Crown XTI 2000 fries RT6 at turn on

[Bro Lurcha]

Immediately after the AC Input switch there is RT6. I bought this amp not working cheap, and they told me that it just stopped one day. After replacing burnt out RT6 turn on showed it heating instantly at turn on. I have replaced D24 , and C57 thru to 60. with no success so far. I have gone through Crown's destructive at turn on guide but I am thinking that there would be other parts of the amp that would blow according to that guide. The fuse on mains doesn't blow ever. What has me tossed is for such a current draw through RT6 I would expect something else to blow instead. . I am hoping that someone may be able to point me in the right direction with some pointers. Oh and also with C57 through to C60 removed I can turn on and RT6 survives. Thanks

 

[sajti]

Check if the relays (K1+K2) are swith on few seconds after the start! If not, this is the reason of the overheating.

Sajti

 

[Bro Lurcha]

I think the penny just dropped. RT6 is actually meant to be sacrificial ? ie if there is something wrong elsewhere it cooks. Pity the fuse just doesn't blow. If that's the case I'll look further into the power up circuitry. Bit hard to test Voltage points if RT6 is left too long. Flying blind trying to test. Suppose I'll get there.

 

[JonSnell Electronic]

RT6 limits the turn on current and is an NTC type resistor. The relay should pull in when the supply rails start to rise. If the sensor is not working, the NTC will take the strain for too long and go open circuit.
Does the SMPSU work without a load? If not it could be the power supply is faulty.

 

[Bro Lurcha]

I think now the light has come on with what is happening as the power supply runs up, there is something happening in the Rail Sense, and the Kick Start circuit. I'll do some more hunting around in those regions and post my findings.
Can't believe that it's been staring me in the face for so long and after reading just the right thing the whole schematic on that page makes more sense 😛

 

[Amraks]

You ever get to the bottom of this have a AMP doing the same thing.
I replaced c197.
Tested voltage on u16 VCC looks its being unstable.

 

[Amraks]

after spending some time going thought the soft_startup circuit, tombstoning resistors and diodes to test transistors, I decided to measure what voltages where at a1 and b1 well the voltages where present meaning the relays should be kicking in but you don't hear that.

I thought I had found a stuck on relay but turns out I had the switch on