[fabi]

Hello, my MP573 died a few weeks ago. I opened it and discovered the C13 cap was blown and leaked. I attach some images at imgur:

(https://imgur.com/a/wGRzk3T)



Apparently there's no cold solder joint. I don't know it this was just a bad luck cap death, or if it hides something. Maybe the L7909 is damaged. I can try to replace the cap, but the one I have is shorter, and most likely not low ESR. Any advice on how to repair it is more than welcome.

Thanks.

[JPK]

The dead cap could be the symptom of something else.
Check the DC voltage on TP2 against the 0V pin. It should be -9VDC.
If OK, replace the cap and make a check of the other DC voltages: 

[fabi]

Thanks for the support! Here goes the measurements:

TP2: 13.1
A: 12.8
B: 14.2
C: 15.3
D: 13.7
E: 13.1
F: 13.2
G: 12.8
H: 14.2
I: 13.1
J: 14.6
K: (it's plugged, cannot measure)
L: 14.1


I saw the values I read are very different from the expected ones. no clue why.

[JPK]

TP2: 13.1
The minus voltage doesn't go through.
Check the diode D4 (DMM in diode mode), resistor R27 (ohmmeter), U1 and U2 (by replacing).

[fabi]

Do you mean to replace the regulators? I don't have spare parts for that, and surprisingly there's no LM7909 on mouser. Do you suspect the 7909 and diode burned out? Why this happened? Could be caused by a bad input?

- R27 is 7.5 Ohm.
- Both diodes seem funny. They don't conduct, Just a small beep and then I see the voltage drop between its leads (around 0.56 Vdc). If I reverse the meter, they don't conduct but I see voltage drop, in reverse. So I guess both are dead.
- As per the regulators, I don't have a spare LM7909 nor LM317T. I'm checking mouser and they don't have it, oh gosh I'll have to buy them in China?

In any casse thanks for the support JP

[JPK]

Could be caused by a bad input?
No.

I would start by just replacing the cap which is the most likely to fail.
The other parts replacements are only if this doesn't work.

[fabi]

I changed the cap, now the measurements:

TP2: -9.1
a: -6.4
b: -5.1
c: 13.4
d: 4.4
e: -7.1v


I unplugged the unit at this point, since the measurements seemed bad. Now TP2 seems fine, but not the others. If you need some other measurement please let me know, the card is over the table now.

Thanks!

[JPK]

These voltages must be measured against TP2.
So they don't look so bad.
Can you redo these measurements relatively to TP2 from A to L ?

[fabi]

Yes, they look pretty good now:

TP2: -9.1
a: 2.6
b: 4.0
c: 22.5
d: 13.5
e: 2.0
f: 12.9
g: 0.9
h: 4.0
i: 0.4
j: 3.4
k: (cannot measure, it's plugged)
l: 2.9


Mmm, could it be just bad luck with that blown cap? Or do you think there's another issue? Just FYI I have another MP573 working just fine in the same lunchbox, so not sure why it happened

[JPK]

Yes it happens. Electrolytic capacitors are the most fragile components, especially if their temperature is high.