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Old 20th May 2013, 12:51 PM   #1
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Exclamation TL494 output frequency dropping

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?
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Old 20th May 2013, 09:42 PM   #2
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the last time such thing happened to me, it was the timing capacitor (Ct) of the pwm chip.
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Old 22nd May 2013, 01:41 AM   #3
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Quote:
Originally Posted by ultra View Post
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.....
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Old 25th May 2013, 10:31 PM   #4
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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...

Last edited by ultra; 25th May 2013 at 10:37 PM.
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Old 26th May 2013, 05:38 AM   #5
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Quote:
Originally Posted by ultra View Post
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....
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Old 26th May 2013, 10:03 PM   #6
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Did you notice/check it to see if it was getting hot (beyond normal temperatures) before replacing it?
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Old 27th May 2013, 01:57 AM   #7
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Quote:
Originally Posted by jeanious2011 View Post
Did you notice/check it to see if it was getting hot (beyond normal temperatures) before replacing it?
Which component ?
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Old 27th May 2013, 11:27 AM   #8
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Quote:
Originally Posted by Leolabs View Post
Which component ?
The IC 494.
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Old 27th May 2013, 01:33 PM   #9
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Quote:
Originally Posted by jeanious2011 View Post
The IC 494.
Never,that's how it caught me out...
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Old 28th May 2013, 06:19 PM   #10
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So it never got hot? Am confused by your last post.
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