Creative S750: back LED & control pod LED are okay, but unable to power up

[moontide]

Half a year ago, I repaired my S750 woofer.

Half a month ago, I bought a failed S750 woofer to repair for fun (I'm still a newbie). There were some obvious failed parts on the PSU board I checked, and I may fixed them.

• D9 - 1N4004 - short circuit, replaced.
• D13 - 1N4148 - short circuit, replaced.
• R9 - 6.8Ω, 1/4W - open circuit, replaced.
• U7 - TOP243Y - it was replaced with a TOP249Y by the previous owner, I don't know if it's okay, but I replaced it with a new TOP243Y. And the solder pads are off except 'C'. And a piece of copper on the other side is missing which makes an open circuit, I fixed this too.
• C59 C60 C69 C70 - 200V/470μF - most of them are bulged, so I replace them with 4 Chemi-con KMM series capacitors (not long-life series, but should be okay).
• C61 - 450V/68μF - no obvious bulge sign, but I replaced it with a Rubycon TXW (long-life series) 450V/82μF capacitor.
• C62 - 25V/47μF - It was replaced with a Rubycon YXA 63V/47μF by the previous owner, since YXA series only have 1000 hours life, so I replaced it with a Rubycon ZLH 63V/47μF capacitor. Also, there's explosion sign under it's legs, a piece of copper missed.
• C63 - 50V/0.1μF - no obvious failed sign, but I replaced it with a Nichicon UMV 50V/0.1μF, it was an extra components from my previous repair.
• C7 - 250V/470pF - looks like it was replaced by previous owner, I leaved this one wihout replacement.
• FUSE - 250V/3.15A - It was replaced with a 250A/10A one by previous owner, I'm afraid it can't protect the boards inside, so I replaced with a standard 250V/3.15A one.

The current issue
After fixing, I plugged power line, the green LED on back plate lights up, the red standby LED on control pod also lights up.

Exciting! (Donald Trump's WINNING face emoji)

I continue to press the 'STANDBY/POWER' button on control pod. The 'MASTER' LED lights up!

Exciting! (Donald Trump's WINNING face emoji)

I plugged audio line to a radio, but no sounds came out.
Hmmm... it reminds me that I didn't heard the click sound of relay when press the 'STANDBY/POWER' button on control pod.

So I measured the voltage of each pin of CN6 connector, I found that the reading value is about half of expected value it should be.

Now I'm lost, what reason it could be?

Extra information I don't know if it can help:

• T3 - After took this woofer home, open the back plate, I smell something like humidity, so I leave it opened for several days. But it still smells like that now, especially when put my nose close to Transformer3 - the one related to the standby circuit. Could it be transformer 3's fault?
• U2 - optocoupler - Since there's explosion sign under C62, U2 is closed to C62, could it be affected and failed? By the way, the voltage of Vce(4 pin to 3 pin) is around 5V.
• U3 U4 U5 U6 - The input voltage and output voltage (obviously) of these regulators are low too.
• C64 C65 C73 C78 C74 C66 C75 C68 C76 C77 - I didn't replaced those electrolytic capacitors, could these capacitors caused low voltage?

One more thing, how could low voltage makes back LED & control pod LED works?

[moontide]

Re: Creative S750: back LED & control pod LED are okay, but unable to power up

Okay, after weeks struggle, I may fixed this subwoofer after replaced the optocoupler (U2) in the feedback circuit.

Since I suspect U2 (optocoupler) and U3 U4 U5 U6 and T3 and electrolytic capacitors in the feedback circuit, I began to replace those components from easy (electrolytic capacitors) to hard (Transformer 3).

1. I replaced all the electrolytic capacitors in the feedback circuit, not helping
2. Desoldered optocoupler, clean some dark gray explosion dust/stuff, solder it back, something interesting happened:
   When click the POWER button, I can hear the relay sound now -- the main power is up. But still, the voltage output is low -- I got +17V (+15.xV before optocoupler been resoldered) on the +24V ouput, and I got no sound when playing music.
   
   
3. Planned to replace U3 U4 U5 U6. Fortunately, I didn't reached this step.
4. Planned to replace Transformer 3. Fortunately, I didn't reached this step.
   
   I didn't found a proper replacement for Transformer 3. If I reached here, I'm dead (or I must buy another failed one to get the replacement).

This make me focus on the optocoupler now, the resistance between #3 and #4 leg of this optocoupler is around 1040 Ohm in both direction, it indicated it's failed, so I bought a SHARP PC817 A (rank must be A, because when I bought Avago ACPL-817 C, it didn't work), replaced the old one, now the voltage on +24V output is about +23.7V, -23.xV on -24V output, so it can give enough voltage to the STA575 chips on amplifier boards -- hence it can make sounds now.