TL494 output frequency dropping

[Leolabs]

Repairing a Zapco i-2100,which all the IRFZ44N at DC-DC converter were burned.

Already replaced them,measuring the gate drive,frequency is dropping start from powering up,around 28kHz and decreasing even lightly loaded.

Anywhere to recheck again?Capacitors?

 

[ultra]

the last time such thing happened to me, it was the timing capacitor (Ct) of the pwm chip.

 

[Leolabs]

the last time such thing happened to me, it was the timing capacitor (Ct) of the pwm chip.

After replace the Ct(6.8nF for this case),the frequency stay around 35kHz for some times,then decreasing again.....

 

[ultra]

What about the timing resistor (Rt), is it within tolerance?

6.8nF seems to be a high value...what is the value of the Rt resistor?

I think it should be around 10K...

 

[Leolabs]

What about the timing resistor (Rt), is it within tolerance?

6.8nF seems to be a high value...what is the value of the Rt resistor?

I think it should be around 10K...

Fixed,it's the TL494 chip problem....

 

[evianamp2011]

Did you notice/check it to see if it was getting hot (beyond normal temperatures) before replacing it?

 

[Leolabs]

Did you notice/check it to see if it was getting hot (beyond normal temperatures) before replacing it?

Which component ?

 

[evianamp2011]

Which component ?

The IC 494.

 

[Leolabs]

The IC 494.

Never,that's how it caught me out...

 

[evianamp2011]

So it never got hot? Am confused by your last post.

 

[Leolabs]

So it never got hot? Am confused by your last post.

yes,it NEVER get hot

 

[evianamp2011]

Ok got'cha.