Disclaimer: I'm not a professional electronic engineer, not even an amateur, so, take the advice at your own risk.

Okay

Seems okay, but I'm not 100% sure, I didn't found a proper datasheet.

I'm not sure if it's capacitors' fault, that beyond my knowledge and skill to diagnose it.
All I can tell from your previous test result is:
　　Vgs of Q3 & Q4 is either too low (0.285mV, not enough to open the gate) or tooooooooooo high (280V, it's just too high).

If it's not hurt, you can buy new capacitors and probably other components from a trusted supplier, and replace those capacitors onboard.

This post has been edited by asenrzhang: Sep 4 2016, 04:21 PM

As OP(@lex) and others bought:
Replace all electrolytic capacitors on PSU board, and other failed components.

Several users in this thread had already made a buy-list, you may have a look and make some changes if components are out of market.

For other failed components, you may check others posts (one by one) in this thread to see which components they had replaced, and check yours to see if you need to replace them (or wait for a professional engineer to diagnose which components caused your IRF740 been burned).


You can buy from

• DigiKey
• or Mouser
• or element14
• or others

Those are suggested suppliers by OP and others.

The price of components on their websites are expensive, but they guarantee components are genuine.

This post has been edited by asenrzhang: Sep 5 2016, 11:41 AM

I live in China mainland as written in my profile.

It's actually @rsseco are going to buy components, not me.
Anyway, good to know RS Malaysia, now we have more choices.

Yes, it was world factory before (probably still is), and there're many capacitor suppliers on local internet market (taobao.com). But I had to bought from mouser to get a reliable supply, that is awkward .

Disclaimer: I'm not a professional electronic engineer, not even an amateur, so, take the advice at your own risk.

I know how frustrated it is when get into puzzle. I was there before, I'm a beginner in electronic engineering too, I spent a year to find out what caused my fuse blown again and again.
But hey, we're not in AA meeting in Hollywood movies, "AA is for quitters ", said by Bubba J.

You may read @lex (OP)'s posts 2.5 years ago. Those are two replies to another user's "resistor blew" issue after he replaced Q4,
I don't know if it can help, but you can have a look, and check the components listed in the posts.
- https://forum.lowyat.net/index.php?showtopi...post&p=66540534
- https://forum.lowyat.net/index.php?showtopi...post&p=66570064

Congratulations for the revival of your two sets!

About the "double click" issue, I hope that article I mentioned is right. "double click" issue of my subwoofer get worse now, it happened almost each time on power resume operation in the past week. So, please let me know if the "double click" issue of your two sets does not happen anymore after 3 month using, so that I will replace it with a higher voltage rating capacitor.

Sorry to hear that. I actually have same issue before, see my posts
https://forum.lowyat.net/index.php?showtopi...post&p=80007050
https://forum.lowyat.net/index.php?showtopi...post&p=80199476

When this issue happened, I can't hear the click sound of the relay, so, definitely no main power.
But if I plug the earphone to the control pod, I CAN hear the music, so standby part works.

It seems been self-healed now, but I don't know what caused this,
I just leave it for several days with power cord unplugged, then plug the power cord and then repeatedly power on / off.

This post has been edited by asenrzhang: Sep 14 2016, 07:05 PM

After I repaired the second power amplifier board (the one with 3 STA575 chips), and reviewed this phenomenon, it reminded me that there's a sign of capacitor leaking on one photo.

(I had replaced all electrolytic capacitors on this power amplifier board)

I don't know if it's the cause. Maybe, leave the issue there with back plate opened for several days, the leaked liquid may dry in those days, and hence self-healed? I don't know, just wild guess...

If my guess is right, maybe you can try

• Unplug the CN4 & CN5 connector on power supply board (unload the load), they try power on to see if you can hear the click sound of relay. If you can hear the click sound, then the cause is located on power amplifier board(s).
• Check signs of capacitor leaking on power amplifier boards like mine (It's hard to see these signs in dark environment, even in bright environment, you still need a angle of view to see it). If there's any sign, clean it, plug the power cord of AMP boards to PSU board, and try power on to see if you can hear the click sound of relay.

This post has been edited by asenrzhang: Sep 16 2016, 09:50 PM

Sound fading in/out is new to me, I never heard this before.

I don't know if you should do a throughout search for leakage, because I'm just guessing here. (By the way, I leaved it unplugged for not just 3 days, but more, say one week or ten days or two weeks.)
But you can check the voltage of CN4 & CN5 connector to make it clear that which board is not working well.

1. Unplug power cord
2. Unplug the power cable from the CN4 & CN5, this make the power amplifier boards unloaded
3. Plug power cord
4. Power on, and as you say, you can hear the click sound of relay.
5. Measure the voltage of CN4 & CN5

(1) (2) (3) (4) (5)RED wire
V₂₁ should around 70V (or -70V, I don't remember which pin is high voltage)
V₄₃ should around -24V
V₅₃ should around 24V

• If the reading values are all okay, it means the PSU board is okay.
  Then measure again after the power cable of AMP boards plugged to CN4 & CN5.
  • If the new reading values are all okay too, then I'm lost too.
  • If the new reading values are not okay, it means the AMP board(s) is/are not working well
• If the reading values are not okay, it means the PSU board does not work well.

This post has been edited by asenrzhang: Sep 30 2016, 01:39 AM

It's located on audio I/O board, there're seven connectors in a row for audio output on audio I/O board, and the center one is for center speaker. See my photo:


By the way, the age of this thread is actually 3+ years, not fourteen years.

May I ask where exactly you checked and got the short circuit to GND result?
And, do you follow @lex's guide to clean all degraded glues, especially the hard glue arround C62 & C7 in your case?
And the result you got is after you replaced C62, or before?

I guess you're measuring the voltage between the left(1) & right(5) pins of CN4 / CN5? If so, then it should be okay (mine is only 86.xV.

CN4 & CN5 should only be measured and get the reading values like the following:

(1) (2) (3) (4) (5 RED)
V₂₁ should around 70V (or -70V, I don't remember which pin is high voltage)
V₄₃ should around -24V
V₅₃ should around 24V

This post has been edited by asenrzhang: Sep 30 2016, 02:02 AM

V₂₁ = 90V? hmm, that's not seems right. I don't know the reason either , sorry.

Congratulations!

Disclaimer: I'm not a professional electronic engineer, not even an amateur, so, take the advice at your own risk.

Happy new year!

Did you replaced all the electrolytic capacitors on PSU board?
Since we are beginners, no enough experience and equipment to check, I would like to follow @lex's guide to replace all electrolytic capacitors on PSU board first, then do the test.

Not sure it's AMP board's fault or not. The glues for biggest capacitor on AMP board are 'white' glues, I think it don't need to be removed.
I don't know what the 'pfff' sound came from, maybe it came from the blown fuse itself.

I would like to do the following before testing

• Replace all electrolytic capacitors on PSU board
• Check CN4 CN5 voltage before testing

And when testing, add one test case: plug CN4, unplug CN5, then test.

-------------

By the way, I bought a failed subwoofer, and try to repair it for fun.
The previous owner said it does not power at all, no LED light, nothing.

After fixing, LED on back plate and control pods are back, press the "STANDBY/POWER" button, the "MASTER" led turns on, but it doesn't actually power up: no click sound came out from the relay.
I measured the output voltage of CN6, all the voltage reading values are about half of value it should be.
I suspect it's the fault of T3 (transformer 3), but I'm not sure (even I'm sure, I can't found a proper replacement now), and I'm stuck here.
Hope somebody can help me to find out the source issue.




All the electrolytic capacitors on PSU board had been replaced (but the 35V/100μF were replaced with 35V/220μF, because I forgot to bought them...)



Update 1 (2017-01-07 02:50)
I just desoldered the opto coupler (U2), clean it, check it (seems fine, but I'm not sure), and I soldered it back, luckily, I can turn on the main power now.
There's explosion sign under C62 which is near to the opto coupler, I guess the explosion had affected the feedback circuit somehow.

Update 2 (2017-01-08 00:50)
The subwoofer does not produce sounds even the main power is on. After careful check, I found that the +24V and -24V voltage are still low (around +17V and -17V), and 4 terminals of VH connector of power lines from/ AMP boards are broken (the tongue is broken/missing).


So I bought some opto couplers and terminal components, waiting for the arrival...


Update 3 (2017-01-10 20:30)
Good news, after I replaced the optocoupler (U2), the subwoofer can produce sounds now !

In update 1, I thought it's okay, but it's not!
The resistance between 3 and 4 pin/leg is about 1040 Ohm, in both direction, without powered on.

-------------

Just for fun:
My PSU of desktop computer failed last week.
Symptom: Keeps rebooting without BIOS booted.

I disassembled the PSU, found two capacitors in it, and their rating are the same as S750's: 200V/470μF/22mmx35mm.
I measured the capacitance of them, only 320μF (68%) left. So I replaced them with two capacitors which were desoldered from S750, guess what, IT WORKS!


But for long term consideration, I bought two new Nichicon PW series capacitors and replaced those two CapXon capacitors.


This post has been edited by asenrzhang: Jan 10 2017, 09:47 PM

Well, if budget is the primary concern, this could lead you to a dead loop:
1. You don't have the ability to diagnose the issue.
2. You buy least components to replace suspicious failed components.
3. You found more suspicious failed components, or, the previous replacements which may not be genuine introduced more unexpected symptom.
4. You may need buy more components which cost more including delivery cost and time. (dead loop)

So I will suggest to follow @lex's guide, replace all electrolytic capacitors, then do the test. This is because that electrolytic capacitors are the most easy-to-fail components, replace them can clean the road to next step.

Some users had already made a buy-list of capacitors, you can do a quick math according those lists.

If the subwoofer is running in normal condition, then C70 is the last one of the 4 200V/470μF capacitors that will become bulged -- because it located at the bottom -- heat go up, not go down.

So if C70 be bulged now in just testing stage (no heavy load, no long time running), it could be capacitor's fault.

About the unsteady solder joint of + leg of C70, make sure your solder points are completely soldered to the legs.
And the solder pads of these 4 capacitors are easy to get off (I really hate Snap-In legs), if so, make sure the solder pads are completely off and then make a 'L' shape connector to fix the circuit, this will make the solder joint steady.

This post has been edited by asenrzhang: Jan 9 2017, 02:57 AM

Good news, after I replaced the optocoupler (U2), the subwoofer can produce sounds now !

In update 1, I thought it's okay, but it's not!
The resistance between #3 and #4 pin/leg is about 1040 Ohm, in both direction, without powered on.
So I bought some new optocouplers (I actually bought two times, wrong type at the first time: Avago ACPL-817 C, which the rank is not right. It should be rank A, so I bought SHARP PC817 A at the second time), replaced the old one, the failed one, now the speaker can make noise now (although it still have some small issues such as low volume on some channels, I suspect it's like the issue of @lex's second fix: some channels random off).

So, I make a memo here in case I encountered same issue next time:
When got low secondary voltage of transformer 3 (while the input voltage is normal, around 300V), check the feedback circuit, including the optocoupler (U2).

This post has been edited by asenrzhang: Jan 13 2017, 01:08 PM


Attached thumbnail(s)

I can tell the only different is just different connections of those three 0Ω resistors, as you already found it.

With proper modification (the schematics already figure it out), you can make the PSU works on 230V or 110V.

110V to 220~240V modification should be easy, because you don't need to buy another 0Ω resistor.
220~240V to 110V modification is easy too, but you need buy another 0Ω resistor.

Again, I'm not a professional engineer, so take the advises at your own risk!
I would like to do this test (let me call it Test Case for AMP Board 2) to identify the source:
disconnect AMP board 1, connect AMP board 2, then power on to see if the back LED blinking or not.

Most blinking LED issues of S750 are caused by bad capacitors.
I say "most", not "all", because when I repair AMP board 2, there's one time, when I try to mount the AMP board 2 back, I leave one screw loosely, very loosely --> this caused back LED blinking too, I guess the screw make a circuit somehow between the back plate and hint sink of AMP board 2.

CN4 & CN5: [1] [2] [3] [4] [5](pin for RED wire)

• V_2to1 should be 70V
  
• V_4to3 should be -24V
  
• V_5to3 should be 24V
  

I don't know. I mean it could be the fault of PSU board, or fault of AMP board 2, or both.

Let me confirm these information:
(1) You replaced C59 & C60 & C69 & C70
(2) You disconnected AMP 2
(3) You use POD or remote control to power on
(4) Still, you got C60 very hot in seconds (without AMP 2)

And the voltage of your power line is 110V or 220V~240V?

If C60 is the only capacitor which becomes hot, maybe you need to check components in the following picture:

Especially Q2 (IRF740) and D4 (FEP16DT), if Q2 is the one exploded and been replaced in the repair shop, then focus on D4.

If you successfully put PSU & AMP & IO boards to another box, please share your experience. such as

• how to disassemble the bracket without hurt the screws (I try to disassemble the bracket of AMP 2, the screw head is been damaged, but still I can't screw it down)
• how to fill the hole after disassembled audio IO board -- I mean the part on the back plate.
• how to drill a small hole for audio line to subwoofer
• and, the most important thing, how to build the box and put parts in it

This post has been edited by asenrzhang: Mar 12 2017, 04:02 PM