tv retunes every 2 seconds, tuner or TDA?

[Wouter Minjauw]

Carad TV set. Customer said picture had been going odd for quite some time,
picture completely died last night. I Found a big crack in the circuit board
near the flyback transformer. Repaired the tracks, now I've got picture and
sound, but every 2 seconds, the picture has noise and audio crackles, as if
it is slightly mistuned. Picture and sound come back to normal in half a
second.

Personally I think there are 2 possibilities, the tuner itself or the
TDA8362A, which is a TV combi IC that controls just about anything between
the tuner and the power stages, including AFC and AGC. At one of the pins on
the tuner there is a DC with a relatively small sawtooth superimposed. I
think this could be the AFC or AGC signal for the tuner.

Does anyone have some advice before I buy a 30$ IC?

Greetings


Wouter Minjauw
Student Electronics Engineer

 

[JURB6006]

Have a look at any coils near the IF IC, or whatever handles the IF. Not
familiar w/that mdl, but it sounds like what Sonys do when the connections go
bad in the IF circuit, or some other sets when the coil drifts.

When you fix cracks in the PCB, there may be more. The thing was probably
dropped so connections near the IC may be also cracked. They could be near any
shields in the circuit and they might not be visible because of solder mask.

Depending on the architecure of the set it might be yet one more connection in
the horizontal ckt, the one that supplies the coincidence detector. Sets will
sometimes endlessly hunt even if the AFT works because it doesn't know that
there is valid sync.

Only other possibilities are high value resistors used to "fix" a tri-state AFT
output from the IF, the IC itself as you suspect or it may be the uProcessor if
it's not reading AFT and/or coincidence (valid sync)

Even though I'm in the US and only work on NTSC and HDTV sets this info should
be valid, hope it helps.

JURB

PS you might want to try www.bdent.com for the pinout of the IC. Gotta register
but it's free and all you need give them for real is one valid email address to
recieve your password.

Also, although it wouldn't help me, including the model number might help
others help you. By the way, where are you, I've seen the dot be before but I'm
not sure where it is, like nl, jp and others tell what country you're in. I'm
just curious.

JURB

 

[Wouter Minjauw]

Thank you very much for your advice. I'll check for other cracks at first.
There was a shield around the TDA IC, so there could be some little cracks
down there. I have specifications of the IC, but since it's a 54 pin IC it
will take some time to examine all the signals.

..be = Belgium. Huh? Wha? Belgium? What the hell is Belgium? ;-)
Answer: little country between the Netherlands and France.