Zenith TV model Z25A11S apparent storm damage

[Lenny]

The customer says a storm caused this. The set will not find any
channels when autoprogram is selected. There is OSD but no raster. On
video mode the external audio comes through but the video does not. I
have no schematic but I did look up the NTE equivilent for the TA1268N
jungle chip and noticed that it seems to handle all these fuctions,
although I'm mot sure if it also controls switching from TV to video
mode, which the set seems to do. I hate to "shotgun" the jungle chip
but it does look suspect. Does anyone have any info on this problem
with this set? Thanks. Lenny Stein, Barlen Electronics.

 

[David]

Step 1: unsolder the 5v, clock, and data lines off the tuner
Step 2: try a/v inputs for proper video, video returns, definitely the
tuner.
Step 3: once it is confirmed tuner is the problem, 175-2701 variant, order
the updated tuner number and apply the needed 5v line modifications as
applicable to that model to prevent future failure.

Engineering/design defect, well known problem. Causes audio on a/v input
but no video due to corrupted data to the jungle ic from the micro.

Possible, but unlikely a storm actually caused it unless the set has already
had the updated tuner and 5v line modifications performed.

FYI, while you are in the set, check/replace: The two common failure
capacitors, 22uf and 100uf, in the SMPS if they have not been already.
Q5104 on the crt pcb if it has it with a curve tracer, or simply replace it.
The three video output pre-drivers, 2n3906 right next to the jungle ic for
leakage, best to use curve tracer or simply replace. All of about $2.50
worth of known parts failures that often cause repeated call backs.

David

 

[Lenny]

David
I removed the tuner from the set and I have now had external video
and audio working for the whole day on the bench. Its not worth it to
this customer for me to replace the tuner and perform the necessary
modifications to correct the manufacturing defect in the set, if he
can just use it as a video monitor with his VCR as the tuner. So is it
ok to perform the other repairs you suggested in addition to the
tuner, and just leave it this way, as a video monitor with the tuner
removed?
Thanks, Lenny Stein, Barlen Electronics.


----Original Message-----

 

[Lenny]

Hi David
Now my customer wants to fix the set. Go figure!
The original tuner was a 175-2721. I pulled a 175-2701 (which you had
mentioned) from a working set and tried it in this one and it would
not work in this set. Are the two tuners interchangeable? The 2701
came up with symptoms as a tuner with a bad mixer stage although I did
get a raster with this one. I put it back into the set it came out of
just to be certain it had not died in transit and that set worked ok.
Should it have worked in this set? Can you please tell me what the
part number is for the updated tuner for this Z25A11S set? Also I
can't seem to find any information on the 5 volt line modification you
mentioned. Will I need a schematic for this?Thanks again. Lenny stein.




David
I removed the tuner from the set and I have now had external video
and audio working for the whole day on the bench. Its not worth it to
this customer for me to replace the tuner and perform the necessary
modifications to correct the manufacturing defect in the set, if he
can just use it as a video monitor with his VCR as the tuner. So is it
ok to perform the other repairs you suggested in addition to the
tuner, and just leave it this way, as a video monitor with the tuner
removed?
Thanks, Lenny Stein, Barlen Electronics.


----Original Message-----