B
Bob F.
My big screen set, an RCA CTC195, purchased in 1997 died a few weeks ago. I
purchased the schematics from TCE (Thompson Consumer Electronics) and
carefully read the section on the input power supply, then checked all of
the waveforms and voltages. I could see that the B+ voltage was too high
(170vdc vs. an expected 150vdc) and pin 5 of U101 (a PWM IC for voltage
regulation) was NOT oscillating.
My first step was to take the main video board out and clip some capacitors
across the large input capacitors and see if the power supply would start
up, but no response was seen.
Next, I started checking resistor values. All of the resistors on the top
side of the board around U101 checked out okay, so I flipped the board over
and started checking the surface mount components. I went to check R121, a
1K-ohm, surface-mount resistor, but lo-and-behold, it was missing! In fact,
it looked as if the component had never been installed because the glue dot
to hold it in place before solder reflow was perfectly formed, not mashed
down.
Anyhow, I added a resistor to this point and fired it up. Amazingly, the
damn power supply came up. I put the main video board back in the chassis,
connected everything back up, and its been working for the last four days.
My theory is that R121 was never added or fell off during production.
During testing at the factory the set checked out okay, but with the passing
of time, component values in the voltage regulation circuit have drifted
causing the set to shift from a state of marginally operating, to shutting
down.
My lesson learned is that its sometimes necessary to ADD parts to fix a dead
set.
Thanks,
Bob F.
purchased the schematics from TCE (Thompson Consumer Electronics) and
carefully read the section on the input power supply, then checked all of
the waveforms and voltages. I could see that the B+ voltage was too high
(170vdc vs. an expected 150vdc) and pin 5 of U101 (a PWM IC for voltage
regulation) was NOT oscillating.
My first step was to take the main video board out and clip some capacitors
across the large input capacitors and see if the power supply would start
up, but no response was seen.
Next, I started checking resistor values. All of the resistors on the top
side of the board around U101 checked out okay, so I flipped the board over
and started checking the surface mount components. I went to check R121, a
1K-ohm, surface-mount resistor, but lo-and-behold, it was missing! In fact,
it looked as if the component had never been installed because the glue dot
to hold it in place before solder reflow was perfectly formed, not mashed
down.
Anyhow, I added a resistor to this point and fired it up. Amazingly, the
damn power supply came up. I put the main video board back in the chassis,
connected everything back up, and its been working for the last four days.
My theory is that R121 was never added or fell off during production.
During testing at the factory the set checked out okay, but with the passing
of time, component values in the voltage regulation circuit have drifted
causing the set to shift from a state of marginally operating, to shutting
down.
My lesson learned is that its sometimes necessary to ADD parts to fix a dead
set.
Thanks,
Bob F.
