Thanks for all the info - it's more of a project now, a few of my
friends and I are tinkering with it a bit, following all the safety
precautions.
It's hard to narrow down the sound, and I'm not familiar with all the
"exact" terms for each item on the circuit boards.
Hi Guy,
To localize noise techs uses either steroscope or a listen through a
Mcdonald's drinking straw at item in turn. Extremely effective. (I
keep few of them in toolbox and invite one of my tech or boss to help
me find it. I'm deaf and can't hear small noises.
But on CTC203, I rather not blow it up more, order the RCA part number
237470 first. Check the Q14401, it WILL be shorted. Hence the buzz
noise. transistor is marked clearly on the circuit board and this
horizontal output transistor is clamped down with wide spring steel
clamp. If you have a plastic goes from square metal box running
across horizontal transistor to the power supply's heatsink. Take it
out and toss it. This plastic strap chokes the air flow for that
horizontal heatsink which is Bad Thing but good thing yours did not
have it.
Reason you hear buzz or squeal is the all SMPSes either or both
increases the "on" time and or decreases frequency to pump "Scotty,
more power!" This reasoanaces changes and made audible from parts
vibrating. More power to loads if power demands incrases to keep
voltage as stable as possible, some SMPS simply blows that instant
like in Sony. Shorted horizontal transistor is easily found with a
ohm meter (DMM).
Heating up is also consistant with bad parts that is shorted but this
is very misleading because the good parts does heat up more trying to
pump more power to something else. Also this is how some cases lets
out magic "smoke" that stink that goes with it.
Heating up pointing to the small board is the pincushion circuit to
correct pincushioning error on some CRTs. Not all models has this
active pincushion. Very nice set indeed.
Below that pincushion board, right at the mainboard you will see a
small coil which is L14401 right next to base of heatsink of
horizontal transistor where that small board is mounted to that said
heatsink. On other side of that heatsink is the Q14401 horizontal
transistor with wide steel spring clamp which you will find shorted.
Fix the L14401's glue and replace Q14401 (part number 237470) will get
that tv fixed.
That Q14401 grounds B+ through flyback tranform to put energy into the
magnetic field of the ferrite core in linear fashion and horizontal
yoke winding. B+ is 140V typically measured from cold side ground, the
SMPS transformer is the isolation but it is still hot chassis since
the SMPS circut is still hot direct to line power). Grounding B+ via
flyback transformer during beam motion just before vertical centre of
crt's face and that transistor turns off at end of scan and capacitors
and diodes, windings in both coils and flyback oscillate from back EMF
in orderly way to kick beam from left to right and fall back to the
centre at correct speed, process repeats.
This horizontal circuit exacts very heavy demand on that transistor
and have to be designed carefully during design stage of a TV.
Normally could radiate about 40W during worst cases during horizontal
transistor's turn on and turn off part of pulses, hence the heatsink
for it. Anything wrong in that horizontal circuit that transistor
goes instantly with a snap of fingers. Just one bad pulse or bad part
anywhere in the horizontal circuit is enough. One of those killer
parts like glue that cracked L14401's solder joint will do that. That
L14401 coil is the part between horizontal transformer driver (T14301)
and the horizontal transistor's base pin.
Injecting small amount of current into base pin on BJT transistors
controls amount of current going C (collector) to E (emitter) in turn
vary the voltages, this is how audio amp does or as a voltage
regulator, etc. Also can be controlled fully on for full on or fully
held low for off such as for computer, horizontal transistor for
horizontal circuit applications.
Reason TVs needs horizontal transformer driver is impendance matching
and to turn low current into high current drive with very tiny volts
and ablity to pull base negative for tiny time as required to force
horiozntal transistor off while in satrated mode. Malformed waveforms
on base drive or bad parts anywhere in horizontal circuit upsets the
tuning in turn changes waveforms, this blows that transistor because
transistor isn't turned on or off properly or way too slow or too
fast.
Also that is reason this particular transistor is required for
reliable operation, poor horizontal transistor substutition won't work
and also there is so many fake transistors out there, hence reason for
ordering that one from RCA.
I've been burned by fake parts in past. Wastes time and makes
customers unhappy with repeats (aka call backs). Choosing good
supplier is important for us and knowing which supplier does and what
doesn't is good to know.
Cheers,
Wizard
