can anyone explain how this protect circuit operates ?

[petehall347]

its in a samick smp-4000 ..its driving me nuts .. 3 resistors lifted and q117 still affects drivers bases when i freeze it .. cant get it bias .. positive to negative swings with heat and cold ... am missing something obvious here .i am going to pull r159 to see what happens ..if that dont work ic101 is going . ... its very odd as good channel only works with bad channel removed although bias is still ok on that channel ... https://elektrotanya.com/samick_smp-.../download.html

[Agent24]

Re: can anyone explain how this protect circuit operates ?

Does Q117 actually have anything to do with bias? Or is it just for driving the protection LED?

[petehall347]

Re: can anyone explain how this protect circuit operates ?

Does Q117 actually have anything to do with bias? Or is it just for driving the protection LED?

thats what i am trying to figure out .. it certainty affects those driver base voltages when i freeze it

[Agent24]

Re: can anyone explain how this protect circuit operates ?

Well... from what I can make out from it:

The output from the protection IC, IC101 provides feedback to Q120 via R108(?), and by extension, Q101/Q102. That seems logical enough. Normally that IC101 (TA7317P) is designed to switch a speaker output relay, but they seem to be using it to switch off Q120 and thus cut power to Q101/Q102 in the event of a fault instead. So it would seem that this would effectively disconnect the audio input signal to the amplifier, and that's about it.

That output from IC101 also drives Q117 through the voltage divider of R159 and R160, lighting the protection LED (LED 2) if there is a fault.

I'm thinking that C123 is probably used to prevent LED 2 from flickering, and Q118 simply used to provide a faster way to discharge C123, but keeping the discharge current low enough that in an actual fault condition, LED 2 will still be able to light. Seems a bit overcomplicated but that's all I can think of as to its purpose.

The next stage of that circuit with C124, Q119, I think may only come into play at switch on\off, as it has a larger capacitor, and probably again, to prevent some spurious LED 2 output at power up or power down.


I really see no way that Q117 could affect the actual amplifier stage, unless you're accidentally freezing\heating IC101 and\or one of its support components.

[petehall347]

Re: can anyone explain how this protect circuit operates ?

at present i have r142 and r155 lifted and cooling and heating isnt changing voltages at the drivers bases like it was before .so at least its now stable but no bias drive .. i went over all resistors in circuit and transistors and nothing jumped out as yet .

[petehall347]

Re: can anyone explain how this protect circuit operates ?

case closed for now .. found broken trace by q107 collector .previous work is shoddy .. bias will turn up now . still work to do yet but it looks like its going to work ..