HiFi Power Fault

[room237]

OK i am new to this forum so firstly hi to everyone here.
I have a problem on a hifi the problem started when the unit was switched on one day and nothing happened? I suspected it was a blown fuse of which was replaced but still no power. However the standby light was just barely visible. Now i checked the internal fuse which was not blown. I took the fuse out and put it back in, switched everything on and i had all power. However after a few seconds or more it all died again. Taking the fuse out cures the problem but as soon as it is put back in the unit goes off again. Is this a bad capacitor? on the power board which also has the diodes. Anyhelp please.

 

[davelectronic]

Hifi power

Hi and welcome from me. If you remove the fuse, and you say every thing is ok, how is power getting through ? No fuse in. can you use a multi meter ? but only if your familiar with safe mains voltage practice, Check the fuse rating and continuity in the fuse. Other problems can be looked in to once you eatblish the power with in the power boards secondry voltage, or lack of it. Dave.

 

[davenn]

OK i am new to this forum so firstly hi to everyone here.
I have a problem on a hifi the problem started when the unit was switched on one day and nothing happened? I suspected it was a blown fuse of which was replaced but still no power. However the standby light was just barely visible. Now i checked the internal fuse which was not blown. I took the fuse out and put it back in, switched everything on and i had all power. However after a few seconds or more it all died again. Taking the fuse out cures the problem but as soon as it is put back in the unit goes off again. Is this a bad capacitor? on the power board which also has the diodes. Anyhelp please.

Is it the mains fuse or the DC fuse thats blowing ? .... this is an important Q as it quickly isolates the fault.

if its the mains fuse thats blowing, then you have 2 primary causes 1) the bridge rectifier, very commonly dies. or 2) a transformer fault, maybe a shorted turn ... it happens but not so common.

If its the DC fuse thats blowing, then you need to start looking at the power output devices in the amplifier. They will either be IC modules or discrete transistors

if thats the case you may be able to remove/ test the transistors/module and find out which channel has died. possibly both, but unlikely, most likely just one channel.

cheers
Dave

 

[room237]

Yes sorry for the dleay in reply. It is the fuse that is on the board. The fuse is not blowing if i said that no that is incorrect i meant to say i thought it was blown. The fuse was tested and was found to be ok. No what it is when i remove the fuse on the board then put it back in and switch everything on there is power to the unit and everything works for a few seconds and then it goes off again. If i remove the fuse again i get the same results. However the last time i put the fuse back in the unit did not switch on? I can hear the transformer so know there is power going to it. I do know how to use a multi meter for some things however i do not know how to test transistors or even rectifyers or capacitors. I am told that one must be very careful testing capacitors and that to unsolder one side of the capacitor but i do not know how to test them. This would be intersting to know just what the problem is. I do have some electrical background so know how to work safely. Thankyou to you both for your replies i trust i have cleared the mist a bit.

 

[room237]

i see now where the confusion was. I meant that i replaced the fuse in the plug i never had my tester with me at that point so just replaced the fuse in the plug. Yes it is the DC fuse on the board i was later refering to which was not blown. Just took it out put it back in then we had power but only for seconds then all power off.. Transformer buzzing very low. Pull the fuse from the board put back in switch on power for a few seconds then off again??

 

[davelectronic]

Hi fi fault

Hello and welcome to the forum. This is a maybe, as it could be many things, the fuse carrier is normally soldered to the pcb circut board, if a solder joint has a bad or broken connection intermittent faults as you describe is possible, its only a maybe and you would need access to the underside of the circuit board to check visualy and with a meters continuity function, not powered up. But it needs to be ruled out as a fault, then other problems can be looked in to. Dave.

 

[room237]

OKthe fuse on the board is totally secure as is its carrier2634

2635
here are some pics of the power board itself. Thanks

 

[room237]

two more pictures

 

[room237]

I have checked through the diodes on the board, i would not have thought these would have gone anyway but i checked them and all seem to be fine. I am wondering if it is one of the capacitors that have gone bad? None are bulging or leaking but i am wondering if it would be a good idea to remove them all and check them.
2639

 

[Resqueline]

The pic's are too blurry to be of any use. A big & sharp pic of the underside is of particular interest to spot any bad solder joints or cracks.
Keep at or beyond the minimum distance of your camera and shoot straight down at the surface, having illumination from several angles.
Then check the quality and crop the picture to the area of interest before posting it. Windows Paint can do this if you have no better pic' editor.

 

[room237]

The pic's are too blurry to be of any use. A big & sharp pic of the underside is of particular interest to spot any bad solder joints or cracks.
Keep at or beyond the minimum distance of your camera and shoot straight down at the surface, having illumination from several angles.
Then check the quality and crop the picture to the area of interest before posting it. Windows Paint can do this if you have no better pic' editor.

Sorry i only put up the pictures as some kind of reference really. I have checked the board for dry solder joints or cracks already and the solder joints are in fact perfect.

 

[Resqueline]

Ok, but they are too poor for us to see what the circuit does, and to refer to points to measure on. Also, what kind/type # of IC is that?
Do you hear the transformer humming even when the power goes out?
The first thing to check for then is AC presence on the secondary side.
The next thing to check is the presence of DC voltage on the big capacitor.

 

[room237]

Ok, but they are too poor for us to see what the circuit does, and to refer to points to measure on. Also, what kind/type # of IC is that?
Do you hear the transformer humming even when the power goes out?
The first thing to check for then is AC presence on the secondary side.
The next thing to check is the presence of DC voltage on the big capacitor.

Ok well if you do need pics i can probably get better ones that is no problem.
However the IC i googled the number on the back of it and it comes out as a La4728 which all i can find is a BTL 2 channel power IC used for Car stereos? This is a small household hifi.
Now is it possible my problem could be due to this? How does one test such an item.
Also about the big capacitor how do i check the presence of DC voltage using say a Multimeter?Does it have to be unsoldered fully extracted to test it?

The buzzing of the transformer i recall yes it was buzzing but very low hardly audible when switched on.

I have heard that some problems due to power faults such as mine have been down to an IC but the problem then is why did it blow in the first place? Thanks for your help though. If you can be more precise in explaining things as i am not an electronics wizard as you may already have guessed. Thanks for your time though i appreciate it.

 

[alfa88]

What I'd Do

Somehow you're going to have to take a voltage reading on pin 14 of your La4728 chip while the unit is powered up. The safest way would be to tack on a temporary wire and carefully put things back together. Test clips come in handy here so you don't have to hold on to anything but the power switch. The chassis would make a suitable ground. It looks like you should be seeing 12 to 18 Volts DC. If you have the voltage you know the power supply works (no duh) and the La4728 isn't loading things down. In that case it may be the standby circuitry and in that case it's bad news because that's probably controlled by a propitiatory control chip that's gone bad. The fact that the amp works right after resetting the DC fuse is a hint to me. Of course I'm just armchair troubleshooting.

 

[Resqueline]

The big capacitor has two pins that are connected across the power supply.
In order to test for the presence of a DC voltage you just turn on power and poke the two solder joints with your multimeter probes.
While you're at it you may also want to check for the same voltage across pins 3 & 14 on the IC. Pins 3, 10, & 12 are all ground btw..
There should be at least 2.5V on pin 4, and some voltage on pins 1, 2, 5, 6, 7, 8, 9, 11, & 13. Be careful not to slip the probe and short two pins.
There's no reason an IC mainly intended for car stereos can't be used for home stereo systems.

 

[room237]

Thankyou to all who have replied. I have decided to throw the unit out now but it would have been nice to have found out what went wrong. I decided to do a google on the IC and was going to try another one in the board just to see? But it seems that this part is not easy to get hold of unless your in China and some of the distributors there seem to be unwilling to sell me just one? So have decided to call it a day and scrap it.
Thankyou though for all your help very much appreciated on this brilliant site!