Repair Grundig T63-540TF/E --- CUC5310

[Jeroni Paul]

Hi people, I'm about to give up on this one!

Symptom is white short streaks (1 cm long) visible on picture, similar to
interference from a motorcycle that appear only at turn on for few minutes
and then go away and don't return anymore. This interference at turn on is
very random, coming and going and not sensitive to any mechanical stress.
When it happens the colour reception comes and goes, as if the channel had
bad reception.
The facts are:
* have resoldered everything
* also happens when using scart input - then tuner is not culprit
* unsoldered inputs to RGB output IC (TDA3566P), luminance, chrominance and
RGB inputs: problem still persists. This rules out lots of things:
electronic switch, IF stage and l/c separator.
* [contradicts the previous fact]: interference also appears on scart
output, which would lead to belive problem is on tuner/IF stage that was
previously ruled out.
All in common is the power supply, so that would indicate this is noise
coming from 5V or 12V line. I can't check it since a scope is not available
but voltage readings on DVM are rock solid with or without the interference:
5V is 4,9V (from a 7805) and 12V is 12,00V (from an LM317T). Note RGB out
(TDA3566P) is powered from the 12V rail.
I've carefully traced voltage sources - 12V comes from an LM317T (that I
replaced already) and it is powered by a standard switching supply, with a
diode and cap filtering output from switching transformer (cap replaced). I
didn't replace the diode because it isn't a common diode.
It is very frustrating to have only two minutes every 5 hours to
troubleshoot. Indeed, when interference is gone it doesn't show again unless
TV is at least 5 hours powered off! If set is kept running it works fine all
the time.

What do you think about this? Any experience... I don't know what to
suspect...

 

[Jerry G.]

Take your scope and monitor the power supply rails, signal paths, and scan
pulses to look for any non normal signal conditions or interference. It is
very possible that you may have some arching in the high voltage multiplier
or flyback assy. The break down may be a bit thermo sensitive from time to
time. It is even possible that the CRT itself may be arching inside the
electron-gun.

By using deduction you should be able to resolve this. If the set is worth
it to you, take it to a local service centre, and pay the estimate to find
out what the fault is, and let them fix it for you if the cost is
reasonable.

--

Greetings,

Jerry Greenberg GLG Technologies GLG
=========================================
WebPage http://www.zoom-one.com
Electronics http://www.zoom-one.com/electron.htm
Instruments http://www.zoom-one.com/glgtech.htm
=========================================


"Jeroni Paul" <JERONI.PAUL@terra.es> wrote in message
news:bkctq9$8af$1@nsnmpen2-gest.nuria.telefonica-data.net...
Hi people, I'm about to give up on this one!

Symptom is white short streaks (1 cm long) visible on picture, similar to
interference from a motorcycle that appear only at turn on for few minutes
and then go away and don't return anymore. This interference at turn on is
very random, coming and going and not sensitive to any mechanical stress.
When it happens the colour reception comes and goes, as if the channel had
bad reception.
The facts are:
* have resoldered everything
* also happens when using scart input - then tuner is not culprit
* unsoldered inputs to RGB output IC (TDA3566P), luminance, chrominance and
RGB inputs: problem still persists. This rules out lots of things:
electronic switch, IF stage and l/c separator.
* [contradicts the previous fact]: interference also appears on scart
output, which would lead to belive problem is on tuner/IF stage that was
previously ruled out.
All in common is the power supply, so that would indicate this is noise
coming from 5V or 12V line. I can't check it since a scope is not available
but voltage readings on DVM are rock solid with or without the interference:
5V is 4,9V (from a 7805) and 12V is 12,00V (from an LM317T). Note RGB out
(TDA3566P) is powered from the 12V rail.
I've carefully traced voltage sources - 12V comes from an LM317T (that I
replaced already) and it is powered by a standard switching supply, with a
diode and cap filtering output from switching transformer (cap replaced). I
didn't replace the diode because it isn't a common diode.
It is very frustrating to have only two minutes every 5 hours to
troubleshoot. Indeed, when interference is gone it doesn't show again unless
TV is at least 5 hours powered off! If set is kept running it works fine all
the time.

What do you think about this? Any experience... I don't know what to
suspect...

 

[Jeroni Paul]

It seems I finally found the culprit. The repair will be confirmed after
some days of testing.
It turns out Focus/G2 divider was previously replaced as noticeable from
solder joints. Someone attempted to fix previous broken divider by placing a
1M resistor in series between flyback output and divider and fitted the
resistor inside the gap where the wire is introduced on top of the flyback
(this set has no tripler), that resistor was not visible from the outside.
Then, the divider was replaced but person mounting it did not notice the
resistor was left inside and just introduced the wire, two things happened:
a) did not make solid contact with resistor terminal
b) resistor was open, probably internally arcing
Which caused that intermittent arcing noise in picture. The responsability
here is from that person mounting that divider, s/he should have realized
that the wire did not properly enter all the way as it should, and should
have checked the flyback gap. The wire even has a mark that shows its
correct position.
And you still wanted me to bring the set to such people...
However I must thank you, your suggestion about the arcing causing the
interference was correct.

Jerry G. wrote:

Take your scope and monitor the power supply rails, signal paths, and
scan pulses to look for any non normal signal conditions or
interference. It is very possible that you may have some arching in
the high voltage multiplier or flyback assy. The break down may be a
bit thermo sensitive from time to time. It is even possible that the
CRT itself may be arching inside the electron-gun.

By using deduction you should be able to resolve this. If the set is
worth it to you, take it to a local service centre, and pay the
estimate to find out what the fault is, and let them fix it for you
if the cost is reasonable.