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Old 13th May 2003, 06:05 PM   #1
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Default Help! resistors burnt after capacitors replacement

Hi All,

I helped a friend to replace or upgrade some electrolytic capacitors on one of the circuit boards of his sony projector. Upon powering up, it tripped its power supply, burnt off 2 resistors.

All orginal caps were normal blue rubycon 85deg stuff. Value in the picture shows the orginal values.

I had replaced C508 and C509 to 47uF 250V, 105deg low ESR. And C527, C502 and C506 to 22uF 63V, again 105deg low ESR.
After powering up, resistors R568 & R570 which are parallelled burnt.

There are 6 of these circuits on the same board, only one have the resistors burnt.

I am clueless on pinpionting the problem. Can anyone please help me out?

Thank you very much
Yeo
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Old 13th May 2003, 06:10 PM   #2
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Old 13th May 2003, 06:29 PM   #3
tiroth is offline tiroth  United States
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Even if one of those caps was reversed or shorted I don't see why you would have lost the emitter resistors...in the absence of someone more experienced posting I would look to an inadvertant error made in the upgrade (after checking the caps for orientation and to see if they are shorted).
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Old 13th May 2003, 07:11 PM   #4
Netlist is offline Netlist  Belgium
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Does Q501 stills measures OK?
Is there DC at the Output?
/Hugo
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Old 13th May 2003, 08:17 PM   #5
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Quote:
Originally posted by tiroth
Even if one of those caps was reversed or shorted I don't see why you would have lost the emitter resistors...in the absence of someone more experienced posting I would look to an inadvertant error made in the upgrade (after checking the caps for orientation and to see if they are shorted).
I confirm the orientaton of the capacitors are correct.


Quote:
Originally posted by Netlist
Does Q501 stills measures OK?
Is there DC at the Output?
/Hugo

I only have the board without the power supply. How do I actually check if Q501 is ok? Is it possible to do it with a multimeter? I only have this measuring equipment with me at this moment.

Thanks
Yeo
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Old 13th May 2003, 08:56 PM   #6
Netlist is offline Netlist  Belgium
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If the resistors burned directly after power was applied, it means a big current was drawing.
If for some reason Q501 is short circuit between collector and emitter, the full current flows trough the burned R's.

Try this link to learn how to measure transistors:
http://www.eatel.net/~amptech/elecdisc/ampfail.htm

/Hugo
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Old 15th May 2003, 07:08 PM   #7
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Do not underestimate the transient that can flow when you power on a low esr capacitor.

Consumer electronics are designed to get the last cent out of it. So by doubling the capacity and going to low esr you may have quadrupled the transient. Maybe not in peak current, but in duration. Having done measurements on a similar problem, I measured currents up to 30-40 Amps, blowing even surge rated resistors.
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Old 15th May 2003, 10:09 PM   #8
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Netlist....that was a good link that you gave to that website...well done mate

Cheers!!The DIRT®
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