Thanks Jerry for taking the time out to write your reply. I tried putting a
20mA load instead of the 100W bulb prevoiusly mentioned, on the output of
the B+ rail and guess what...it doesn't like that either. Now I'm wondering
if maybe there is possibly some sort of shutdown circuit which causes the B+
not to come up when any external load is applied????
Does anyone know???
"Jerry G." wrote in message
...
I have not worked on your model of set, but I will suggest a few things. I
was mainly servicing many of the Sony, and some Panasonic products,
including a little of some other makes of sets.
I would use a 60 Watt, or a 40 Watt light bulb for the load test. For the
output of the supply, 100 Watts is a bit much on these sets.
When testing for a short, and you want some protection, the recommendation
was to put a 100 Watt bulb in series with the complete set, not to put it
as
a direct load on the power supply. The supply would most likely not be
able
to supply a load of a 100 Watt bulb at 140 to 160 Volts. The lamp would be
144 ohms at 120 Volts AC. In reference to this, as an approximation the
load
at 160 VDC would be approximately 1.1 Amps which is over 175 Watts. The
power supply in any of these TV sets would not be able to take this type
of
load. I would use a 40 Watt light bulb at the very maximum to load test
the
supply.
The typical horizontal output stage alone pulls about 45 to 65 Watts on
the
average, depending on the design of the set. Most of the power supply load
is reactive, and not directly resistive, which adds to the complication of
this scenario.
A very common fault that causes the horizontal output stage to draw too
much
current is a defect in the flyback assembly, and or including defects in
the
high voltage multiplier. If there are shorted windings in the flyback
transformer, or if the core has an internal crack in it, its power
consumption will be highly increased, and thus cause the problem you are
describing. Also, if there are failed components in the multiplier, this
can
also result in what you see, therefore causing excessive loading at the
output of the flyback.
Other things that can cause the horizontal output stage to pull too much
current is if there is excessive loading on any of its output supplies,
such
as leaky (resistive, and not visible leakage) caps that are causing
resistive loading. Improper drive from the scan generator to the
horizontal
output device is also common, due to failed components in that area.
Various sections of most sets that are fed from the flyback supply
outputs.
These sections may be the tuner, IF section, audio pre-amp sections, and
etc. Something in these areas may also be drawing too much current, is
another possibility. There is also the possibility that there is a short
in
the deflection yoke, or in one of the components, if any, that are in the
yoke assembly.
As for the RCA set, when these types of faults happen, I have heard that
the
EPROM may have to be changed or re-programmed. This is something that you
would have to refer to in their service manual, or their tech support for
details. Since I have not been doing RCA service support, I would not be
able to give you any in depth information about the RCA product to the
extent of details such as the EPROM set-ups, and etc.
--
Jerry G.
======
"Rocker" wrote in message
...
I have a RCA PTK169PGA RPTV that I have been working on and it has been a
real dog. I can finally get the TV to come on, the audio is fine, but I
do
not have full deflection in either the vertical or horizontal direction
and
the convergence is way off. I have found that at "stand-by" I have a B+
of
143.4V and I have the 13.6V on the cathode of CR4118, 6.6V on CR4117 and
24V on CR4119. I also have the +5V "stand-by". When I turn the TV on,
the
B+ drops to 114.5V and the 13.6V drops to around 11V. I decided to take
this chassis out of the cabinet and disconnect all the boards to see if I
could fix this using the 100W light bulb trick. Here is what I have done:
I completely isolated pin 1 of the HV Xfmr (T4401) so now the sweeps
circuit
and the HV transformer is out of the circuit. Next I placed a 100W light
bulb across the B+ 143.5 Rail. Then I plugged the TV in without any of
the
other boards attached. At this point I am simply trying to figure out why
the B+ is being loaded down. I have the light bulb in a switched socket,
so
when the Light bulb is switched off I have all the proper "stand-by"
voltages and as soon as I switch the light bulb on, the light bulb
flickers
off and on and therefore the B+ is not coming on at all. The problem has
to
be either in the chopper circuit or the feedback circuit for the B+. I
have
already replaced the following: U4101 (Chopper IC), C4104 (15uf cap off
pin
8 of chopper IC), C4102 (2.2uf cap off of pin 6 of chopper IC), C4108
(39uf
cap to the base of Q4101), CR4116 (B+ diode), CR4118 (13.6V didode),
C4141(470uf cap on the B+ rail), and the following transistors in the
feedback circuit: Q4105, Q4111, Q4106, Q4109, Q4107. I'm just trying to
get
the B+ to stay up when the TV is plugged in without any of the other
boards
attached and when I have a 100W light bulb on it.
This B+ should come up with the load on it right??????
Thanks!
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