Kortex 25inch ktm-f voltage regulator.

[Fugus]

Hi all.

Can anybody tell me why there's a resister on this voltage regulator.

Is it a brightness mod? At ic601

[Jomac]

A lot of people ask me this,

It's used as an over voltage clamp, an add on we did here in Australia for any chassis that used the STR-S5741 after there were a stack of failures caused by the B+ going way to high.
When C610 ( 10uf 160v ) and C609 ( 33uf 200v ) start to fail it causes the output frequency of the chopper circuit primary winding ( for the switch mode transformer ) to increase meaning the output voltage would increase proportionately and could go up as high as 180vdc.
Pin 8 is an unused "ref" pin so we put a 120K resistor from here to the positive side of C610 Pin2 , this monitors the frequency and stops it exceeding 130vdc.

Obviously when the caps are new there is no problem for a little while but this even changed as the operating temperature changed, so that one little resistor saved a huge amount of these being destroyed.
 

[Fugus]

Thank you jomac.