18 Jul
2007
18 Jul
'07
4:42 p.m.
Tony Duell wrote:
>> Get the cap as close to the chip as you can: think of the trace as a
>> (really low resistance) resistor: the resistance PS-to-cap is is much
>> larger than resistance cap-to-chip and bingo! low-pass filter.
>
> Actually, it's the inductive reactance of the trace/PSU connections
> that matters here. With high-frequency signals -- this doens't mean a
> high clock rate necessarily, it means a fast ris-time edge -- you can
> get significant voltage drop from very small 'inductors', like the
> connction from the chip to the PSU.
>
> -tony
In my 5 years of TV repair only one I needed your experts to figure
out and inprove the reliablity of these sets as they are premium
series.
I have a bad time with PARTICULAR switching power
supply design that likes to make a diode HOT also capacitor on that
output to self-heat as well. This makes frequent repairs that lasts
from 9 months to 2 years at the most on that capacitor.
Even this
capacitors is designed for high freq SMPSes.
This is usually found in JVC AV-xx980, 820, 985 series, more worse if
it is equipped with guide + module.
This SMPS is based around STR-F6626 IC. I don't fault this if it's
designed according to the Sanken suggested circuit design and I
really liked these STR-xxxxx series as they tend to gracefully shut
down without any blow ups. Only this JVC made this extremely
complicated and problematic when it finally give me problems like
this with hot diode and self-cooking capacitor.
On hot side, there is 2 extra circuits that
sanken sample circuits don't use: one regulator fed with DC from
SMPS transformer and there is extra circuit for the off/on standby
mode that ties into the error/feedback regulation pin on the STR IC.
This pin is also fed from the optisolator for the feedback from cold
side again complex too. Two circuits sharing this optisolator.
One is for 13V supply used for standby and supply for many low
voltage devices so it takes fair current but I doubt this is a issue.
And 135V error feedback circuit using SE135 IC that is disabled when
in standby.
Pretty complicated.
When new, JVC sets usually lasts several years then this C926 1000uF
35V finally gives out on that 13V standby/supply line and STR-F6626
shuts down in self-protection. Also this diode (around 3A RU3YX
diode) runs hot. Even hotter in standby! Same thing with capacitor,
it is not heated by that diode, the capacitor also self-heats but not
as severe. Eventually cap fails.
I have tried all kinds of capacitors, 105C, 85C, jellybean cap,
special cap, even low ESR, even high freq. They simply die only on
that 13V supply. Farther from this on same 13V supply is not a
problem. Even I resorted to replace this diode with TO-220 diode
with small heatsink, still get hot.
There is other cold side 3 outputs also, the two outputs are directly
fed by their seperate diodes and have no issues with heat, and 3rd is
via relay to a diode and a capacitor for the 135V supply for
horizontal circuit. Again, no issues with this.
Only the 13V capacitor/diode is the thorn!
There are plenty of makes that successfully used STR-F66xx series
with no issues but just this particular! Oh my.
Help?
Cheers, Wizard