Hello Folks,

I've been following the forums for a while, and already read through the textbook. ( Helped me through Physics II, thanks!)

However now that I am confronted with a problem involving a more-complex circuit, my skills are not enough to solve this issue.

In short, I found a 37" LCD by the dumpster (yeah, really) and big surprise, it didn't work. However probing voltages on the power supply board and on a specific IC - "Click-Click" TV turns on, and works until I turn it off and leave it for a few hours. This was repeatable for several days... So that's problem one.

Problem two was induced by me more than likely, as it came about only after a few days of the above use. I can still get it to "Click" as if to turn on, LED's on secondary boards flash for an instant, then nothing.

Attached is a photograph of the board in question, and circled in red is the IC that will turn on the TV (Contact labeled as "B", right most contact)

I thank you in advance for any help in resolving these issues. I have a DMM, and follow directions well.

 

your question does not provide any usable information.
how about model number?
can you read part number of the component you circled?
can you show bottom side and can you make pictures in higher resolution?
this sounds a lot like loose connection, which is rather common type of fault.

 

Picture provided is as good as its going to get at the moment., cell phone. I will provide an underside photograph ASAP.

TV is a Panasonic TX-37LZ80, Board is TNP4G440.

Circled part is labeled D2396.

Also I have checked for loose connectors, burnt traces, and charred parts.

 

that is better.

the component is japanese transistor 2SD2396
it is a high gain NPN transistor (hfe=1000) and medium power (30W).
http://monitor.espec.ws//files/2sd2396_840.pdf

B is Base.

 

"Also I have checked for loose connectors, burnt traces, and charred parts."

how did you check? just visual?

 

Yes, just visual.

Please advise.

 

evidence (your description of a problem) suggests possible intermittent failure. common example for such failure is poor connection. sometimes such poor connections ("cold joints" for example) may not be visible by naked eye. therefore finding such connection can be time consuming and/or fruitless effort. it is quite possible that all components are ok (because device can work, at least sometimes) and that culprit is merely cold joint. Cold joints may look perfect on surface because little drops of solders cover pins of components. if the pin was oxydized or moved during soldering, it may not have proper connection.

some methods that could help trace down or fix the problem are:

1. measuring voltages; for example comparing readings in normal operation and when failure occurs. this usually works well when one has schematic of the device.

2. introducing stress (gently of course) to provoke failure or operation. this can be done by appperature change (hot air, cold spray).

3. just blindly go over the board and resolder all joints or at least those in section of interest (power supply).
etc.

of course, one has to be well aware of risks and dangers and know how to safely work with circuits with higher potentials.

 

This sounds reasonable, however I'm looking for some more targeted advice. For instance if I were to chase down the start up problem first (doesn't click/start unless I probe voltages as discussed previously). How would you approach this?

On the other hand, would you advise I try and solve the second problem first? as it appears to me, once the unit 'clicks" it goes into a protection mode of some sort, as voltages drop to 0.0 at the circled Base pin. In addition, I cant induce another start up until power is removed for ~ 2-5 min.

Thank you for your advice thus far, I do appreciate it.

 

your probing around it using DC source may have caused the change you mention. it will be hard to offer anything constructive since I don't have it in front of me. i'd check if the transistor in question is used as SMPS. one option would be to get schematic from online repositories (usually few $). be careful, the fat line on silkscreen indicates different zones.