Power boards buzzing sound

[Zeno]

Hi guys! I'm trying to fix two devices: one is a TV, one is a guitar amplifier.
The problem is that they stay on for some time and then go off. In both of them we narrowed down the problem to the power board, which is making a strange buzzing sound (when on) that apparently comes from the transformer!
I'm guessing it's because of a condensator but don't know about mosfets or something like that. Everything looks good, no evidence of damage in both cases.
My actual plan is to desolder capacitors, charge them and measure voltage with multimeter. It may be long but don't know what to test other than them.
What do you guys think?? Any experience in that?
Thanks
Francesco!

 

[ramondo]

Welcome. We are always willing to help with any problem from any one. SOOOO.

What Models are they? Do you have schematics? How old? How many Capacitors? Send photo of suspected boards.

Testing caps the way you suggest may not lead to any usable results. Hum is usually from AC ripple traveling thru the cap while still blocking dc and rising to proper voltage.

Next if there are not too many caps then you might consider replacing them all as caps have a limited life span compared to other electronic parts. If there are many then a proper "ESR and Capacitance test should be done. Did both units fail at the same time (voltage surge?)

 

[starLED]

caps have a limited life span compared to other electronic parts

What is the life span of caps?
Can you make a list of electronic parts by their life span, sorted as which is most likely to fail first?

 

[Nigel Goodwin]

What is the life span of caps?
Can you make a list of electronic parts by their life span, sorted as which is most likely to fail first?

Electrolytics have specified life spans, depending on their temperature when in use, details are provided in their datasheets. However, good quality ones (I like Panasonic and Nichon) will pretty well out last the rest of the equipment.

Electrolytics are the only component that really has a 'life span' - but with all components heat is the killer.

However, the massive failure rate of electrolytics is in switch-mode PSU's, linear supplies rarely give a problem (due to the much lower running temperature)

Hi guys! I'm trying to fix two devices: one is a TV, one is a guitar amplifier.
The problem is that they stay on for some time and then go off. In both of them we narrowed down the problem to the power board, which is making a strange buzzing sound (when on) that apparently comes from the transformer!
I'm guessing it's because of a condensator but don't know about mosfets or something like that. Everything looks good, no evidence of damage in both cases.
My actual plan is to desolder capacitors, charge them and measure voltage with multimeter. It may be long but don't know what to test other than them.

You might try posting the make and model of the units, and we can try and find schematics for them, it's impossible to make suggestions for completely unknown items.

However, randomly removing components and trying to passively test them isn't how you go about repairing things.

If the TV is anything remotely modern it's likely to give error codes by flashing the standby LED (count the flashes).

 

[Zeno]

Thanks all of you! The TV is not so recent. It's a Philips model 32PFL3605H/12.
It stays on for some time and it shuts off. I guessed buzzing was involved because it's not the first time it happens.

The ampli i mentioned has a similar problem. It goes well for a little time, variable. Then buzzing becomes strong and it enters protection. So i connected buzzing = something broking.
My grandma has an LCD which is beginning to have this sound (before it didn't) and i thought the same thign is going on.. (just like trying to figure out).

However here are the pics!

VIDEO LINK OF THE BUZZING SOUND


I also found this site - REPAIRING SERVICING TV PHILIPS

And the pdf manual attached in pdf. Don't know if is good but i've tried!

 

[Zeno]

However, randomly removing components and trying to passively test them isn't how you go about repairing things.

Ehm.. bruteforce attacks ahahah

Did both units fail at the same time (voltage surge?)

They are not related at all. Just same noise in the transformer and same shutting down behaviour.

 

[Nigel Goodwin]

So do you get error codes from the TV? - and what make and model is the amplifier?.

 

[Zeno]

So do you get error codes from the TV? - and what make and model is the amplifier?.

No, unfortunately no error codes from the tv. It works as normal until it shuts off (not like if you press the button, just like if you unplug it).

The ampli is a cheap entry level guitar amp. It's my friends' so i don't have right now, but i will post pics and videos of that also when i will have the chance!

 

[ramondo]

Have you checked the solder side of the board for poor connections? Can you restart the unit after a cooldown period?

Lifespan of caps depends on temp, original quality, design load and amount of usage, SO, NO list , It just depends; and shotgunning with few caps is not "proper diagnostic procedure" it may well serve a non-technician as the easiest path.

 

[Nigel Goodwin]

Check the voltages coming out the PSU, there should be 5V when it's on, or in standby - then there's 24V and 12V, which are off in standby. This is switched on/off by Q907 through R943, the connection labelled SB - this should be high for ON, low for OFF - so if the 5V is OK, and the other two are missing, check that voltage.

 

[rherber1]

Nothing looks burnt on the board. My guess is it will almost certainly be due to dried out electro caps. The shut down will be due to excessive ripple on the supply rail/s.

 

[Zeno]

I'm going to check everything soon! Thanks