Creative S750 AMP board, there's reading value on one board, no reading value on anot

[moontide]

I'm trying to repair the subwoofer (MF7010) of S750 speaker.

I managed to replaced some obviously failed parts on AMP board 2, after that, when I use diode switch of multimeter to measure two pins, something interested happened.

The power input of AMP board has five pins:
[+24V] [-24V] [GND] [70V+] [70V-]

• When I put RED probe on [GND], BLACK probe on [-24V], there's a reading value (1.98x or 1.99x).
• If I measure same pins on another AMP board of another speaker, there's no reading value (actually, the multimeter displayed '1' which indicated it's out of scope)
• If I use resistor switch (20K) of multimeter to measure same pins on both AMP boards, the reading values are very closed, one is 10.7xK(the failed one), another is 10.91K (the good one)

So, I'm confusing now.
Does it means there're still issues on this AMP board?
Or, it's just the smaller resistance caused the reading value occurred when using diode switch of multimeter?

[stj]

Re: Creative S750 AMP board, there's reading value on one board, no reading value on

the difference is minimal, probably just caused by age or slight differences in parts.

[moontide]

Re: Creative S750 AMP board, there's reading value on one board, no reading value on

the difference is minimal, probably just caused by age or slight differences in parts.

Yes, I think that too.

Just did the following test:
Clean solder of [CD-1](#1) & [CD-2](#27) & [GND](#13) pins of three STA575 power amplifier chip, measure again using diode switch, then there's no reading value now.

Then I measure [GND] pin to [CD-1] pin of each STA575 chip, there's no reading value too, which means these new STA575 chips (old chips are failed, and were replaced) are not failed.

And then, I tried to push the pins of STA575 to touch the solder pads, when three CD-1/CD-2 pins touched solder pads simultaneously, reading value occurred again. So yes, I think you're right.

So, I'm going to put it back (probably two days later), and try to power on to see what will happen. Hopefully it will get fixed.

[moontide]

Re: Creative S750 AMP board, there's reading value on one board, no reading value on

Sad story, power it on and got at least one STA575 chip fried (See attachment 2, video). This is the same phenomenon when I got this subwoofer.

Resistance between +Vs and GND and CD-1 pin of one STA575 is less than 100 Ohm in both direction. (See attachment 1, photo)

In the video, you can see when I switch power on, it goes to standby mode. In standby mode, +24V and GND and -24V already reached the AMP board, so I assume there's no serious issue on +24V and GND and -24V on PSU side or AMP side.

The issue occurred when I click the button to switch the main power on from standby mode. The PSU board works fine on another speaker, so I guess there's something wrong before STA575 circuit. I found part schematics of the AMP board from http://www.tgahinfo.homecall.co.uk/S...heets/Buck.jpg.



Before powered on, I had desoldered D4(MUR1520) Q1(IRF540) D3 D5 D1, they seems okay when I use multimeter to test them.
Q5 U1 was failed when I got this subwoofer, and I had replaced them. After powered on, they seems okay.

Currently, I don't know which failed part could cause STA575 conductive between +Vs GND CD-1 pins, I guess there is high voltage after L2, but which could caused this?

Any help is appreciated!