Hello everyone !

I am trying to repair a desk lamp for a friend, it is supposed to light up 2 CCFL gradually then stays on (from bottom to top, like an old thermometer as if temperature was rising)
The behaviour is a bit erratic : it lights up gradually nearly to the top, lower a bit, sometimes ramp up again then doesn't light up at all.

Here is a picture of the circuit with some references :

Things I have done so far :
I have replaced the 1KV, 220uF and 100nF caps : no change in behaviour
I have replaced the S8050 transistor : no change in behaviour
(I don't have replacements for D882 and D1616, Checked them with a components tester they seem fine)
I have checked the resistors, they are good
The inductor has continuity
I have checked all the traces : there is continuity where it should
I have checked all solder joints

This leads me to 2 potential culprits : the transformer or the DIP8 component. As the transformer looks good and those rarely fails (as I've read here and there).
The DIP8 component has no reference on the top, only some markings underneath. I tried to search, it doesn't return anything, sadly.


What kind of component is it, a controller? Could it be the problem or am I missing something else?
I just have some basic knowledge, any help would be greatly appreciated !

 

Hi,
the transformer is under suspection.The high voltage winding sometimes breaks. And the brown capacitor close to the trafo.

Desolder red wire (+5V IN) and connect it to the bottom pin of inductor L102, power the PCB again. If behavior is the same, the upper part of the PCB is defective

 

Hi,
the transformer is under suspection.The high voltage winding sometimes breaks. And the brown capacitor close to the trafo.

Desolder red wire (+5V IN) and connect it to the bottom pin of inductor L102, power the PCB again. If behavior is the same, the upper part of the PCB is defective

Thank you for the reply, Pyrex.

I've tested the capacitor : 90.76nF (for a supposed 100nF), so it's good.
Other test I did was to check for continuity between the transformer pins:

I also tried you suggestion : connecting the 5v pin directly to the bottom inductor pin : same behavior.

As I can see (correct me if I'm wrong) just after the inductor is the transformer.
Does that mean, as you've supposed, that the transformer is dead ?

I don't think finding a replacement will be possible as pin numbers/disposition vary greatly and I don't have any info on the internal windings, nor a reference number anywhere except this generic one on the PCB ( LH209 ?):


Is there something to do ?

 

It seems, the transformer is bad.
Inter-winding transformer failure is possible. In most cases, such transformers are manufactured with a sectioned bobbin,
the secondary winding is divided into several sections.Those transformers are more resistant to inter-winding break.
As i understand, you do not have another lamp to compare the resistance of the secondary winding to be more sure?

By the way, check the resistance between the upper outermost pins of the transformer, not those you marked in short bend.

Adjacent pins are shorted on the PCB, no reason to check them.

If the trafo is faulty, the only remedy is to rewind it. It's not a simple procedure having in mind the secondary should withstand voltage of about 1kV. Several layers of insulation in the secondary winding are necessary

 

Thanks again for the reply and following, Pyrex.

Sadly, I only have this one lamp. I have nothing else to compare the values.

I have checked the resistance (transfo still soldered on PCB) as you asked, I also did some other measurements if it could provide any information.


If you confirm the transformer is dead, I will not try to rewind it. It is way above my skills and knowledge.

I have made some other research and I did find a 5V inverter for CCFL with the same kind of transformer ( same number and disposition of pins).
I don't think I could use it as a direct replacement though ?





Now for the second option, if I get this new 5v inverter, what would be the best way to have the CCFL light up gradually like they did before?

 

It's seems to me, pinout is the same. Try to desolder old one and put in the new, if trafo size is the same.
If the transformer size will be different, solder short wires to the pins to fit the board.

The bottom part of the PCB does the slow voltage ramping to the inverter. It seems to me, it's Baxandall inverter, on both PCB's the same Baxandall inverter is made, only two transistors are different ( ignore that).

On the other hand, rewinding a transformer is not an insurmountable task.This will require a machine for winding the secondary winding. The primary can be wound by hand. To disassemble the transformer, place it in a closed container with a little acetone for a day. The wire diameters may differ slightly from the original, it will still work

 

Again, thank you very much for the reply and guidance, Pyrex !
I've placed an order for 3 inverters (just in case and to study a bit), I will try to make a direct swap when I recieve it.
Also, I will try to check resistance between the pins to have a comparison with the broken, original one.
I will make an update once they are in my possession ( in 2 weeks).

I was kind of sure the ramp up was being done on the bottom part of the PCB on 5V, with transistors slowly feeding caps (If I understand the circuit correctly).

Thanks for the informations about the transformer, I may try to do it someday for fun, but I have to study this field a bit beforehand.
Right now I don't feel comfortable handeling this kind of thing.
I'll make an update when the new inverters are here, thanks again !

 

By the way, are you sure the lamps are OK? The lamps to be checked first of all due to the limited lifespan

 

The CCFL are good, I wanted to edit my first post to add this information but I don't have access to this option yet.
When making my 2nd post I forgot about it.

I have known working CCFL that are the same size (25cm), I've put them in the circuit high voltage output (in place of the original ones) and the behavior is exactly the same.
I also hooked the original ones to a known working ( 12v input ) inverter I have laying around (just to be sure), and they light up normally, just without the ramping up, as expected.