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Dead RCA CTC195 - Fixed!!

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.
 
J

Jason D.

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.

AMAZING! I had a JVC horizontal shrieking at 22KHz 27" set (narrow
raster). It was missing resistor for the bus controlling that digital
jungle. :-O

I think that small electrolytic cap in the primary side SMPS went old
making the problem show up in yours, just in case change that small
cap.

BTW, R121 is partial location number, The RCA has number of different
areas designations: In your case for SMPS is R14121. Horizontal
circuits is numbered with x144yy where x is type of components, yy is
location numbers. For example L14401 is inductor coil for base drive
between HOT and horizontal transformer driver.

Bravo! & Cheers,

Wizard
 
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