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Old 09-21-2021, 11:29 AM   #13
Join Date: Mar 2015
City & State: Sweden
My Country: Sweden
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Posts: 59
Default Re: Dell 2209wa - momentarily blinks every two seconds [ Video + Audio + Service Manu

Thanks for the intuition! I couldn't find a mention of ESR in the datasheet so I decided to put the caps back. They at least seem to be high reliability 125 deg C caps.

The next thing I did was to remove one tube and plug in a standard ~11 W commercial tube used in desk lamps (of a type mentioned in that other thread that recommends using old energy saving bulbs to test for bad tubes -- see photo). I also tried to connect it in parallell with each existing tube to see if I could aviod pulling out the old ones while testing.

At this point I noticed some more gaps in my understanding of what I was looking at (I don't know much about high voltage AC):
  1. When I replaced a tube, the replacement test tube (tested, working) didn't seem to light up at all. Did I have the wrong expectations? Why doesn't it visibly light up? It was undoubtedly connected (see #3). It was not connected via the conductive rubber sockets, but directly to the metal contacts.
  2. When I connected the test tube in parallel with the old tube the old tube didn't light up (expected), but neither did the test tube (see picture). Is it a bad idea to test for bad tubes this way? With my limited understanding I imagined the good test tube would "take over" if it were in parallel with a tube that is bad enough to make a difference. This time the test tube was connected via the conductive rubber sockets.
  3. In both of the above scenarios the backlight did not turn off, which it would have done had any of the tube sockets been empty, suggesting that the replacement tube in #1 actually "worked", without lighting up. Is this to be expected?
  4. When, as a test, I disconnected one of the pins of the test tube (while it was in parallel with the old tube) it lit up more than when both its wires were connected. I'm not sure I can explain that. Granted, the test tube had its glass resting on the presumably grounded metal chassis of the backlight assembly. Does that explain it sufficiently? If I were to guess it's basically the same as holding the glass and touching one of the contacts to the transformer output, as described in another thread here, as a means to test inverter transformers.

Is there any point in testing with a test tube that doesn't light up? If so, I'll start replacing the tubes one by one (breaking a lot of plastic).
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