"Gareth Magennis" wrote in message ...



"N_Cook" wrote in message ...

On 01/11/2014 15:21, Gareth Magennis wrote:


"N_Cook" wrote in message ...

Surprisingly strong "electrical" smell around the vent ports.
Blown TOP244YN , 20 ohms across rectified mains cap.
Considering crack to its encapsulation is just observable, on one side
only and then under x30. Distorted top to the sillypad (TM) , what may
be a puncture hole at the non-rounded, but then not sharp, rear upper
edge.
Hang on a bit , although the tab holding bolt has an insulating washer,
the heatsink is isolated, anti-EMI ?. So if it did puncture , why
leading to failure?
Any known problems with these presumably fairly generic SMPS sub-units,
all else checks out (cold) ok. Mica will be going back in there.






Oops, I just remembered, my TOP chip wasn't "blown" as such, it was
overheating and shutting down.
I rang Peavey to try and quickly establish if they knew of anything that
might be overloading the chip, or was it just the chip.



Gareth.

This could be your over-temp drop-out failure mechanism.
Sitting quiescent for 1/2 hour and an IR thermometer to the TOP body 38
deg C. But if you point it to the rear it picks up 78 deg C of the 330R
dropper that is enclosed on all sides by the ps pcb , heatsink,chassis
and main board. Perhaps over hours and lack of through draft that
radiator heats up the heatsink and so TOP , enough to trigger temp protect.
Hopefully the holes I ground through the ps pcb and the staircasing of
the h/s vanes will preclude that.





My TOP chip was obviously overheating, you could tell immediately by putting
a finger on it.
It would shut down in 10 to 15 minutes.

The new chip went back in the same place on the same heatsink at the same
angle.
This one did not get finger hot.



Gareth.







Just to clarify, these symptoms were observed with the unit dismantled and
the PSU in free air, so I could put my finger on the chip/heatsink.


Gareth.