Active subwoofer troubleshooting

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
Hello gurus,

I got this malfunctioning active subwoofer from a guy, who says he gave it too much power and probably burn it (didn't say what exactly was done and his electronics knowledge seems to be None).
I would like to get it back to life, to serve me as his new master (I mean...the subwoofer)

Need your advice to troubleshoot the sub's circuits - would appreciate your help on this.

It does power on - there's a Green led on, however it doesn't produce any sound (if 'HighLevel Out' speakers are connected - they do produce a sound)
20210916_163509.jpgBack panel20210916_180815.jpg

After I opened up the sub
- I measured 5.4 Ohms on a speaker/driver. I connected some old woofer I had laying around - it also didn't produce any sound
- the transformer is functioning (in 220V, out 2X 21.4V)

2 PCBs are inside
(One screwed to the heatsink and another is kind of glued to the back panel - that has a Phase switch and knobs)20210916_181111.jpg
The PCB that's screwed to a heatsink:20210916_192328.jpg

I see 2 suspicious points:
1. the missing Q5 on the 'heatsink PCB' (can be seen on a pic) - looks like it had some thermal issues - WDYT about that ?
2. The 'white block' - a cement resistor - has some brown paste on it's base - I guess it's not kind a 'leakage' , am I right ?

Thanks in advance.
 

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
(the fuse checked - not blown)
I connected the sub's speaker directly to another amplifier - I can hear the music coming out of it.
So, the speaker itself is ok
 

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
Well, after trying to tackle this for a couple of days - I gave up on fixing the existing sub's circuitry.
(After checking/measuring/reading/youtube/books/forums I still don't have enough knowledge to solve this and also the board seems to be handled by someone, not sure at which stage, after or before the 'overpowering' occurred)

I want to buy an amplifier for subwoofer and to install into the existing chasis.
What do you think about putting a car sub amplifier ? something like this (12V 600W Mono Car Audio Subwoofer Amplifier)
car sub amp.pngThe problem is, it should get 12V dc....I have 220V in my walls. So I thought of 'reusing' some functioning components from the existing-non-functioning circuit (center tapped Tansformer ac21.6V-0-ac21.6V/BridgeRectifier/big Caps).
With those components I get (+18.5V) -GND- (-18.5V) DC ...but, the car amp needs 12V...hmm
What do you think about this setup ?
Is there a good way feeding 12V from 18.5V ?
 

Thread Starter

2Hexornot2Hex

Joined Apr 16, 2020
54
I checked some 240V plates versions online - the overall cost (incl. shipping, I'm not in US) will be too high (relatively to the expected financial investment in this project).

Is there a way to know the current amplifier wattage ? (the fuse is 0.5A at 250V...so I guess it's MAX 120W ? - Is there a better/more precise way ?)

Should I consider soldering by myself the Amplifier and Crossover circuits for this sub - what do you think ?
(I can reuse the existing components, from the non-functioning....It probably won't be that good like the original one)
 
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