Kenwood SW-505D no sound issue and pops from time to time when it works

[ex250]

Hello mates,

I bought this subwoofer a few days ago used and blind and it works erratically, as far as I could see, it has been tampered with, the speaker is not the original one and of 8 Ohm when it should be of 4, and there are some welds like reworked by the boards.

It doesn't have much power (expected more from it) and it distorts if you increase the volume over the middle, maybe it's because of the high impedance of the speaker. The problem is that it has sound when it wants, the led panels all work correctly, you can hear the power relay on power on, but right now no sound.

I thought that maybe some mosfet was burned (there was a previous rework on the solders there), I desoldered them but they seem to activate and measure correctly with the polymeter.

But I don't know what it does so that sometimes it works and sometimes it doesn't, there doesn't seem to be a pattern.

It wakes up correctly when turning on the sound source and goes into standby mode if there is no audio source some minutes after, but right now after trying on and off multiple times it does not work, I do not know if tomorrow it will... when it works, sometimes very occasionally you hear one or two loud "pop" in the middle of the music, without having the volume very high.

Could someone give me some guidance on what to check?

Thank you very much

[petehall347]

post pictures

[ex250]

I am checking the power board voltages, on CN5 pin 1 there should be -12.4v, and there is 0v. The linked IC303 is -12,4v on the out pin, suspect about the cap C317. The B+ voltage is 5.02v,

[ex250]

Cap c317 seems fine 11,7uf, I just realized that the line goes to the speakers and line in connection board...

[ex250]

Sorry I mean before the IC304, out pin 12,4v, in the IN pin should be -21v, but there is 0v. The cap was fine, just changed with another just to try, but same issue. Today the sub not worked to check any differences on the voltages.

Also checked all the voltages on the IC301, all seems on range, except the pin 5, with 4,92v and there appears that should be 0, I don't know if its a typo on the scheme

[ex250]

On the CN30, the pin 6 should have 10,2v, and reads alternative low values between 0v and around 2v keeping oscillating, with no static value, would check for C135, feel free to give me ideas please, I would appreciate it

[CapLeaker]

The 5V on the pin 5 on ic301 is probably a fault protection pin. With that being “high” it knows something is wrong and disables the amp. If you got nothing (-21VDC) going into ic304, there won't be any -12.4VDC coming out of it. Either that -21VDC rail is shorted or something is open. Follow the -21VDC rail from ic304 backwards and see what gives…
Check around D301 and the AC voltage on CN7

[ex250]

Happy new year!

Thanks to point that, now the sub is working, in such a low power mode (vol knob on the middle to high side to hear some sound from), I still think, (without much confidence) that could be for the 8ohm impedance of the speaker used instead a 4ohm one, in IC304 there is -21.64v in and -12.1v out, seems OK there, just reading on the outer pin, and IN is on the middle... I'm feel stupid...

On CN7 the readings are 28,71v and 29,40v
On CN5 pin 1-2=5v pin 3-4=75v

On IC 301 there is still 5v on pin 5.


I just found one cold welding on the IC205 12v out, I reworked it to see if it does not reproduce the fail.

Is there any safe easy way to measure the power the amp is doing?

[ex250]

Well, there must be some other problem, I thought that the amplifier line pre out output of myAV receiver was not giving enough signal for the amplifier on the SW line sockets, however I connected one of the outputs for a speaker through the subwoofer, and even with a low volume the sound is distorted, so there is something else, I guess in the line in / speaker input card.
It probably has to do with pin 6 of CN30 not reaching the 10.2v indicated in the schematic, any idea where to look for it?

[ex250]

Also, could anyone point how the hell that 10,2 v are generated? and on the TR106 should be -5,1v and I have +5,4v on each side.

[ex250]

I have not found the problem yet, I checked all the components of a board, I found Q212 with some high measurements in one of the outputs compared to another one that has the same board. I will replace it.

A question that maybe if you can help, the mosfet is necessary to match them? one of them gives a reading vt:3,26v while the other 3 oscillate between 2,91 and 2,99v (2,91v, 2,94v 2,99v)

I find that by adjusting the off set of the speaker as close to 0v (-0.024v) as the potentiometer does not give, the distortion is very high at minimum volume, however, if I leave the off set at the maximum allowed by the potentiometer -4.x volts, makes less distortion, it is clear that the cone is down with that voltage and will heat up, but I do not understand why it distorts less than with the off set close to zero.

Found also almost 2 capacitors are different than the scheme there is one of 0,22uf50v where should be one of 1uf, and one of 47uf where should be a 10uf one. The solderings seem original on that ones, maybe a production update?

[ex250]

please, some input!