SMPS repair attempt strategy advice sought

[Andy Watson]

From what I can see of the upper side of the board I have sketched-out most of the circuit. Note - it is incomplete - there are tracks that disappear under the IC and under the current transformer - I can't guess where they terminate. I have also had to make several assumptions about the termination of partially obscured tracks - i.e. NO GUARANTEES of accuracy of this schematic. Happy to make corrections to the schematic if anybody has the information.

However, from what is visible it would appear that the tantalum capacitor that you have already removed on the underside image of the tracks went short-circuit and took-out the resistor. The resistor is decoupling the mosfet driver stage from the higher-voltage supply - so it's value is relatively unimportant - I would guess at 47, but it might be 4.7

The diode that you highlighted with the blue arrows has one end attached to a 4R7 (to ground) and its other end attached to the current transformer (also to ground) - so you will measure very little resistance across it in circuit.

[jaunty]

  I see you have removed one of the orange tantalums in the path of the burnt resistor:  I is this one  shorted?

Sorry, I'm still trying to figure out what the burnt resistor does.  It almost appears to be shutting down the regulator via one of those two transistors next to the tantalum you removed by taking away the gate drive, but that's just a half-baked idea.  Since there's a 35v tantalum (the one  of the two near the edge of the board by the adjustment trimmer) in line with it, it shouldn't have more than 35v applied to it.  That should limit it to being 470, 47 or 4.7 ohms or else it shouldn't have burned out.  But that's assuming something very bad didn't come up to it through the BUZ10's gate.  There's no continuity between the gate (connected to that jumper right in front of the BUZ10) and either of its other pins?

Hi Paul - yes the missing tant is the one i removed - just to double check it - but I got zero ohms across it's terminals so it seemed PRETTY likely to be one of the pieces of evidence. And indeed it measured short out of circuit too.

Yes well - i assumed actually that the current limiting on the reg chip failed (I measured short between the current limiting pin and ground - if that's what that means) so - somehow the resistor (and the rest of the circuit is being protected by the current limiting)  - but i guess (if that's indeed correct) there's a deeper reason why the current limiter failed - I think the original CAUSE is that one of the tantalum bypass caps on one of the boards (of which there are many) shorted - and something in the PS gave way (maybe the BUZ 10)

[jaunty]

  I see you have removed one of the orange tantalums in the path of the burnt resistor:  I is this one  shorted?

Sorry, I'm still trying to figure out what the burnt resistor does.  It almost appears to be shutting down the regulator via one of those two transistors next to the tantalum you removed by taking away the gate drive, but that's just a half-baked idea.  Since there's a 35v tantalum (the one  of the two near the edge of the board by the adjustment trimmer) in line with it, it shouldn't have more than 35v applied to it.  That should limit it to being 470, 47 or 4.7 ohms or else it shouldn't have burned out.  But that's assuming something very bad didn't come up to it through the BUZ10's gate.  There's no continuity between the gate (connected to that jumper right in front of the BUZ10) and either of its other pins?

Hi Paul - yes the missing tant is the one i removed - just to double check it - but I got zero ohms across it's terminals so it seemed PRETTY likely to be one of the pieces of evidence. And indeed it measured short out of circuit too.

Yes well - i assumed actually that the current limiting on the reg chip failed (I measured short between the current limiting pin and ground - if that's what that means) so - somehow the resistor (and the rest of the circuit is being protected by the current limiting)  - but i guess (if that's indeed correct) there's a deeper reason why the current limiter failed - I think the original CAUSE is that one of the tantalum bypass caps on one of the boards (of which there are many) shorted - and something in the PS gave way (maybe the BUZ 10)

ah okay - maybe i'll pop the diode too and re-measure.

Hey thanks for your efforts - that's 'damned christian of you' as a friend used to say, amusingly ... i guess i'll print that out later and see if i can verify or add any detail etc

[Andy Watson]

(I measured short between the current limiting pin and ground - if that's what that means)

Do you mean between pin 11 and ground ? I would expect to see 4.7\$\Omega\$. The other end of that diode goes through the coil of the current transformer - to ground - so I would expect to measure, in engineering terms, sod-all resistance to ground.

I would replace the offending tantalum and the 47\$\Omega\$  and test it - if possible with a dummy load for the +5V line. Change as little as possible to get it to a functional state - the worse that could happen is that you fry another resistor - and you did purchase a few spares ... didn't you?

[jaunty]

pin 11 and ground yes - i got a dead short - rather than 4.7 ohms.

ok point taken - though I'm a bit hesitant to fry anything else - it's psychologically easier for me to just replace as much as I can when i'm 'in there' so I thought I would just replace whatever might be suspect (I suppose such a 'shotgun' approach won't really help my skill building though ... what do you think about the mosfet? replace it?

i guess i could fire it up with a current limiter (light bulb) in place too? or no?