Saturday, October 1, 2011

NEC TV 21” CRT sound not ok but picture Okay


 This TV was brought in my shop with the picture ok but the sound was not stable…sometimes good and other times not.

I opened the set and went straight to the speaker (power off and power cable disconnected from the power outlet).


Checking on the speaker label I noticed the speaker was rated 8 ohm and a quick test with my ohmmeter got 8 ohms…this confirmed to me that the speakers are okay and therefore I should look elsewhere for the problem.

So the next hypothesis considered for checking is the sound amplifier ic and since I did not have the circuit diagram of this TV, I decided to look up for the sound amplifier ic, To know which is which I followed the speaker wires backward from the pins of the speaker to the main pcb and followed closely the PCB lines and this directed me to an ic TDA 7245.

I did a Google for this IC TDA 7245 for the datasheet and here what I got.


Looking at the circuit you can see pin 3 is very important because it is the power supply pin. I set my meter on 100 VDC range and did a voltage testing on this pin with my meter red probe on the pin 3 and black on cold ground.

To my surprise the Voltage was well within my expected because from the datasheet I was able to see that this IC needs a working voltage between 14 VDC minimum and 30 VDC maximum.

Up to here things are ok, now what else? I decided to check for the signal output for the presence of a signal using a scope and if found no signal and I concluded that the IC TDA 7245 must be the culprit.

Below you can see the pin connection of this IC


I replaced the IC with a new one and after applying the Power I loved what my ears heard…very nice sound…

This Report is brought to you Courtesy of Mr. Beh.
A renowned Technician of all time
City: Tehran Country: Iran
Mr Beh can be contacted directly at email: bsedigh@yahoo.com

3 comments:

  1. HI MR KIMATHI

    i want add to this report that there is no any voltages in the pin no 2 ic to check and find out that ic is good or bad but with scop we can check the RF signals that are coming form the ic or not?
    regards
    beh

    ReplyDelete
  2. Thanks a lot for this very helpful topic....just want to ask,what is the very typical specs of scop that a technician needs have in his bench? Thanks

    ReplyDelete
  3. Hi Bogie, check out this link on what to consider when buying a scope.

    http://www.picotech.com/applications/oscilloscope_tutorial.html

    Regards Humphrey

    ReplyDelete