[lcaillo]

I continue to see people having problems with convergence repairs, and with what they believe to be convergence problems. There are some important facts that many do not seem to understand, most importantly that these repairs are often not just a matter of replacing a couple of chips. What is most distressing is that I see posts from techs who really do not understand how these circuits work nor how to properly troubleshoot a problem.

First some perspective. A majority of convergence repairs in CRT based RPTV can be completed with only the replacement of the output ICs and maybe a resistor or fuse or a couple of each. That means that a minority of them, but a significant number involve something else. Hopefully this thread will help to sort out what some of those problems are, both generally, and with respect to some specific products.

Second, there are many problems that get created when one is attempting to repair a set. Some of these issues are covered in the Convergence Repair sticky in the first 10 posts, but I will try to be more complete here. Some information may be duplicated, but it is useful and often never clearly stated in much of what can be found online, so I don't think that is a problem.

Finally, if any of you techs are reading this who constantly criticize me for giving this information to consumers, I suggest you simply read on through and learn something if you can. Some of the very critics that I have experienced have also posted some of the most embarrassing questions that, IMO, a professional in this field could ask. It continues to shock me how little many techs know about how these circuits work, and how many assume that they can fix every set by replacing ICs and pressing some auto-convergence button. The level of discussion on these problems on the tech lists and forums is not much better than on some of the consumer sites. The difference is that there is a lot less mis-information in the responses, generally.

I hope this makes some problems easier to solve for someone.

 

[lcaillo]

The Power Supplies

One of the most common problems that I see people struggle with that goes beyond the output ICs are the power supplies to the chips. These circuits all use a +/- supply referenced to chassis ground and if either of them is not present, you can easily damage new parts or you won't get convergence operation.

It is not always easy to check, because in some sets the location of the supplies is not well labelled on the board, and it may be difficult to get to the bottom of the board where the chips are soldered in with the set operating. Most of these circuits are fusesd, so the easiest thing to do is just to check the continuity in the fuses. This does not tell you that the supply is working for sure, but if you have an open fuse you will know before installing new parts. The vast majority of blown fuses are due to a bad chip, so it is pretty safe to guess that replacing it along with new chips will solve the problem without having to troubleshoot the power supplies. Most of the time anyway. If possible, it is a good idea to make actual checks of the power supplies to the ICs with the set running and the ICs removed.

Most sets use both + and - supplies to the convergence circuits at voltages between 16 and 45 volts. If either is missing or noisy, you can damage new parts or not get convergence control.

Occasionally, there will also be other supplies to convergence circuits, depending on the age of the set, the chips used, and the specific design. There may be muting lines that require another supply or peripheral circuits that require a different supply, like in some of the older Mitsubishi sets with the 8 volt supplies that can cause problems. Having a schematic and knowing how to read it can be essential to solving some of these more complex problems.

 

[lcaillo]

The Resistors

Resistors on the output lines of the ICs are often damaged when the chips fail. The reverse can happen as well. A resistor can open and take out the IC. Regardless, anytime one services a convergence circuit, it is important to verify the resistors are not heat damaged, cracked, open, nor off value. Occassionally, manufacturers will have updated values for some resistors if they find that the circuit is inefficient or ICs are upgraded and require different operating parameters.

Check them with the chips removed AND the convergence yokes disconnected. The yoke may alter the readings on some resistors on the outputs. There are also other resistors that can fail, so it is a good idea to check all of the related resistors in the convergence amplifier circuit.

 

[lcaillo]

The Convergence Generator

This gets a little more complex. The convergence generator may be a discrete module or may be built onto a larger board. It can be easy to locate the lines from it on some sets (like Sonys that have them on the bridge connector near the back of the set nicely labelled). Others are near impossible to find without a schematic.

There are six channels form the convergence generator to the ICs, horizontal and vertical for each color, red, green, and blue. There are two components to the signals coming from the convergence generator (often aka DCU for Digital Convergence Unit). The complex waveforms that allow correction to the geometry and size of the image for each color are impossible to evaluate without an oscilloscope. Even with one, it can be very hard to make sense of them unless you know what to look for. Fortunately, lots of problems can be identified with a simple check of the d.c. voltage on these six channels. If you measure several volts on one, you can bet you have a leaky op amp or bad convergence generator IC. There should not be more than a few hundred millivolts. If you have a problem in a convergence generator, it is not likely a DIY job. Most service techs don't even know what to do other than replace the module, though I have found that many can be repaired, mostly the ones with a single channel leaking d.c., as the op amps in these circuits are commonly available chips that are not too bad to change.

Most people never get to this level of testing, however, and it may not be necessary in most cases. If you are being thorough, however, and want to make sure the set is operating most efficiently, it can be an important thing to check. It is not uncommon in some sets to find up to a half volt of d.c. on these lines into the output ICs. That means that there will be about that much on the output to the yokes. This d.c. bias is used to shift the image up and down or left to right (static convergence). The same can be done with the magnets on the back of the deflection yokes. The problem is that too much d.c. offset can stress the chips by increasing the temperature. It may only make a few percent difference in the power dissipated but in circuits that run hot anyway, every bit of efficiency should be considered important. When I note more than about 100mV of offset, I zero the static convergence by measuring as I adjust the position, then reposition the image wth the magnets on the yokes. THe hardes part is often getting to the points to measure, but if you do the final adjustment measuring the offset at the convergence yoke connectors you can usually get to them more easily. It is still a good idea to measure at the inputs or coming out of the DCU if you can to be sure that you are not having a problem there before installing new parts.

 

[lcaillo]

The Convergence Generator

This gets a little more complex. The convergence generator may be a discrete module or may be built onto a larger board. It can be easy to locate the lines from it on some sets (like Sonys that have them on the bridge connector near the back of the set nicely labelled). Others are near impossible to find without a schematic.

There are six channels form the convergence generator to the ICs, horizontal and vertical for each color, red, green, and blue. There are two components to the signals coming from the convergence generator (often aka DCU for Digital Convergence Unit). The complex waveforms that allow correction to the geometry and size of the image for each color are impossible to evaluate without an oscilloscope. Even with one, it can be very hard to make sense of them unless you know what to look for. Fortunately, lots of problems can be identified with a simple check of the d.c. voltage on these six channels. If you measure several volts on one, you can bet you have a leaky op amp or bad convergence generator IC. There should not be more than a few hundred millivolts. If you have a problem in a convergence generator, it is not likely a DIY job. Most service techs don't even know what to do other than replace the module, though I have found that many can be repaired, mostly the ones with a single channel leaking d.c., as the op amps in these circuits are commonly available chips that are not too bad to change.

Most people never get to this level of testing, however, and it may not be necessary in most cases. If you are being thorough, however, and want to make sure the set is operating most efficiently, it can be an important thing to check. It is not uncommon in some sets to find up to a half volt of d.c. on these lines into the output ICs. That means that there will be about that much on the output to the yokes. This d.c. bias is used to shift the image up and down or left to right (static convergence). The same can be done with the magnets on the back of the deflection yokes. The problem is that too much d.c. offset can stress the chips by increasing the temperature. It may only make a few percent difference in the power dissipated but in circuits that run hot anyway, every bit of efficiency should be considered important. When I note more than about 100mV of offset, I zero the static convergence by measuring as I adjust the position, then reposition the image wth the magnets on the yokes. THe hardes part is often getting to the points to measure, but if you do the final adjustment measuring the offset at the convergence yoke connectors you can usually get to them more easily. It is still a good idea to measure at the inputs or coming out of the DCU if you can to be sure that you are not having a problem there before installing new parts.

 

[lcaillo]

I keep blowing fuses, resistors, or ICs!

I hear this a lot. The solution is to check the obvious, check it all again, and assume that you either did something stupid or missed something. Most experienced techs have done both, and know that you have to double and triple check yourself and NEVER assume anything. Many, if not most, problems that come up in servicing relate to assumptions that are incorrect, at some level.

OK, so you have checked the supplies, checked all of the fuses, checked all of the resistors, checked for d.c. offsets on the inputs and outputs, verified no missed solder connections or bridged solder joints, verified that you used reliable suppliers for your parts, applied heat sink compound properly, and prayed to the convergence gods. And you still blow parts when you fire it up...

You could have a convergence yoke that is shorting. You could have bad data causing the correction waveforms to over drive the ICs and causing failure. On sets that have a convergence data reset you can try that (this is sometimes necessary on Samsungs) but it should be considered a last resort. You will have to rebuild the geometry and convergence from scratch, and that can take hours for even the most experienced tech. You may need a convergence template to get it right.

Generally, if things blow right away, you either have missed something or made a mistake somewhere. Yokes and bad data generally overheat the chips and you have at least a few seconds to see the problem coming. Sometimes, you can have a severe short in a yoke, but the set will not likely run with the convergence chips out of circuit if this is the case, or you may measure voltages on the convergence yokes where there should not be much.

The vast majority of repeat problems come down to the servicer missing something. Don't jump to conclusions and don't assume. After 30 years of service, I keep learning those lessons over and over. Less than in the early days, but we all tend to get in ruts and make assumptions. DIYers generally don't appreciate the number of assumtions that they are making, and unfortunately, they can be hard to identify with all of the "just follow these instructions and fix your set" postings on the internet.

 

[lcaillo]

Should these things really run this hot?

Yes, and no. Ideally, the convergence ICs should not get as hot as many do. The circuits could be designed better and the sets could be assembled and aligned for optimum efficiency and improve many of them. Some do get hot enough to be very uncomfortable to touch. Is the solution fans or additional heat sinks? Maybe. I have never found the need to use these solutions.

Probably the most notoriously hot convergence chips are many of the STK392-010, -020, and -040 in Samsung sets. Even in these I do not resort to fans. Careful alignment using minimal correction, both d.c. and with the convergence waveforms has been adequate for me to have virtually no recalls. I think that most of the repeat problems that I hear about with these are due to excessive correction, particularly at the edges. If you are not getting results with a correction and you keep pushing it harder, you are sending a more exaggerated waveform to the output amp that is going to work harder to try to output it. These ICs are basically big op amps. They try to drive their output to match the input, and if the load will not respond, just work harder to make it happen. They are not very smart, so the person adjusting the set has to be. I have seen lots of Samsungs with convergence pushed to its limits with little effect other than to overheat the ICs. The bottom line is, if you are not getting much effect with a control, back off and try something else, or live with it. These sets will not have perfect geometry and convergence. You can usually get decent trade-offs, however, that result in a more effcient system and better than OOB performance.