"playedataritoday" wrote in message
...
I was given this TV today to try and repair. I know the power
supplies on this model have been a big problem, but before I buy a
replacement I wanted to run the symptoms by you guys.
It was working for a while on tuner mode and then got stuck on one of
the inputs and would not allow for it to be changed. It was used like
this for a few weeks and then got stuck running in the test pattern.
Every 5 seconds or so a different color screen would show up and then
sometimes the full color bar test pattern. I took the back off and
took the following measurements at power supply connectors both when
the unit is on and when it is in standby.
CN102
On (Standby)
1 - 12.09 (0)
2 - 12.09 (0)
3 - GND
4 - GND
5 - GND
6 - GND
7 - 5.11 (5.11)
8 - 5.11 (5.11)
9 - 5.11 (5.11)
10 - 3.47 (.062)
11 - 3.57 (.627)
12 - 4.87 (.006)
CN101
On (Standby)
1 - GND
2 - GND
3 - GND
4 - GND
5 - 23.75 (0)
6 - 23.75 (0)
7 - 23.75 (0)
8 - 23.75 (0)
9 - 3.57 (.627)
10 - 4.87 (.006)
I removed the power supply completely and can see no evidence of
faulty capacitors and the voltages all seem to be in range. My only
concerns are the On/Standby voltage being 3.47V, although I don't have
the schematics so I am not sure what it should be. I might have
expected something closer to 5V. If that is the expected value I may
try to strap an external 5V supply on and run as a test.
At one point using the top switches I was able to change the input
mode to VGA, and I am now driving it with a netbook and it gives me a
beautiful picture, I just am unable to change it to anything else. An
the OSD window for "Main:" stays on the screen. The voltages at P701
(top side switch board) are as follows:
On (Standby)
Orange - 0 (3.77)
Blue - 3.74 (3.77)
Red - 3.74 (3.77)
Green - 3.74 (3.77)
Yellow - 3.74 (3.77)
Black - 0 (0)
Brown - 3.74 (3.77)
Has anyone seen similar symptoms with their set? I have no problem
dropping money on the replacement supply, I just want some confidence
the problem isn't with the motherboard before I invest anything more
than my time. No post that I could find described symptoms similar to
what I am seeing. Also does anyone have the schematics or repair
manual for this set? If so please send me copies via email.
All help is appreciated.
Stephen
The voltage that you see at 3.74 is almost certainly the 3.3v standby supply
for the system control micro. I would NOT under any circumstances, hit this
rail with 5v. Although it sounds a little high, such rails often are a few
decimals above the 3.3v nominal value. The 23.75v is the 24v nominal supply
for the backlight inverter. Other 12's and 5's are the general supplies for
the analogue and digital circuitry.
Whilst it is true that electrolytics do cause a lot of problems in
switchmode power supplies, I have never been an advocate of just blanket
changing them for the hell of it. It is easy - particularly if you are not
highly skilled with a soldering iron of the right power rating, and with the
right size and shape of tip, to create more problems than you think you are
fixing. By all means, suspect electrolytics - they can be responsible for
some bizarre fault symptoms, although I have to say that this one doesn't
'feel' like an electrolytic problem - but do the proper checks on them
first - that is initially, an ESR check, which in my experience, will find
over 90% of bad caps, followed by looking for hash on the rails with a
'scope. This will find any that have gone low value without their ESR
changing significantly (rare, but it does happen. Much more usually, the
other way round) without having to remove them from the board to check with
a capacitance meter.
Before going any further, I would also check that you haven't got any
problems with any of the user switches sticking or leaking, particularly if
this TV is used anywhere moist or greasy like a kitchen. I have a 'cheapo'
LCD in my kitchen, and from time to time, the volume 'UP' button gets leaky,
and the volume then goes up and up, until you rush over and switch off. When
it's doing it, a couple of buttons still work, but most are locked out. How
the micro responds to this type of fault condition, depends a lot on whether
it is a 'straight' xxx by yyy switch matrix connected to a pair of digital
ports, or a series switch - R - switch setup, connected to an ADC input on
the micro. With this type of arrangement, I have also had leaky decoupling
caps along the chain, causing the micro to sense erroneous 'button push'
conditions. Good luck with it. Try to use a structured and disciplined
approach to troubleshooting the problem, and try to avoid shotgunning
component groups, unless you are confident that you have one or more
problems among them, and are totally confident in your soldering abilities.
Arfa