Hi !
Sorry for the delay, but with these hot summer days, not very easy to do some checks !
By the way, here are the new checks of today !
I re-check Q3 and Q4 completly again.
Q3 S->D : 152V, no more fluctuating !
Q3 S->G : 283V

Q4 S->D : 148V
Q4 S->G : 280V

C59 : 157V but decreasing slowly

I wanted to re-check Q2 and Q1, but on Q2 I ripped the red probe between G and D and what should happen, happened ! Shortcut, electric arc and Q2 explodes !

For the other 2, I already threw them away...

But I don't understand why this time I haven't no more fluctuating values !? ! So next step is to replace Q2, then Q3 ?

Hi !
Sorry for the delay, but with these hot summer days, not very easy to do some checks !
By the way, here are the new checks of today !
I re-check Q3 and Q4 completly again.
Q3 S->D : 152V, no more fluctuating !
Q3 S->G : 283V

Q4 S->D : 148V
Q4 S->G : 280V

C59 : 157V but decreasing slowly

I wanted to re-check Q2 and Q1, but on Q2 I ripped the red probe between G and D and what should happen, happened ! Shortcut, electric arc and Q2 explodes !

For the other 2, I already threw them away...

But I don't understand why this time I haven't no more fluctuating values !? ! So next step is to replace Q2, then Q3 ?

Vgs of Q3 & Q4 are around 280V, are you sure? That definitely burn IRF740 -- Vgs should never lower than -20V or higher than +20V, usually it should around +10V.


Be careful when checking components with power on, I burned two tiny components on AMP board when I checking other component with power on, literally burned -- I saw white smoke flow up.

What's next step? I think you need fix the "Vgs too high" issue first if it's really that high.
Vgs in your previous check is only in mV level, I don't know why you got such a high reading value this time, it closed to the reading value of voltage of D1 you last checked!


This post has been edited by asenrzhang: Aug 29 2016, 01:33 AM

You are maybe right, it was perhaps not 280V but mV. I probably have not read well !

Edit : but, if it's mV, you have said it's too low too, so where is the matter now ?

This post has been edited by rsseco: Aug 30 2016, 03:49 AM

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

My guess is

• D23 and/or D24 may failed.
  
• Q6 may failed, can't provide enough voltage.

(My guess could be totally wrong!!!)
You can use your first multimeter to test them after unsolder them.

For D23 & D24, if your battery in your first multimeter is 9V, they should not be conductive in one direction.
You may check D22 & D25 too, just in case.

For Q6, I never test a transistor, but you can try switch multimeter to 'hFE' position, then plug the unsoldered transistor to PNP (Q6 is a PNP transistor) to test it ( just make sure each leg is in right position )

This post has been edited by asenrzhang: Aug 30 2016, 11:39 AM

Hey, I'm working on soldering on my first set now that my components have arrived. Looking over my list I haven't actually found the location for two of the components.
* I've only found one of the 47UF 25V, the one next to the 35UF 100V, where is the other one?
* The only 0.1UF 50V I can find is a cylindracal one (similar to the others), but the one I've got on my list is a small yellow ceramic capacitor (this one: http://www.digikey.se/scripts/DkSearch/dks...81768803203599). Is this the one I should replace? Is that ok that they are so different?

Thanks for any help!

The other 47μF is closed to the 0.1μF one, it's wrapped by black stuff, see the photo below.

And yes, @lex use a ceramic capacitor like yours to replace the 0.1μF one(C63) instead of original electrolytic capacitor.

This post has been edited by asenrzhang: Aug 31 2016, 08:48 AM


Attached thumbnail(s)

Interesting! I actually looked under that rubber tubing and found a Yageo 47UF 63V. Could it possibly be that this one isn't an original? This set was actually in for repairs a number of years ago, but I only noticed one other fix that they actually did, but could I be sure that this one is also part of that fix then? Should I leave it be or put my 47UF 25v there? See picture. Also, is this tubing important to get back?

Also one more question: On my list I had 3 1000UF 100V, but looking around I can only find two of these, one on each amp. Is it just 2 or am I missing something?


Attached thumbnail(s)

The original one is 25V rating, so it had been replaced in previous repair.

You may leave it there, because I saw someone in another forum (Chinese) said: This one is related to the power reset issue (press standby/power button, power is on but go off immediately, then go back in standby state again), and should be replaced with a higher voltage rating capacitor.
(I didn't verified if it's true or not, I left this issue there, because I can still power on my subwoofer)

Or you can replace it, if power reset issue occurs, then put it back.

The black tube, I don't think you can put it back again, it's some kind of shrinking tube via heat. @lex and others didn't put it back, me neither.

You only need two 1000μF 100V. I guess you bought an extra one as you said before.

This post has been edited by asenrzhang: Sep 9 2016, 09:47 AM

Hi, here I am again !

So, here's the results of tests you asking me to do :



In conclusion, for diodes, all seems to be fine/OK.

For Q6, I first done the test with the multimeter, as shown on the next picture :

The value gaves me 199 to 197. I don't know what it means. If I place the PNP in other way (E on C and C on E, it gaves me 0)
Then I measure it with the diode function, and the result is shown in the first picture.


Question : if these values are correct, should I not rebuy some "real" capacitors ? It could be simply - are they seem to be fake ones - that they are the main problem ?

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

For the 280V, as said before, I think I've just read it bad (must be mV as the first check)

OK, what should buy finally ? And where ?

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

asenrzhang, are you live in Malaysia?

You can also buy components from RS Malaysia

This post has been edited by tezro: Sep 5 2016, 02:05 PM

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.

You live in China Mainland? That's much better. Most electronic components is manufactured in China.

We come again to my dilema : as I'm not sure it will work after all the replacements, I'd not like to spend a lot of money.
On one hand, I'd like to repair it, but on the other hand, I'm not ready to replace every component if it's not necessary.

At the beginning, I was exited to began the repair process ; but with the time, I'm disappointed not able to found the problem. And more, as my capacities in electronica are limited.

If you haven't no more idea from where's the matter could be (beware, it's not a reproach, I'm very happy you help me until now), I will try to reset all the components and do a new try !

This post has been edited by rsseco: Sep 5 2016, 07:56 PM

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

Just wanted to give an update on my progress. Got both sets working now! One has been running for a week, and the other just got finished and has been running for a half an hour or so, so it seems like they both got at least a bit more life!

Huge thanks and appreciation to everyone helping out in this thread! It's amazing that there are people like you

It will be interesting to see if this "double click at start" error comes back, I've had it on both. So it will be interesting to see if the 63V instead of 25V helps as asenrzhang mentioned it could be.