inverters with C5707 benq hp 1940
I just found out about badcaps and am amazed at the amount of information here. I have been going through all the old posts and seeing how the experiences of other tech compares with mine.
One of the subjects I have seen discussed a lot are the inverters based on the C5707. Often these are found in 4 lamp combo inverter / power supplies that have a simple tank circuit with 2 C5707s driving 2 transformers.
Most often these boards are made by Benq and are found in Dell and HP 17",18" and 19" monitors.
My experience has been that one C5707 blows and shorts the power for the inverter. Then I replace the pair of C5707s and recap the whole thing and maybe half the time the same 2 transistors blow out after a few hours of use.
Some people have suggested this indicates a bad transformer or bad solder joints on the transformer. Almost always the solder joints look fine to me, but I flux them and touch them up anyway.
As far as testing a transformer is concerned, the only way I know is to see if the lamp attached to the transformer is nice and bright and fires up quickly. They always do.
I just pulled a handful of transformers and the primaries always measure dead short (as far as my meter can tell) and the secondaries measure 180 ohms or so.
Does anyone know a better way to test transformers?
Are there any other ideas out there of why this happens?
The attached pic is the power/inverter out of an HP 1940. One thing that might be a clue: Look at the brown discoloration on the circuit board. This is much darker and larger on the side that blew out. This is the area of the circuit board that has resistors (4 470 ohm) that are in series with the primaries.
Any thoughts, fellow geniuses? (genii?)
-Michael
One of the subjects I have seen discussed a lot are the inverters based on the C5707. Often these are found in 4 lamp combo inverter / power supplies that have a simple tank circuit with 2 C5707s driving 2 transformers.
Most often these boards are made by Benq and are found in Dell and HP 17",18" and 19" monitors.
My experience has been that one C5707 blows and shorts the power for the inverter. Then I replace the pair of C5707s and recap the whole thing and maybe half the time the same 2 transistors blow out after a few hours of use.
Some people have suggested this indicates a bad transformer or bad solder joints on the transformer. Almost always the solder joints look fine to me, but I flux them and touch them up anyway.
As far as testing a transformer is concerned, the only way I know is to see if the lamp attached to the transformer is nice and bright and fires up quickly. They always do.
I just pulled a handful of transformers and the primaries always measure dead short (as far as my meter can tell) and the secondaries measure 180 ohms or so.
Does anyone know a better way to test transformers?
Are there any other ideas out there of why this happens?
The attached pic is the power/inverter out of an HP 1940. One thing that might be a clue: Look at the brown discoloration on the circuit board. This is much darker and larger on the side that blew out. This is the area of the circuit board that has resistors (4 470 ohm) that are in series with the primaries.
Any thoughts, fellow geniuses? (genii?)
-Michael
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