Sickly NAXA NPB-273 Repair Help

[bravozulu]

New to the forum here. Also new to attempted electronics repair. I bought a NAXA NPB-273 boombox a couple of years ago and it started smoking and producing a burning smell about a month after purchase. Unfortunate life events kept me from just returning it, and, being a bit of a pack rat, I just stored it away for the future. I liked the unit while it worked, and would like to fix it if possible.

I hate sending things to the landfill if they can be fixed, I just lack the knowledge to fix it. I took the covers off the unit and found the burned component. I will attach a a picture of the main board and the component.

Can someone tell me what the component is, what it does and if it can be removed with minimal impact to the unit's function. Or replaced (I can solder), and where I can buy this item?

I appreciate any advice. Also, if there's a recommended online repair guide for these types of electronics I'd like to read up on the subject.

Thank you,
Chris

 

[Superduper]

It's a power transistor. Likely in the power supply section, probably as a voltage regulator. The boombox most likely can not function without it, especially on AC. Without a service manual or schematic or parts list, its impossible to know whether it is a PNP or NPN transistor, so I can't recommend a replacement part without knowing the original part #. I would need to take a closer look at the PS design.

 

[caution]

I can almost make out the part number.... C078? C278? Likely a Rohm power transistor

 

[bravozulu]

I can almost make out the part number.... C078? C278? Likely a Rohm power transistor

I can make out "CD780." There may be another character after the 0, but the material is deformed from the heat.

Thank you both for taking a look. I have attached some follow up photos if they may help.

Thanks,
Chris

 

[caution]

Ahhhh yes I see it too
Probably a CD7805, CD7806 or CD7809

 

[Superduper]

You guys have much better eyes than I do. I couldn't see anything but a big charred mess. Without having access to high res closeup of the area and underneath and hopefully see how the circuitry runs, I'm just going to presume that it's either a power transistor (likely NPN since it's by itself) or if it is indeed a CD7805, I'm going to presume that it's a 5-volt regulator IC. Google doesn't show anything except Aliexpress chinese vendors for that part. For that reason, it would seem that it's probably an LM7805 clone manufactured in china by CD. If it's an NPN power transistor, I have tons of 2SD2023 and TIP29 (T0-220 NPN transistors) in my stock which you can have for free (you pay for shipping) but if it's a voltage regulator IC I only have the LM7812 or LM338T. The LM7812 is a 12V regulator so that won't work since there's clearly a "0" behind 78, and the LM338T is a variable voltage regulator which can work to output any voltage your circuit needs but it needs additional components to regulate, so it won't work in your case since it won't be a drop-in replacement. I suggest you just look for an LM7805 which are common and cheap locally and cross your fingers that additional components do not go poof if it's the wrong application. It's less likely a 7806 or 7809 was in there since those are far less common than the 7805 (5V rails are common on digital applications, 6 and 9 volt are less likely and if they do exist, there would usually be multiple rails with at least 1 5v or 3.3v. Finally, if you're going to solder in another regulator to try, I highly suggest adding a heatsink to it. The charring in that area suggests either the transistor really needed a heatsink to begin with, or the current being consumed was way beyond design limits so be mindful of that. There also seems to be some charring around the amplifier area of the board.

 

[bravozulu]

Thanks again Gentlemen. I have been able to source the transistor locally.

I guess the last $64 Question I have is cause. Is it most likely that this transistor was bad from the start, or is there likely another issue that caused this transistor to be immolated? I guess it's probably tough to answer since you can't see it in person, but your opinions are appreciated.

Thanks,
Chris

 

[Superduper]

.......I guess it's probably tough to answer since you can't see it in person.............

Generally speaking, current overload due to circuitry failure, poor design, cheap chinese parts, no or insufficient heatsink for circuit design, operating the unit at too high level continuously, blown output amplifier, etc.

Add a TO-220 heatsink. They are cheap. Don't think, just do it.