more than likely the transistor oscillator has packed up, or on board fuse blown.

 

more than likely the transistor oscillator has packed up, or on board fuse blown.

there are 2 transistors both 13005N...which one to check how?

 

Either one of them can become defective, sometimes both. They are connected like push-pull to the transformer. Also check the high voltage capacitors for open or changed value.

Allen

 

Either one of them can become defective, sometimes both. They are connected like push-pull to the transformer. Also check the high voltage capacitors for open or changed value.

Allen

One thing that could be missed - they sometimes have a diac relaxation oscillator for startup, a high value resistor charges a small capacitor up to the diac breakover voltage, the discharge current is injected into the lower transistor's base to get it going, there is also usually a clamp diode from the inverter's output node, every time the lower transistor saturates it discharges the diac's capacitor to prevent it firing.

The diac and its charging resistor are up there on the list of suspects.

 

Replace the two 1uF electro caps.

Lift one leg of the two 470k resistors (yel/purp/yel) and then test with an ohmmeter.

If those two tests don't fix it, lift one leg of every resistor and diode and check the diodes with multipmeter on "diode test" and test the resistor values.

 

Ideally you need to copy the circuit out, and we can see how it works. Is it 12v AC in?

some circuits here to compare

 

Ideally you need to copy the circuit out, and we can see how it works. Is it 12v AC in?

some circuits here to compare

That looks suspiciously like "13005" on the screen print under the transistors - if that's short for MJE13005, they're rated for 400V.

 

thanks guys for all replies checked according ur suggessions....now it working ...

 

Can you say what the fault was?

 

Such gratitude. We helped him fixed his problem and he wouldn't even say what was the problem and how he fixed it.

 

Can you say what the fault was?

ok...1st picked every components resistors diodes and transistor from board (except transformer,toroid,yellow cap)... checked its value ... resoldered...only 1 resistor showed different value 470k...and replaced fuse wire ...that's all...it worked...

Such gratitude. We helped him fixed his problem and he wouldn't even say what was the problem and how he fixed it.

i replied ...i did it according to everyone suggestions...i forget to reply actual problem since the actual problem may be that 470k and fuse..

thanks for all replies...

 

@ RB , You were so close! If only you would have told OP which 470k was bad.

@ian may have nailed it too!

I for one never suspect resistors first, I'm learning though!

Bad timing on these threads.

Couple months ago I went through about 20 unpotted T8 ballasts. My brother did a routine replacement after huge failure problems. Picked out two or three that were good and dumped the rest. I'm betting all would have been easy fixes,

 

I'm betting all would have been easy fixes,

i was sure it will be simple fixes..since toroid and transformer will be never (90%) bad ...rest components are cheapless...check and if bad replace...if that 470k is ok then i should go for capacitor replace since i don't know to check caps....all ideas got from aac members.. thanks

 

@ RB , You were so close! If only you would have told OP which 470k was bad.
...

Yeah I'm getting slack in my old age.

...
I for one never suspect resistors first, I'm learning though!
...

Those yellow band resistors 330k/390k/470k in high voltage circuits like SMPS and inverters are generally used for high voltage feedback, and a good clue is that there are two of them in series (to cover the fact resistors are only good for 200v max, because they have will HV across them in use).

Even with two in series they still commonly get HV failure to open-circuit (or high resistance).

 

Hello,

When you look at the construction of a carbon film resistor, you can imagine that it is not really capable of handling high voltage:

This image comes from this doctronics page:
http://www.doctronics.co.uk/resistor.htm

Bertus

 

One needs to check the flourscent tubetoo. An open filament at either end will also make the setup not to function.

Ramesh

 

Hello,

When you look at the construction of a carbon film resistor, you can imagine that it is not really capable of handling high voltage:

...

Yeah, metal film are similar spiral construction too.

I'm still not sure why they go open circuit though as the HV fault? Maybe they arc in one spot, then get a "hot spot" which soon fuses out to open circuit?

Metal film resistors usually have one very thin spot in the spiral, where they are laser-trimmed. I know when they fail from over-current they always fail at that spot, but haven't checked with HV fail ones.