 mm and solder the leads to the board,
then trim the leads. This allows the leads themselves to become a heat sink so that the heat energy doesn't conduct into the
actual guts of the cap as much.
RADAR
Another alternative possibility is:
In the case where the capacitor is overheating when power is applied - If the rectifier feeding the capacitor is shorted, then the current on the capacitor will be AC, Not DC, and the capacitor will fail, as it is designed to filter DC, not AC. It has a very low impedance to AC and will there for act as a load and overheat.
In the case where the capacitor is overheating when power is applied - If the rectifier feeding the capacitor is shorted, then the current on the capacitor will be AC, Not DC, and the capacitor will fail, as it is designed to filter DC, not AC. It has a very low impedance to AC and will there for act as a load and overheat.
Excellent point Migold! Particularly when we know that the original capacitors failed! Why did they fail? Electrolytic caps can be prone to death by natural causes (i.e. they only live so long), but was that the case here? Or did something or someone "kill" them?
I think the butler did it.
RADAR
Yes. I agree. I could see the originals fail just because they may have came from a lame production run, but to have the NEW replacements fail right off, too? Not a good sign.
I'd desolder and remove the crispy remains of the caps and leave all cables disconnected and power the unit up. Then get on the terminal points with an O'scope and see what kind
of voltage is present there.
RADAR
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It can be very dangerous to poke around in a switching power supply with a grounded 'scope.
Even a VOM or DVM can get the inexperienced into trouble.
AC powered switchers are not a place to learn troubleshooting techniques
Even a VOM or DVM can get the inexperienced into trouble.
AC powered switchers are not a place to learn troubleshooting techniques
Anole,
That's no problem. Just get one of those grey grounding adapters which elliminates the 3-prong ground and has the wired ground terminal with the spade connector and cut that off. Then the O'scope ground will be floating (not attached to true ground) and you will have no issues. I use this all the time so that I can put the "ground" clip on a hot lead and scope out the voltage signal across a component.
OSHA hates these little tricks, but you must utilize them to get the job done properly.
RADAR
Ke4est, I see you caught your typo! 
Don't worry, I knew exactly what you meant without you having to correct it.
"Sometimes I don't ..... speak.... right... But yet I know what I'm talking about. Why can't we be friends, why can't we be friends"
"I know you're working for the CIA, they wouldn't have you in the MA FIA"
visit: h--p://www.youtube.com/watch?v=5DmYLrxR0Y8
RADAR
Don't worry, I knew exactly what you meant without you having to correct it."Sometimes I don't ..... speak.... right... But yet I know what I'm talking about. Why can't we be friends, why can't we be friends"
"I know you're working for the CIA, they wouldn't have you in the MA FIA"
visit: h--p://www.youtube.com/watch?v=5DmYLrxR0Y8
RADAR
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I believe he shorted his rectifier in a previous post using his meter on a current scale to try and measure voltage.
re: gray 3-prong Ground adapter
That could still leave the guy behind the 'scope floating at 110v.
Not a situation I'd put a neophyte in.
The first rule of working on line-powered switchers, is to run it on a reliable isolation transformer.
As there might still be leakage, I use two robust 24v transformers, back to back.
I'm all for giving tricks to the newbies, but not if it might kill a stupid one.
And when it comes to strangers on a forum, ya just can't tell.
edit:
And that makes my point!
That could still leave the guy behind the 'scope floating at 110v.
Not a situation I'd put a neophyte in.
The first rule of working on line-powered switchers, is to run it on a reliable isolation transformer.
As there might still be leakage, I use two robust 24v transformers, back to back.
I'm all for giving tricks to the newbies, but not if it might kill a stupid one.
And when it comes to strangers on a forum, ya just can't tell.
edit:
Oh, you might be right!
And that makes my point!
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Most modern switch mode power supplies will have hot and cold sections. Test equipment must be grounded to correct section of the pcb (printed circuit board). That's why an isolation transformer is a must. You can apply a temporary heat sink such as a suture clamp or something like that to each lead of the cap when soldering to prevent overheating the cap. This will leave the cap raised slightly from the pcb, so make sure you have room to do that.
You don't have to take his advice, you don't even have to read it. But you should not flame anyone for trying to help.
I was going to Flag the post, but I think it's funnier to let it stand to humor others as well.
Yes we will leave it, for now....but To the OP: Flaming is against the rules here! So watch yourself.
Don't bite the hand that tries to help you. Anole knows what he is talking about, he has been doing electronics for years.
Don't bite the hand that tries to help you. Anole knows what he is talking about, he has been doing electronics for years.
Pull the cap's back off and if you can take a picture of both sides maybe we can help you. Also if you don't see any marks then look at the trace on top and you should see one that is either square or maybe a circle which will normally be the negative side for the cap. Also look at another cap that you haven't touched.
fhorta,
I have over 60 years involvement in electronics... And I have seen people injured AND KILLED. Anole is concerned about safety. The post advising to use an ungrounded non-isolated O-Scope is unacceptable! Differential inputs are designed for this very use. Ignorance and carelessness are Darwinian selectors. Say and do as you wish.
As pointed out, switching power supplies can supply over 700 volts -RAW- above chassis ground. If someone copied or used an off the shelf supply without designing it for the particular equipment, you could have a disaster. Did they use a portable defibrillator power supply in you STB? Three thousand volts? Un- explained blown caps? CAUTION.... ... ... ... ... or not. Your choice.
I have over 60 years involvement in electronics... And I have seen people injured AND KILLED. Anole is concerned about safety. The post advising to use an ungrounded non-isolated O-Scope is unacceptable! Differential inputs are designed for this very use. Ignorance and carelessness are Darwinian selectors. Say and do as you wish.
As pointed out, switching power supplies can supply over 700 volts -RAW- above chassis ground. If someone copied or used an off the shelf supply without designing it for the particular equipment, you could have a disaster. Did they use a portable defibrillator power supply in you STB? Three thousand volts? Un- explained blown caps? CAUTION.... ... ... ... ... or not. Your choice.
The main thing here is if he's not sure what needs to be done then check everything without power or find someone to help.
Now I will say that when working on computer mainframes and the drives we did remove the ground which was common practices for working on the equipment and that included all types of devices.
To help others understand using a scope and this issue check out this link as some might find it interesting.
[h=3]Oscilloscope operation differential voltage measurement[/h]http://www.youtube.com/watch?v=VWodjUSkYVE
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