Home amplifier blowing fuse - HELP!

[uengin]

Hi,
Firstly, I hope I am not reposting a thread that already exists. I am a newbie, so please be nice to me

Secondly I am an electronics engineering student in my third year for background information purposes.

Now, the problem I need help with is this; I have household/domestic amplifier which keeps blowing a fuse, obviously because the circuit is drawing too much current, but I need help to diagnose where and how it is drawing too much current so I can rectify the problem. This unit has a toroid transformer with various outputs. I have tested if the rated unloaded output is actually as rated on the toroid, affirmative. Because there are several outputs there are 3 fuses. The rest of the outputs are unfused.

The output voltages are 0-29V (unfused), 0-6V (unfused), 0-11V (single fused - it's fine doesn't blow), and 29-0-29V (dual fused). The one fuse that keeps blowing is the fuse that is on one out put of the 29-0-29V output. Now there are rectifier circuits following the fuses (as per any power circuit I guess). The fuse that keeps blowing is the centre fuse that is missing in the photo. I have also tried shorting the fuse. What happens is the toroid begins to heat along with the rectifier bridge.

Any ideas and diagnosis procedures clearly outlined will be very much appreciated. Thank you in advance. Included are photos of the fuse section of the board. The track side photo has been flipped horizontally to correlate to the view from the component side.

 

[Resqueline]

You're on your third year and can't fix a stereo on your own? Just kidding..
Since only one of the two fuses blows there's a short in one of the diodes in the bridge rectifier, or a short in one of the two storage capacitors.
Leave both fuses out and ohm from + to both ac, and from - to both ac (on the bridge rectifier). If one of the four measurements are low ohms (less than 10) then the bridge is out. If it's ok then ohm both capacitors. If there's a short in one then replace both.
Since the bridge heats up I doubt there's a short in the small diode on the upper right side of the bridge, but ohm that as well to be sure.
If both fuses were prone to blow there could have been a short in the output amplifier.

 

[niftynev]

uengin,i tried to see the pics you refer to in your post, but it seems i just get referred back to your original post!

i must be doing something wrong, but what?

 

[Resqueline]

The previous links were no entirely complete. You'd have to copy & paste them into your browser address line with http:// in front of them.
Modern browsers seems to activate a search engine when they're being fed an incomplete address. And this is for your convenience..

I'm afraid I offended ungin with my joke, I'd be surprised if he turns up again, especially after all this time (over two years)..

 

[niftynev]

The previous links were no entirely complete. You'd have to copy & paste them into your browser address line with http:// in front of them.
Modern browsers seems to activate a search engine when they're being fed an incomplete address. And this is for your convenience..

I'm afraid I offended ungin with my joke, I'd be surprised if he turns up again, especially after all this time (over two years)..

that'd be a shame if we don't hear back from uengin! i recognised immediately that your 'dig' at him was just a bit of sarcastic play. i mean, you did say - "just kidding!".

the other thing is i'd be interested in hearing if he fixed his amp.

the pics he supplied were very good too.

he's probly completed his engineering course by now, so perhaps he's gained enough knowledge & experience to fix it.

if it was my amp, i'd be putting my meter on the ohms range, & measure resistance from the blown fuse line to earth. if the short is still there (little doubt, though, that it'll be a dead short), i'd remove, say, the rectifier bridge & measure the 4 diodes within it. bridge ok? test the line again. short still there? remove & test the big filter cap. short still there? then i'd suspect one of the driver or main output transistors, or any diodes around there. progressively removing components & testing them out of circuit reduces the chance of ambiguous readings. testing the line after a component has been removed is a good double check as well. semiconductors, annoyingly, (D'oh!), seem to nearly always go short circuit when they die. they also seem very good at blowing fast enough to protect the fuse that is supposed to be protecting them! other components can go short circuit when they fail, but my main suspect would be a diode or 'tranny'.

my meter has a diode test, which shows the junction voltage. this function is useful for comparing transistors & diodes, but since he said the main transformer was getting hot when he shorted the fuse out, there's obviously a dirty big short there somewhere! it's not wise to do that anyway, as something else may now have been damaged.

the other danger is that some toroidal transformers have an in-built thermal fuse, & once that blows the transformer is toast.

(cups hands around mouth to form a funnel shape - "hey uengin, come back mate! how did you go with your amp? we really would like to know?)

 

[Resqueline]

Well, it's very common for people on forums to not report back, even if no jokes have been made - and tons of help have been offered. So we're quite used to it.
Yes, he provided good pic's. People are usually hopeless with cameras and post the most useless pic's you can imagine - not even bothering to check & re-shoot.
You know, the thing about engineering education is that they usually don't get taught the practical side of electronics, so they're usually unprepared to repair things.
But by now he's either fixed it (somehow), or dumped it - we may never know. Thanks for contributing.

 

[PsyX]

Well I'm also an engineering student... although I'm a masters degree student doing chemical engineering

And I stumbled upon this forum and thread when I was googling a similar problem I have with one fuse blowing.

A friend told me to check the output transistors, but they appeared to be fine. I also checked the rectifier diodes and they checked out as well. Then I checked the big caps (filter caps I guess?) and the one gave a short... So I'm off to the electronics store to get some new ones. The stock ones are 3300 uF 50V Sanyo caps (Electrolytic). So I'm going to replace both with high temeperature caps and see what happens!

I'll check back to tell u guys what happened

Here's a pic of the area where the culprit is... (Fuses removed from slots on the right side)

 

[(*steve*)]

I would recommend that you check the diodes in the bridge rectifier for shorts.

See that muck under the caps? It looks like they've vomited all over the board. I suspect that changing them is a good idea anyway.

Clean up the board once you have them removed.

 

[PsyX]

What is that muck on the caps? I see it on various spots on the board.
It's a budget unit, but the sound quality it provides is quite nice...

Also, the bridge rectifier diodes are the 4 diodes located toward the right of the caps? Ive checked them and they all have similar resistances so they appear to be fine.

 

[PsyX]

Well the fuse lit up again after I replaced the caps...

 

[Harald Kapp]

You can try to replace the fuse by a resistor (temporarily) to limit the current to a few mA. Then follow the traces on the PCB measuring the voltage versus GND at any available solder spot along the traces. If you're lucky, you can identify the short by a comparatively sharp drop in voltage. Or by a component that gets hot.

 

[(*steve*)]

With a decent multimeter you can trace the power supply rails until you find the minimum resistance. When I say "decent" I mean a multimeter that can measure milliohms.

Whilst looking for a piece of test equipment that does this (there's a really good one that costs about $200) I found this.

I realise this may not be useful to uengin, but others may find it interesting.

 

[WhiteFro]

Aura Power Amp - PA - 50

Ok, so a mate gave me this Power Amp for nothing and I did a bit of research and it seems to be a good one, made by B & W back in the 90s. I opened it up and the fuses were blown, both of them. I bought four fuses, and inserted two, powered up the amp while it was connected to speakers with volume turned down very low. I also had some music playing through off my iPhone. On power up the speakers immediately began to buzz and no music could be heard after about 5 or 10 seconds one of the fuses blew and no sound was coming from the speakers. Here are some pics of the PCB. I found two very burnt out looking resistors seen in the last picture, the diodes of the bridge were also stuck together possibly from the heat over the years. I can't see anything wrong with the large capacitors but would like advice on how to test them. I have access to a cheap multi-meter.

I need to know what I should do next basically, Those resistors need to be swapped out, but I'd like to know where the short is before I start changing stuff out. What is the best way to test the diode bridge, the capacitors and transistors? Appreciate your help here guys!

 

[eKretz]

Well it looks like the previous guy disappeared again, but the "muck" appears to be what almost universally mucks up the board when an electrolytic cap fails...electrolyte from inside the capacitor.

For the latest guy: it kind of looks to me like you might have a leaky capacitor also. The board appears to show some more of that mysterious "muck" under the cap on the left in your 2nd picture, unless that's just a dust bunny. And you're right, those resistors (R43 & R49) are showing definite signs of a short/overheating also.

The best way to check the caps is an ESR meter. If you haven't got one you can check the cap terminals' resistance for a short using an ohmmeter after discharging them. Use the diode test mode on your multimeter to check the diodes and transistors. The transistors should be compared to each other and any others of the same type. Check (power off!) across all the legs in every orientation and compare, look for any readings that are different, shorts or opens.

I'm still fairly new to this stuff, so that's about as far as I can go into detail. If you find something report back, if not, one of these gurus will hook you up, just remember to post back!

 

[WhiteFro]

Yeah sorry the PCB is very dusty, need to give it a clean reckon that's just dust, I've blown a few caps in my day and this one looks ok.

 

[differential equasions]

I know that this is no way to start out a relationship, but 14 posts and not a single person thought to ask for the make and model number of the unit to look at the schematic.

and the OP did not think to ask his professor for help..

Most colleges has a lab where you can take things to diagnose problems and others that you can rely on and a service manual with the information necessary to diagnose the problem.

Someone doesn't want to fix this amplifier - they just want to talk about it.

Lisa Simpson once said something about the power of prayer - when her brother Bart refused to study and didn't do his homework. Don't remember which episode it was - but is very relavent here.

You never replace the fuses when the fuse is blown, you first diagnose the problem and fix the problem and the cause of the problem.
Working with microwave equipment - all you get is a big bang when you don't fix the problem first.

Work outwards from the power supply to the pre amp and eventually you will find the problem.

I read somewhere in a previous post where the op said there was resistance in the diodes.

Diodes do not have resistance other then the POV .3 - .7 and should show power in one direction and nothing in the other.
You can't just touch the leads on the diode and see a reading and say - oh it's ok!

I have some antenna matches here that has bad torroids - chased the problem for about a week and couldn't find anything wrong on the board, but it still didn't work.
Those are the type of problems that will drive you nuts!

 

[(*steve*)]

I know that this is no way to start out a relationship, but 14 posts and not a single person thought to ask for the make and model number of the unit to look at the schematic.

Probably because, at this stage, nobody thought it would help.

 

[KrisBlueNZ]

I'm not sure if there's any point saying this at this point in the thread, but that goop under the electrolytics isn't electrolyte, it's just hardened reinforcing goop that's put there during manufacture to hold heavy components so they don't vibrate around and cause dry joints.

You can tell this from the fact that it's formed a web between the two electrolytics, part way up. If it had just leaked out, it would just spread out on top of the board. Also you would expect to see some bulging at the top of the electrolytics.

 

[eKretz]

The "goop" in the picture that I'm referring to, (that was referred to as "muck") is the orange-ish residue in the pic posted by Psy-x in January. Sure looks like leaked out electrolyte on the board to me. None of the caps on my electronics are fastened to the board with water-thin orange crap. Failed caps don't always bulge at the top, I have seen plenty that only popped on the bottom and puked all over the board.

You guys do realize there are 3 separate failure questions on this post by 3 separate posters, right?

A service manual with test points would be great, but in my experience not too many stereo equipment makers have made them available for recent stuff. I've never been able to find one for my Integra DTR-9.1 for instance.

 

[KrisBlueNZ]

The "goop" in the picture that I'm referring to, (that was referred to as "muck") is the orange-ish residue in the pic posted by Psy-x in January.

I was looking at the picture in post #7. That's dated January so I think it's the one you're talking about.

Sure looks like leaked out electrolyte on the board to me. None of the caps on my electronics are fastened to the board with water-thin orange crap.

I don't think it looks like leaked electrolyte. The shape of it looks like it has been applied deliberately, and there doesn't seem to be any damage to the board, or corrosion of the link in that area, as you would expect with electrolyte. I don't think it looks water-thin, and I've seen a similar appearance from the goop they use after it has had a decade or two to deteriorate.

Failed caps don't always bulge at the top, I have seen plenty that only popped on the bottom and puked all over the board.

True. The only way to know for sure is to desolder them and look at the underside. If I'm right, all the goop will be around the outside of the capacitors.

You guys do realize there are 3 separate failure questions on this post by 3 separate posters, right?

No, I didn't realise that! I haven't been following it. I was just seagulling the thread

A service manual with test points would be great, but in my experience not too many stereo equipment makers have made them available for recent stuff. I've never been able to find one for my Integra DTR-9.1 for instance.

Yeah. "No user-serviceable parts inside". In fact, "No serviceable parts inside", full stop! Bury it in the ground and buy a new one.