repair desktop fluorescent lamp

[senderj]

I have a National desktop fluorescent lamp that failed to work. Every time I press the power button, it blink once and doesn't light up. When I then press the button again (intended to put it to off state), it blink once again after 8 seconds. I trace the PCB and found that it is almost the same as this one.

except that the values of the components are different. The C3 is 1.25kV 5.1nF while the C6 is 400V 470nF. I suspect that it is the C3 that failed. Photo here

I don't have exact replacement, but I have two 630V 10nF

and thinking of connecting them in series to replace C3. I think the calculation is correct, but I don't know if the type of capacitor and the current loading match as replacement or not. The 10nF is much smaller in size. Can somebody please help me on this.

 

[davenn]

Hi welcome to EP

I wouldn't worry about component value variations ...
they will change from model to model and manufacturer to manufacturer

why do you think it has failed .... looks perfectly OK
when caps like that with those sort of voltages fail, its usually spectacular

I suspect your caps are OK and your fault is elsewhere

Dave

 

[shrtrnd]

The way my desk lamp works, is that I have to press and hold the push-button for a few seconds. (Then release it) The voltage goes to the ballast that fires the lamp, and that takes a couple seconds.
Is this a new lamp you're unfamiliar with? Maybe that's the issue.
My second suggestion, is to check the switch itself if test #1 above isn't the problem.
I had two lamps fail, and it was the switch itself both times.

 

[Tha fios agaibh]

Before I messed with anything, I would relamp it. Could be as simple as that.

 

[senderj]

Thanks for all the hleps.

@Tha fios agaibh, The tube should be fine. An old one failed then I bought this new tube, tested at the shop before I pay, so it should be fine.

@shrtrnd, I've tested with a wire to by-pass the switch, still the same problem.

@davenn, the tube blink once at startup but didn't sustain, so it should be the resonant circuit that failed, i.e. C3, C6, L1, TR1(1-4). I also read this article http://www.pavouk.org/hw/lamp/en_index.html which mentioned the most likely failure is C3. Don't have kowledge to judge so plan to give it a trial.

 

[davenn]

That circuit you have shown above is typical of the circuitry that is in the base of the energy saver fluoro tubes
that is ... its an integral part of the replacement tube that you buy

There's little point in repairing them, the replacement lamp is a few $$
and pulling it apart etc is risking problems

 

[Tha fios agaibh]

I'd stick a CR121 ballast in it and call it a day. About $10

 

[davenn]

I'd stick a CR121 ballast in it and call it a day. About $10

a complete replacement globe is only $5 LOL

 

[Tha fios agaibh]

a complete replacement globe is only $5 LOL

I know, I'd pitch it myself.
Perhaps it has sentimental value to the op?

 

[davenn]

a globe like this ....



having sentimental value ??
surely you jest

 

[Tha fios agaibh]

Thats a "desktop" lamp with a power button?
Lol

 

[davenn]

Thats a "desktop" lamp with a power button?
Lol

NO but it is what he is pulling apart and trying to repair
ALL that circuitry shown above in post #1 is in the base of the globes as you see them above
it ISNT in the actual lamp stand .... look at the photo in the link he provided in post #5

That's why I'm telling him its a waste of time and he's risking doing other damage ....
spend around $5 and buy a new lamp and screw it into the lamp stand

Dave

 

[Tha fios agaibh]

Thanks Dave. Sometimes taking a loss to fix something is worth the investment by what you learn in the process.
But yes, In this case, I agree completely.

 

[senderj]

It's not the $5 type as shown by davenn. It's a desktop lamp, though it is not that expensive (about $70 more than 15 years ago). But money is not the matter, learning is my reason to repair.

I finally spotted the problem. It's the C4. Since replacing C3 wit 2 cap in series sound dangerous. I tried the easy ones first. I got a 100uF 400V on hand so I guess try replace C4 first is much less risky and I hit the spot. The old one is a 22uF 400V with output 293V, which I mistaken as normal. After replacing it the output is 303V. I think the old one is low quality and aged and dried such that the internal resistance drop and hence the output is too low for the rest of the circuit to work. With 303V now it works as new.

Thank you for all the helps here.