Pincushion Issue on Sony CRT Monitor

[vol.2]

I have been fighting with this CPD-1604S for a couple of months now, and I am at a point where I feel stuck and I need help.

When I picked it up, it did not turn on. I tested every single electrolytic in the set and replaced everything that was bad. A good 2/3s of the lytics were quite bad (more than 50% out and 100kohm+ ESR) and needed to be replaced.

Specifically, there is a kind of daughterboard PCB in this unit that handles deflection control. It's Sony's solution to the Multisync thing at the time, allowing it to "bin" the incoming signal by horizontal refresh rate and provide separate size controls for each one. This board (and only this board) was covered in early surface mount electrolytics that of course leaked out as they all do. I won't get into the details, but I treated and cleaned the PCB correctly and inspected the traces under a microscope. Everything looks good and I get continuity on even the worst effected areas. There doesn't appear to be any major corrosion, only the solder mask has come off in a few spots.

When I put everything back together, I found that I had vertical collapse. I traced the collapse back to the 24VDC power supply that feeds the Vertical output IC. It had a blown fusible resistor. Replacing the fusible resistor restored the Vertical completely, but the sides are wavy and no amount of geometry controls fixes them.

I turned my focus back to the DA PCB. I did a number of things.

- removed all the geometry pots and ensured they have smooth operation

- tested waveforms around IC301 (Horizontal Oscillator) and IC303 (Pinamp Control, called an "East West IC" in the datasheet)

-checked many resistors for values around the Pincushion circuit and for trace continuity

The only obviously "bad" waveform I found was at the output of Pin 7 on the Pinamp IC. Instead of a 500mV parabola, I'm seeing more of a noisy-looking sawtooth wave at about 280mV with a very quick spike of almost 1Vpkpk right in the middle.

The actual output pin of IC303 is Pin 5, and it basically looks like what the service manual says it should. It's the correct voltage, and it's a square wave. I guess I have to assume that it's slightly off and that the pincushion adjustments make on very subtle changes to the square wave that are hard to see on a scope if you don't know exactly what you are looking for.

The Pin Phase control seems to be doing what it's supposed to do, as I can push it to one side to compensate for the Side correction issue, but no matter what I do, it has a significant dip near the bottom sides of the screen. It seems to just be the "Side Pin" controls and the "Pin Up" Controls that aren't responding correctly.

After not finding anything obviously wrong, I tried replacing IC303 (East West IC), but it didn't change anything at all.

I think perhaps this issue could be related to the blown 24V source fuse from the main PCB; it's possible that whatever happened to the fuse might have taken out another component along the way.

This is not the biggest or most complex PCB, but it's filled with tiny surface mount parts that hard to test, and I haven't tried to remove any of the tiny diodes or transistors yet. I have done a little in-circuit testing of things, but I know that's not ideal.

Can anyone offer guidance based on the bad waveform I'm seeing on Pin 7 of IC303? If I had a narrower field to investigate, I might make some progress.

TIA

If you right-click on one of the images and view it in another tab, it will allow you to expand the schematic and see it in detail. I attached the full Service Manual as a PDF and the Datasheet of IC303.

[wasedadoc]

Are you saying that the picture width is not the same from top to bottom? That requires a correction waveform at field rate. Possibly that would come from circuitry dependent on that 24 volt rail which was missing. Did you just replace the fusible resistor or did you investigate why?

[vol.2]

Are you saying that the picture width is not the same from top to bottom?

Yes, completely so. The best I can get the sides is something like this:



When I tweak the Side Pin controls or the Pin Phase control in the other direction, the best it looks is something like this:




There is no in-between or "happy medium" that can be attained.

That requires a correction waveform at field rate. Possibly that would come from circuitry dependent on that 24 volt rail which was missing.

That's why I was thinking there could be something wrong with a component on the DA PCB (small deflection board). Perhaps something that is fed by the 24V rail got damaged when fusible resistor blew up.

Did you just replace the fusible resistor or did you investigate why?

I did my best to check components downstream from the 24VDC source, but nothing looked obviously wrong, and the components I tested were fine. I came to the conclusion that any number of bad caps could have caused the fusible resistor to blow. The set wouldn't even turn on, and I didn't want to leave any really bad caps in the circuit to endanger other components before I switched it on again. I thoroughly tested all of the caps with a good LCR meter and only replaced stuff that was clearly bad. There were maybe 2 caps that were about 25% out of spec that got replaced, but everything else was either totally dried up or more than 50% out and extremely high ESR (all tested out of circuit).

[vol.2]

So this is the output of Pin 5 on IC303. I realize now that the top of it is the key to understanding the pincushion adjustment output. If you look at the top of the waveform, it's supposed to have a flat top that extends past the edges of the square wave evenly on both sides.

This waveform is incorrect as it has the top portion skewed all the way to the left. The waveform wiggles around in a manner that perfectly reflects when I tweak the Pincushion pots, and it snaps back to the left as the sides of the raster similarly refuse to be corrected.



Now I can see why it's displaying incorrectly, but I don't know why or where the problem lies. I've already spent a long time checking voltages, and nothing really seems wrong so far. I've more or less confirmed the values of all resistors, and I checked the ICs that control the mode selection.

Where should I focus my attention?