Repairing a T8 lighting electronic ballast

[MrElectric]

One of my 2x36W T8 florescent lighting fixtures started flickering a few weeks ago. First just a little now and then, but after a while it got more and more annoying, with an almost stroboscopic effect. I switched off the light and removed the tubes. Then I've tried replacing one or both, in either position. But it doesn't matter which tube goes where, the flickering starts immediately.

I read somewhere that bad capacitors are often the cause of flickering. I opened the ballast and desoldered all capacitors. True enough, one of them was way off in capacitance, and had high dissipation as well. I ordered a new capacitor and replaced the failed one.

I tried it out, and no flicker! But after being turned on for half an hour or so, I noticed the occasional flickering. Not constant like it was before, but flickering 10 seconds or so, and then steady for some minutes.

It seems like this happens when things get warmed up.
I've been using it like this for a week.

Today I decided to give the troubleshooting another go. I opened it up again and could smell that something had been too hot. I found that a diode was burnt now. It still measured ok, but it was clearly burnt.

I'll replace this once I've bought some FR107 diodes.

I have measured all resistors, and all values seem ok. No shorts. I've measured all diodes, and all seem ok. There's one diac that I can't verify (except that there are no shorts). I've checked the two power transistors. I've desoldered them as well as many capacitors. I can find no failed components apart from the diode.

How should I go about to find the problem?

I have started drawing a circuit diagram, but it's not easy as there are lots and lots of components. I've been drawing for a couple of hours, but haven't finished yet.

I think I'll try measuring voltages when the power is on to see if I can find any problems. Unfortunately I don't have a clear understanding how an electronic ballast works..

I'm attaching some photos.

[SilverSolder]

The purpose of the ballast is to limit the current through the tube once it has "fired".

I have a 1960's all steel Dazor floating fluorescent lamp over my desk:



It was a $10 garage sale item.  Been using it for about 10 years, then I had to replace the wires in it - they got chafed and shorted out.  Now it is probably good for another 50 years of service!

The ballasts in this lamp are simply inductors in series with the fluorescent lights.  This allows the full mains voltage to be applied during startup, yet limits the current once they are lit.

[Armadillo]

The ends of the tube has been darkened.
Replace the fluorescence tube with new tubes.

[james_s]

None of that really applies to the high frequency electronic ballasts used by all T8 tubes over 20W in the US. I doubt a bad tube has caused a diode to fail, I would more likely suspect dried out electrolytic capacitors. It could be a fun project to repair it, but keep in mind you can buy a brand new T8 ballast for around $15 at any decent hardware store or home center, at least you can here, I would assume they've available elsewhere in the world too.

[MrElectric]

Probably not worth the effort economically. But I find it satisfying to be able to fix something, instead of throwing it away. I also thought this must be caused by a capacitor that is turning into a resistor (or a short).

[Nusa]

We've gotten to the point that when the ballast goes, it's usually more cost-effective to apply the value of the ballast and tubes toward a replacement LED fixture. Unless you've got something in a artistic form that's hard to replace.

[edavid]

We've gotten to the point that when the ballast goes, it's usually more cost-effective to apply the value of the ballast and tubes toward a replacement LED fixture. Unless you've got something in a artistic form that's hard to replace.

When I faced this, I spent $11 on a new ballast and $15 for 4 LED "tubes".  I don't think I could have bought an equivalent LED fixture for $26.

[SilverSolder]

It is a pleasure to keep old stuff working instead of participating in the throw-away society more than we have to.

I stuck with the fluorescents, for the same reason people like Nixie tubes and neon signs...   they are kind of unusual and therefore cool!

The F15T8 daylight fluorescent tubes in my old lamp are 15W each.  The inductive ballasts are 50 - 60 years old and will probably outlive me...   

[Nusa]

We've gotten to the point that when the ballast goes, it's usually more cost-effective to apply the value of the ballast and tubes toward a replacement LED fixture. Unless you've got something in a artistic form that's hard to replace.

When I faced this, I spent $11 on a new ballast and $15 for 4 LED "tubes".  I don't think I could have bought an equivalent LED fixture for $26.

I suppose it depends on the fixture in question. A couple months ago I bought a 4-foot LED shop light for $19 from Costco, which I've been very happy with. That was a store sale price; the current price as of 3 days ago is $29. I'm going to grab a couple more next time they have a sale....the remaining flourescents in my shop are dim by comparison.

[schmitt trigger]

I also find satisfaction in repairing older products instead of throwing them away.

Is the ballast dimable?

[james_s]

Me too, so many times I've seen people replace really nice well made vintage items with new cheap disposable junk just on the automatic assumption that it's more cost effective to replace it. It bothers me how incredibly wasteful society has become, most goods are priced far below their true cost as we convert useful resources to landfill at ever increasing rates. Almost everything is a little bit lighter, thinner, flimsier one year after another as cost engineering shaves off pennies. Eventually you can't buy quality at any price because everyone has to either start making cheap junk to compete with everyone else.

[MrElectric]

I desoldered all capacitors and tested them with a leakage tester. I found that two, small, electrolytic capacitor tested as leaking. Not excessively, but still leaking.

I replaced these with panasonic capacitors yesterday, and connected all cables without closing the lid, so that I could check the temperatures. Had it running for 30 minutes yesterday and could detect no excessive heat. There was no flickering at all.

I will do this again another day, and let it run for a couple of hours. If there's no flickering, and no excessive heat, I think I can conclude that the two capacitors were the cause of all problems (not sure about the one I replaced previously - but the capacitance meter told me that one was bad as well).

I also replaced the diode that was burned.

[schmitt trigger]

Do you mean C51, C52?

[MrElectric]

Do you mean C51, C52?

Yes, that's correct. :-)

[jdragoset]

Here is a patent from UF days.