Phoenix ZX475Ti question

rickcl

Member
Paid Member
2003-03-12 6:01 am
California
Maybe one for Onemoreamp. This amp had a power supply meltdown. All the PS fets, gate resistors (fuseable), and 1 SF302A replaced. Upon current limited power up the resistor in the pic gets hot. I would imagine its a current limit to the driver circuits? All transistors measured fine. Thank you.
 

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Ok first off power supplies don't melt down by themselves so which channel or channels are blown ?
99% of the time a supply burns up because one or more of the channels popped. very rarely by themselves do they fail, very rarely...

The 470 ohm 1/2 watt resistor in question ties the raw 40 vdc rail supply to the driver supply of 50 vdc. There are two of these resistors and one in each polarity rail setup. So the dual Neg rail has one and the dual Pos rail has one.

Clip your ground probe to the center tap of secondary of the toroid and a voltage measurement on each side of the burnt resistor will tell where each rail is and what polarity it is.
This problem will not go away until the other defective issues are remedied...The burnt resistor is a symptom of another bigger issue elsewhere, as there is very little else near by that could cause such a fault to occur on its own, I would check all four channels for high DC offset to find the possibly defective channel.
But just to be safe meter all the MUR120 diodes nearby looking for a short...Something is pulling high current levels thru that little 1/2 watt resistor...hope this helps...:)