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Old 08-04-2010, 11:33 PM   #269
chdala
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Join Date: Aug 2010
Posts: 1
Default Re: Fix for Benq FP737s 17" (Q7T3)

First, thanks to all the previous contributors. The information in this thread lead me to the cause far faster than I would have if I had started from scratch and probably stopped me from making more silicon smoke.

I agree with PlainBill. Open circuits caused by dry joints in the transformers kills the 2SC5707s. They are probably killed because their maximum power dissipation is exceeded. A current limiter will probably not prevent this from occurring.

Explanation....(from an EE of 30 years)

Dry joints (open circuits) around the transformer primary windings cause the oscillation to stop. This causes both Q759 and Q760 to be held 'on' by by R768 and R769 respectively. 2SC5707s are high gain transistors so the collector current will be high, at least 2 amps for a short period of time.

Holding Q759 and Q760 'on' for more than a few milliseconds will saturate the collector windings and cause the entire power supply voltage to be present between emitter and collector (Vce). This 20V or more Vce at 2 amps is a power dissipation of 40 watts in a transistor that is rated to 15 watts (on a perfect heatsink) so they die, usually by going short.

The power dissipation is much lower when the oscillator is running because Vce (on) is almost zero when the transistor is on and the collector current is zero when it is off.

Once the 2SC5707s have died the TL1415 IC detects that the output voltage is low and increases the duty cycle to the FU9024N which causes it to overheat as well. There is then a race, either the FU9024 goes short and blows the fuse or the fuse blows saving the FU9024.

It is not possible to easily limit the current through the 2SC5707s to values that will allow them to operate normally and also prevents them from blowing if the oscillation stops. It may be possible to modify the base drive so that they are not held on permanently if the oscillator stops, but it is far easier to re-solder the transformers.

Chdala
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