Inverter Repair... I could use some pointers from the experts.

This is a bit vague to me. Are you saying there are two CCFL tubes and the transformer has a seperate secondary and coupling cap for each lamp, and that one lamp worked for a while? If only one lamp lights that should tell you the inverter is working but there is a problem with either the relevent secondary winding (check the DC resistance matches the other) the continuity of the PCB (it looks very badly burnt) or the lamp itself. If the two lamps share a secondary winding then that narrows it down even further. Connect the other lamp to the 'working' output and see if it lights up.

The PCB is a mess, and carbonised PCB will have a significant amount of conductivity to a HV high impedance source. Consider cutting away all the offending bits of the PCB and if necessary soldering the capacitor straight to the transformer leg. It won't look pretty bit it's better than the ramifications of presenting HT across burned PCB! It's possible the heat generated during the burning has damaged the transformer.

It's proably not a short on the primary side, more likely HV leakage on the secondary side. The secondary of the inverter will knock out approaching IIRC around 90V or so into the right load, and several thousand volts off load. A relatively high resistance leakage on the secondary could easily draw enough extra supply current draw to pop the 500mA fuse.

Don't worry about that. Get it working then worry about aesthetics.

Dave

I've tried ohming from the fuse to just before the bad capacitor and it ohms

1347ohms on the bad side and 1341 ohms on the good side.. So it doesn't look like I have a short of any kind that I can see. Would this be a good assumption..

Sorry for posting so much but I really want to learn about fixing electronics. This inverter seems like a simple enough project. I just need some help.

I have an old 200khz oscillosocope that needs calibrating but it works. Would this be usefull at all for working on this? I wouldn't immagine it being usefull at all without some sort of wave generator.

- Mike

The original was probably a slow blow at probably 1 or 1.25 amp (but I'm guessing). The ccfl's are probably 4 watt bulbs, 2 of them, total draw 8 watts, inverter efficiency is around 75% which means the whole thing is drawing 10 watts when running correctly. With a 12 volt supply and you get just under an amp draw.

If you have a scope, see what kind of a trace you get on either side of C9. If the transformer is open on the secondary, you'll get a clean 1/2 wave on either side of C9. If it's working, you'll get the 1/2 wave but it will be misshapen and may have some harmonics on it. The voltage on C9 side is low since you are on the primary. The inverter will usually work for about 2 -

4 seconds with the bulbs unplugged. The square chip in the center is the controller and will shut the inverter down if it doesn't sense the bulbs light up. Here's a file that should give you a close approximation of your inverter: go to and take a look at ANP005

The other thing you can do is to check continuity on the transformer. The primary windings and the feedback winding are very low but the secondary winding should be 100 to 400 ohms if I recall correctly.

That charred board may end up being your biggest liability since the voltage on that side of things normally runs around 600vac @ 60Khz or so. It might just be easier to find a cheap monitor with a cracked LCD on eBay and take the boards out of it.

Dave

First off thanks for the detailed reply!

Let me try to clear up the vagueness.. It has 2 transformers one on each end of the pcb and each has a coupling cap. It also has 2 500ma fuses. The fuse which goes to the non working side keeps blowing. I had both lamps powered up and working for a short while, but they were a bit dim.(may be ccfl tubes) Then I shut it off and powered it back on and the fuse blew on the "bad" side. I havn't been able to get the "bad" side to light again. It just blows the fuse imedately. I've ohmed from the fuse to the transformer's output and it is within 6 ohms of the working side.

I've cut away all the burnt PCB and soldered the cap directly to the lamp socket. I tried that and it still is blowing fuses so I guess thats not it.

The transformer might be the problem. It has a resistance of 668 ohms vs 931 ohms on the good one. I'm not sure which is the primary and secondary but I'm guessing the primary is the one which ohms close to zero. Check my picture to make sure I'm ohming them in the correct place.

- Mike

The secondary of the transformer should be isolated from the primary side and therefore the fuse, so such measurements are irrelevent. Try checking the switching transistors for shorts, as the fuse is blowing I'd start there. I think you'll also find the transformer has a shorted winding. Take both transformers out of circuit and measure the windings, they should have very similar readings between pins. If not, you likely have a bad one.

Yes, it will tell you if there is drive to the transformer and therefore confirm whether or not the primary side of the circuit is working.

The inverter is an oscillator and generates its own waveform to drive the transformer, you don't need a 'wave generator'.

Dave

I beleive I have found a shorted zener diode at ZD1 it ohms 119 both ways. ZD2 on the opposite side of the board ohms 343 foward and over 700 reverse. I guess that is why I'm blowing fuses. Now to get a chart on how to read zener diodes. :)

- Mike

Okay... The color charts I found versus the colors on this diode don't make sense.. It is orange, red, black and green.. so that makes it a 1N320E ?? I can't find any diodes with that part number.

Maybe I'm just color blind.. Can someone tell me if I got the colors right?

Color Codes I found

- Mike

I took the transformers off and they both ohm very similarly... I think they are both good, but it is late and I'll give it a more careful check tomorrow.

- Mike

Okay.. I don't know what is going on.. I replaced the shorted zener and the light lights for a split second and then blows the fuse.. Must be a short somewhere..

Could it be that I used a 24pF 1kv capacitor on the output instead of a 22pF

3kv?

- Mike

Okay.. I found where I had a short.. My output cap was shorting to something. I cut the lead off shorter and I no longer have a short and have stopped blowing fuses... Yey! :)

I think I may have the wrong value zener. The inverter comes on for a split second and turns back off. Both Lamps are working now... well at least they are comming on for a split second..

What now? This is the zener I replaced with a 1N5237b738 (supposedly a 8.2v zener)

Someone told me that this was a 8v2 zener in another post titled SMT Diode Identification

- Mike

Does it come on long enough for you to take some voltage measurements on the good side and start comparing with the bad side? Also some scope traces on the primary of the transformer would help. That should help you figure out if the replacement zener is correct.

Another place to start is with the feedback circuit. Make sure the proper voltage is coming back to the controller chip. If you trace the low side of the ccfl plug (not the line with the output cap) you'll see that the low side hits a few small diodes, resistors and caps. Eventually it will end up at one pin of the controller chip. That line has to have a certain voltage on it to tell the controller chip to stay on. If the voltage back to that line is not right the controller shuts the inverter down after a second or two.

Dave

Could my problem be related to having a 24pF cap instead of the 22pF cap that it is supposed to have. The 24pF is only rated for 1kv also..

My local surplus dealer didn't have any high voltage 22pF caps.. I'm guessing I'm going to have to order that part.

- Mike

I forgot to mention that it was the output cap.

"Michael Kennedy" escribió en el mensaje news: snipped-for-privacy@comcast.com...

Check that your replacement capacitor is not shorted again. I repaired an inverter very similar to yours... even the failure mode was the same... the capacitor going to the CFL charred by itself and shorted. While it was shorted, the inverter would power on for half second, and then turn off by itself.

Regarding your Diode question, maybe you will find this page interesting:

Towards the end of this page you will find a color chart for SMT diodes. If my monitor is rendering correctly the colors of the picture you posted, your diode has the following ring bands RED, BLACK, GREEN. Using the table provided, that would be a RLZ12B zener diode manufactured by Rohm.

Here you have the datasheet for all Rohm diodes and their voltages:

As you can see, you really need a 12V zener diode.

I hope all this info helps you.

Good luck!

Check that your replacement capacitor is not shorted again. I repaired an inverter very similar to yours... even the failure mode was the same... the capacitor going to the CFL charred by itself and shorted. While it was shorted, the inverter would power on for half second, and then turn off by itself.

Regarding your Diode question, maybe you will find this page interesting:

Towards the end of this page you will find a color chart for SMT diodes. If my monitor is rendering correctly the colors of the picture you posted, your diode has the following ring bands RED, BLACK, GREEN. Using the table provided, that would be a RLZ12B zener diode manufactured by Rohm.

Here you have the datasheet for all Rohm diodes and their voltages:

As you can see, you really need a 12V zener diode, not an 8.2V one.

I hope all this info helps you.

Good luck!

hmm.. Thanks. I found out how the diode got blown.. If you power the inverter on without a lamp it blows the diode.

I tested the good diode and you're right it was a 12v zener diode.

The odd thing is I had this working and put the monitor back togeather. As soon as I got it back togeather it stopped working and started powering on for only a second again.. err..

I really think this could be related to my improper output capicitor that I used.. No one has commented on that. It had a 22pF cap and I replaced it with a 24pF and I also tried a 20pF.

- Mike

I don=B4t think the 22pf versus 24pf will make any difference. Most caps have tolerances as high as 20%... so a cap marked as 22pf could have a capacitance as high as 26.4pf or as low as 15.6pf... and it=B4s value would be within tolerance. The circuit designer surely took that into account when the circuit was designed.

What worries me is the 1kv rating of the substitute cap... try to get one of 2kv at least, and repeat the tests.

Probably the 1kv cap is shorting under load.

What worries me is the 1kv rating of the substitute cap... try to get one of 2kv at least, and repeat the tests.

Probably the 1kv cap is shorting under load.

hmm.. Cap shorting.. Well I'm not blowing fuses anymore but that does sound quite possible. I guess I'm going to have to order one since skycraft doesn't have any in stock.

- Mike

I think I fixed it!!! I think you were right about a capacitor shorting, just not the one we thought..

Anyway I was at the surplus store today and bought 2 18pF 3kv caps and replaced both output caps on the inverter board. I replaced the one on the "bad" side of the board first to no avail.. I then thought why not replace the one on the "good" side and I did and it turned on..

My only regret is ordering the 22pF caps from mouser that I no longer need.. :) oh well you live you learn.. I'll classify this as an education expense.. It doesn't come anywhere what my tuition for school is and is way more fun..

I probably couldn't have fixed this without the help of Dave, lsmartino, and Afra Dailey.. Thanks everybody..

Just hopefully I havn't spoken too soon.. :-)

- Mike