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-   -   Improving reliability in CRT RP convergence circuits (https://www.hometheatershack.com/forums/diy-repair-maintenance/8516-improving-reliability-crt-rp-convergence-circuits.html)

lcaillo 01-08-08 06:27 PM

Improving reliability in CRT RP convergence circuits
 

I have noted that many people have had repeated problems with convergence amp failures on CRT based RP sets. I also see people who DIY having problems completing the repair correctly. This thread is about the former. Even many techs seem to not understand what makes these circuits fail, so this post will focus on more detailed discussion of the matter of reliability. I suggest reading the series of posts at the beginning of the CRT based RPTV Convergence Repairs thread.

The big problem with these circuits is heat. These are amplifiers with 2-6 channels, not that different from what you would find in an audio amp. Most high quality audio amps do not use integrated circuits for outputs, however. Heat and design flexibility make them less desirable. For convergence correction, however, they have been used to reduce costs and to simplify service. You find integrated outputs like this in boom boxes and compact stereos, not in higher quality amps and receivers. Imagine running a boom box at about half power constantly with a series of consistent test tones at 60hz and 15khz and you can get an idea of the load on these chips.

Assuming you have purchased ICs from a vendor known to supply only Sanyo first quality parts (of which there are only a few, noted in the other thread), you should be able to expect excellent reliability if you attend to the details. If you bought your ICs somewhere else, expect trouble, as many techs have learned over the years.

The first area to deal with is heat sinking. When the chips are installed, it is important to remove all the old dried heat sink paste and apply a new layer. You should have only enough heat sink paste to fill in the gaps where there is no metal to metal contact. A common mistake is to use too much. Heat sink grease or paste is actually a much poorer conductor than metal. You want as much metal contact as you can get. The paste is a much better conductor than air spaces, so you want them filled, but that is all. I apply a thin layer then use a scaper or the side of the heat sink to remove most of the excess. Then I press the chip into place and slide it back and forth until I feel the metal scraping. On chips that are severely uneven, I have even used a flat surface and sandpaper to "mill" the heat sink flatter for better contact. This is usually not an issue with the Sanyo original ICs used for convergene circuits. There are better heat sink compounds available. The Arctic Silver products are some of the best available. I use nothing so exotic, and am just careful with alignment to minimize how hard the chips have to work and minimize d.c. offset. With attention to detail I have never had a problem with repeat failures, even without fans or expensive heat sink paste. IME, most of the problems come from sloppy setup, failure to properly clean and apply heat sink paste, missing bad solder joints and resistors, and cheap parts.

Assuming you have properly installed the chips, replaced any open resistors, repaired any bad solder joints in the related circuits, and replaced the fuses, you should have a working set. Sometimes, a bad digital convergence unit or convergence generator can cause problems, but that is another discussion. At this point, it is important to understand the manufacturer's procedure for adjusting static convergence and centering the convergence yokes. Static convergence is the overall position of the image, and is adjusted by applying a small dc offset to the system. The consumer and service level controls for position should be centered so that there is virtually no dc offset on the inputs to the convergence amps. The dc voltage should be in the tens of millivolts, and rarely more than 100 millivolts. Once this is done, the picture is centered using the tabs on the back of the yokes which orient the magnets to position the image. Most systems should be centered vertically with all three images (R, G, & B) overlapping, and horizontally with green centered and red and blue offset to opposite sides with no horizontal correction. This is because when you correct the horizontal linearity to account for the angle of the red and blue tubes you will be shifting the center of the pix. If you have active correction and have not completely reset the convergence, they should overlap if the convergence is close.

Centering the position controls should bring the dc offset to near zero. If you have to offset a control to get there by more than a little, you may have a problem in the system. I have seen leaky op amps in the convergence generators or defective output ICs cause this. This needs to be figured out or you will likely have more problems.

Once the static convergence is centered and dc offset is minimized, you want to be sure that there is not too much correction at the edges. This can be tricky and varies with manufacturer and model. The bottom line is that if you can make changes in the controls and not get much effect, you probably have a parameter that is too far out of adjustment. This usually happens at the edges, and is often corrected by recentering the control for the edge parameter and making adjustments that affect it at the next point closer to the center of the screen. Sometimes, you have to start over and reset the entire convergence memory to make efficient corrections. If you go that route, mark the positions of the grid with tape if you can or get a convergence template for the set. Getting the geometry right from scratch is a major headache on most systems. Having a guide is invaluable.

Adjusting the shape and size of each image is known as dynamic convergence, and this is what I am refereing to in the previous paragraph. Most systems use a point convergence system in recent years, but some still have more global parameters such as moving entire lines, or entire sections of the screen. These more global controls should be used to get as much correction as possible and minimize the fine and point corrections. When you push the correction to its limit, you are driving the chips harder and getting less results at the yokes and the system runs more inefficiently. Keep corrections to a minimum and rethink what you have done on a more global level if you run out of range on a control.

Finally, some techs add cooling fans. I have never found the need to do so with careful attention to the details above. I have had virtually no recalls on convergence repairs over the years, at least since I have been very careful about my vendors and procedures. As with most technical expertise, it came with a price, and a rather painful learning curve. Fans, IMO, are a patch that should be unecessary, though some sets do run very hot. I know some techs who will not repair a Samsung convergence problem without adding a fan. I see no problem with this as long as it is powered safely externally. I strongly recommend against powering a fan off of the sets supplies. While there may be some efficient ways of doing so, the risk is not worth the benefit. Like I said, I don't use fans and it has been years since a convergence repair I have done has failed on me. Just get the details right.





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