Stubborn MS275 *SOLVED*

[Kurten75]

I was given an rev H 275 some time ago.

The former owner said it didn't work, but i figured that it might just need new caps.

I've got some free time this weekend, so this morning I opened it up.

This is what I see:

20141114_175021548_iOS (2).jpg (196.88 KiB) Viewed 12993 times

A beauty - ain't it?

It took me a couple of hours to clean.
Then did both rail caps and input caps.
Cleaned the heatsink, put on some new silpads - and put it together again.

But; something is wrong.
Fired it up at my testbench.
First red light, then green light (so far so good)
But; I'm missing the four red lights.

Anyhow - i connected my iPhone to the RCA - but no sound whatsover.

hmm

So - I need som help now - on what to test next.

This has been testet:

Idle current : 0,15A !!!! (Not touched the BIAs pots (yet))
DC offset across L/R speaker channel : about 35mV both. But the numbers decreases rapidly towards 0mV ?!?

So - what to measure next?
Any ideas?

[wooferdog]

The power supply to the rails is not working if the red led's are off.I don't have a clue on the repair tho.Did you do any testing before it came apart?

[Kurten75]

Unfortunately no.

The caps were leaking - so thought they were to be replaced first.

[wooferdog]

I like to do a quick bench test just to make sure everything is good so I don't end up taking the board back out for a switch or gain pot.You can always pull the leaky power supply caps off the board if you want and run the amp that way too as long as you have clean power going to the amp.

[zeropoint0.5]

the big 4 output transistors, the 2 rectifiers and the 4 ps fet's MUST have a silpad under them and no cooling paste.......

if one of those make contact with the heatsink, you have a short circuit and/or the amp will act crazy...

little fuses will blow or the amp will make strange noises.......

for the little ms series you need 10 big silpads............

[trickyricky]

Did you bother checking the internal fuses for the rail (if it has any...it should).

Couldn't help but notice the speaker tab/s on the pcb is/are missing....and as zero mentioned those don't need thermal paste unless they are using mica or kapton insulators as the silpads don't require paste. If they didn't have a insulator you can bet your ass you would of seen smoke and LOTS OF IT!! so am sure it has some type of insulators between the sink and those semi's.

[valeks1]

Check +15V and -15V supply.

[Kurten75]

Thank you all

OK - this is how far I've got:

The two fuses were ok - not broken.
However - measuring any voltage between the ends - nada, njet, nothing.

The missing speaker connector broke off when removing the internal speaker connector.
Soldered back on.

I've desoldered the rail caps, just to be sure that I have connection from both sides.
One via was missing - so i used the guides here with a solder wick:
http://www.phoenixphorum.com/viewtopic.php?f=6&t=11910

Now I'm 100% sure that the rails caps are OK. At least the DMM show me that it is ok.
Anyhow - still the same (only green light in the front; missing the four red ones)

Regarding silpads; Not one was found when cleaning. Just the huge amount of paste
So - 1've used 10 new silpads when mounting it together.
And the small ones (don't know what they're called) - some Arctic Silver Thermal Paste.

Is there any way to check the condition of the transistors, rectifiers and fets with a DMM? Voltage or restistance?

[Kurten75]

Check +15V and -15V supply.

Pardon my ignorance; but where do I do that?

[valeks1]

U5 or U6 pin4 pin8

[Kurten75]

U5 or U6 pin4 pin8

Thank you for your kind help

Pin 4 & 8 gives me 00.0V

20141115_223739433_iOS.jpg (99.51 KiB) Viewed 12939 times

[trickyricky]

You should look at datasheets for those op amps so you can CLEARLY know which pins are 4 and 8 (the power supply for these op amps).

Looking at your pic, the bottom right is pin 1 and you count counterclockwise...so the pin on the bottom left corner is pin 8.

Upper right corner is pin 4 and bottom left corner is pin 8....you should have +\-15v on those pins.

[Kurten75]

This is SO EMBARASSING

You're 100% correct - I did measure pin 4 & 5.

Still - Pin 4 & 8 (upper right / bottom left from the picture above) show me 0V.
The same applies for U5 also.

my apologies ...

[Renovatio]

amazing how this amp was destroyed. thous red leaves you can buy them?

[wooferdog]

Did you have any luck getting the amp fixed ?

[Kurten75]

Not yet

Awaiting new mosfets, rectifiers and power transistors.
I'll start with these cause of the mess with paste and no silpads.

[Kurten75]

I've now changed 4 IRFP044, 4 2SA1302/2SC3281 (2 pair) and 1 pair of SF302/302A

Still no joy.

I'm focusing right now on the SG2525 and the KA339.
They both have the correct input voltage, about 12.7V (car battery power) measuring pin 12/13 on SG2525 and pin 3/12 on the KA339.

Not sure how the readings from the outputs of these should be:
Anyone?

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I'm very tempted to toss this amp

[trickyricky]

Perry's tutorial explains everything, what each pin does and what voltage to expect. Can you post the voltage readings from the 3525? Make sure DMM is set on DC, place the black/ground probe on the amps ground terminal.


I know how frustrating it can be, if you do decide to trash it...make sure you keep it for parts (screws, terminal blocks, sink, end covers, bottom cover)...the board also makes beautiful wall art (if you gut it, you can hand the board by itself). Of course this is after giving up on it.

[RayBuck]

Don't trash it and don't give up.

[Kurten75]

Don't trash it and don't give up.

Thank you guys.
I totally respect all the repairguys who do this for a living.
I think I'm way over my head with this one; but I'm normally not the quitting type of person.
I could just throw it away, and buy myself a new one - but thats not the issue.
Here I'm about to learn something new - and for me - thats the trigger.
SO; again - respect to all you that do this for a living (I'm just a computer sales guy)

Perry's tutorial; is this the one: ?
http://www.bcae1.com/repairtutorialcdinfo.htm#cott

I've read out SG2525' pins:
Are these readings to be expected - or are they way off?

PIN
1 0.2mV
2 1.938V
3 94.7mV
4 48.9mV
5 1.075V
6 178.0mV
7 1.061V
8 3.496V
9 6.02V
10 12.0mV
11 17.8mV
12 0.2mV
13 12.6V
14 11.8mV
15 12.6V
16 3.82V

[Kurten75]

Finally solved.

After much troubleshooting, I decided to desolder the railcaps for some testing with my capacitance tester.
I don't know how - or why - but these were bad.
They might have shorted due to the missing via earlier - or these were DOA.

Anyhow - installed new good railcaps (tested first this time) - and voila - all good.
red lights - and producing sound again.

Thanks to all who have contributed - in this thread and via PM/email.

[NOT 2 OLD YET]

WOW! That's kind of odd. Rail caps don't normally fail like that! Bet your glad you checked them before giving up on it!