RCA Projection TV Convergence - P46150WK

[RCAOwner]

Hello and Happy Holidays. Hoping someone can assist me with a repair on a 1991 RCA 46" projection television. I am the original owner, can solder, read schematics, but have no TV repair experience or service manuals.

The color convergence fails intermittently, sometimes correcting itself while the set is on, other times the set must be turned off for a while. At first the convergence only failed once in a while, now it is fairly persistent. The failure is rather extreme, more than just a colored halo or blur around objects.

To narrow down the problem, I placed the set in the user convergence mode via the on-screen menus. Normally a white "tic tac toe" pattern of crosshairs appears in the center of the screen. When the convergence fails, I see a green "tic tac toe" pattern offset about 1/4 of the screen width to the RIGHT of center. Red and Blue crosshairs remain centered, and of course appear as violet. Both of these can be adjusted up/down/left/right via the TV's remote. On this set, the green convergence is not user adjustable, only red and blue.

I have disassembled the set, it contains RCA's CTC169 PTV circuit cards. I believe the failure may be on what is known as the Auxiliary Board (7xxx) which appears to contain the R, G, and B convergence amplifiers (discrete components so unfortunately I can't R/R a module). There are heat sinks on this board. There is also a separate convergence generator circuit board (8xxx) with a bunch of pots for adjusting.

Incidently, the set has been to the shop twice before for convergence problems, first Red, then Blue. Each time they replaced $15 worth of components (if that) on the Auxiliary Board and charged me a few hunderd dollars.

In the set I found basic service documentation - a component layout diagram with parts list/description. I suspect the problem is with one or more of the components involved in the green horizontal (G H) convergence due to the offset I see described above. This is where I need some assistance.

Is there a way to test the resistors, diodes, transistors involved in this circuit? I have a basic VOM as my test equipment. Not knowing much about television operation, which transistor circuit (GH+OUT, GH-OUT, GH CONV) should I focus on? Interestingly there is a RH+BOOST and BH+BOOST but no GH+BOOST. Maybe there are test point voltages I could take to help pinpoint the problem.

Have not removed the circuit card yet from the chassis, but did not immediately see any obvious charred components. However, during operation of the set, it does smell like an electonic component is overheating.

Thank you for any assitance!

[RCAOwner]

Question for someone with TV repair knowledge:

The picture painted by the green CRT will occassionally shift (jitter?) horizontally to the right, so it is not superimposed on the images painted by the red and blue CRTs. Then it will "lock" back on. Rate of jitter occurance seems to increase as set warms up. Eventually the entire green image will stay horizantally shifted, and when this happens, the green convergence horizontal output transitors and heat sink get warm to hot. I don't think they are "blown" because after a rest period, picture is OK again... for a while. I should mention that the green image appears to shift a little vertically as well, but not much compared to the extent of the horizontal shift.

I have located a schematic of the set's green vertical and horizontal convergence circuits. The horizonal circuit consists of two HOTs stacked electrically (top and bottom) with the output being the common connection. The bases of the HOTs are driven by a two-transistor amplifer which is fed from the output of an omp-amp (classified as an "error amp"). The inverting op-amp input is grounded, the non-inverting input is connected to the output from the two HOTs via a 300K resistor as well as the green horizontal (saw waveform?) input from a separate convergence generator board. Appears to be some kind of feedback/correction mechanism.

Any advice on which of these parts might be causing the intermittent picture shift? I am assuming the G horizontal output of the convergence generator is OK (this is the input to the op-amp mentioned above). The schematics indicate this G horizontal signal is identical to the R and B horizontal signals generated...almost. Each R, G, and B signal is buffered separately by its own op-amp (could this be the source of the problem?), and there are a bunch of convergence adjustment pots in the circuit. Mostly for red and blue, only a few for green.

I am thinking about replacing the two HOTs but not sure this will solve the problem...unless they exhibit this type of behavior when defective. My other thought was to try replacing the error amplifer op-amp which is in the convergence feedback loop.

Any insights appreciated!

[Guest]

[minnie]

Transistors not IC's. Usually the regulator on the convergence board.

[RCAOwner]