AV56WP74 Convergence Issue

[derekll]

This question is mostly directed to lcaillo as he seems to know about everything there is to know about this, but help from anyone is greatly appreciated!

The Details:
The set is a JVC AV56WP74. My friend has had it for something like 3 or 4 years.
The green has a major downward bow.
As to your questions about the tech, lcaillo, my friend did not mention details, but said the guy wanted several hundred dollars just to look at it. He pretty much told the guy he'd look for a better price. So the guy didn't give a diagnosis or actual repair estimate.
From inspection, we're about 98% certain the problem is in the IC.
I have metered the resistors, and they are all within tolerance.
My friend did a lot of research on his own, after he contacted the technician and before he contacted me. He found the most common (and therefore most likely) problem was with the STK392-110 convergence chips. The suggested fix is to replace the STK392-110 with STK394-160. He says he found something online that suggested that the resistors should be changed to different values when replacing.
In my research, I have not found that and am a little skeptical. I am wondering if he misunderstood something about changing bad resistors. It seems to me that if the IC presents a similar enough effect that it can be used w/o different resistors on some sets, it should do so on all sets compatible with the original IC. Unless this is something to do with tolerances of other components, I don't see the set requiring different resistors.

The Question:
In your experience, does the JVC AV56WP74 require different resistance when switching from STK392-110 to STK394-160?

Thanks!

[lcaillo]

First of all, let's set the record straight. I do not kow everything there is to know. We dome here to share information and experience. I happen to have lots of experience and try to share some of it that relates to what are often routine problems. I learn a great deal from the experiences that get related here just like everyone else. In fact, though it may not make some of my tech friends happy to hear, I actually learn more here sometimes than on the technical lists and forums that I read.

I have replaced the ICs in your model with the -160 several times and had excellent results. It is uncommon on these to have to replace resistors, but not unheard of. Just be sure to check them with the yoke wires disconnected while you have the chips out. You are likely correct on your conclusion about the values of the resistors. I have not found it necessary on any set, but there are some that I do not use this sub for and many that I have likely never serviced.

IIRC this is one that can be completely accessed from the front and is one of the sets with the "booby trap" that can disable the digital input if the back cover is removed while powered. There is a light sensor in the rear of the set that detects when the rear cover is removed, or possibly if light is shined in from the front with the cover removed and wipes part of an EEPROM to disable digital input. See the attachment.

I generally do not provide service manuals or service bulletins, as most of our contracts with manufacturers require us to hold this information confidential. In this case the information is already circulated on the internet, I am not contractually obligated to JVC, and it is such a clearly unethical and harmful design that it seems reasonable and fair to make the information public. This is an example of why I have resisted doing business with JVC for years.

[derekll]

Sorry if that sounded facetious or sarcastic. I actually meant that you are essentially the master of this stuff! You have a lot of knowledge specific to the inner workings of A/V equipment. It was hyperbole, perhaps, but honest hyperbole. Sorry for the misunderstanding!

Thanks for the tip on the trap! That seems like a dirty trick on JVC's part.

Well, now I just have to get the ICs. Thanks a lot for the advice.

[lcaillo]

I appreciate the compliment, and know that it was meant that way. I just like to be realistic. Lots of people come to these forums with no skills nor knowledge and expect someone to be able to tell them what part to change to fix their set. It has to be understood that it is rather rare that this is the case. These convergence repairs are SOMETIMES this form of symptom-repair relationship, but there is often much more to repairs, and much more to be researched and learned, even for experts. I spend at least 25% of my time doing research, training, and learning.

Be sure to get the ICs from one of the vendors that I have identified as reliable in providing OEM first quality parts. This set is usually an easy fix.

[derekll]

Hey lcaillo, I've got some more issues you might be able to help me with. I replaced the convergence ICs with the STK394-160. I reinstalled the convergence board yesterday.

The convergence is off on all three colors. They seem to have a sine-wave-like irregularity and are shifted away from each other about a quarter cycle (of the irregularity). Also, the convergence is unadjustabe via the user menu. The service menu doesn't appear to be correcting it either. I was wondering if you have any knowledge about this. I can post a picture and more details if you need some.

[lcaillo]

Read the convergence thread again, and verify that you have what you need at the ICs, i.e. supply voltages, no d.c. offset, no open resistors, etc.

[derekll]

OK, I reread that thread. I rechecked the resistors, and they are still good. I will take the chips out and check the V supply and DC offset, as you suggest in the thread. If they are outside the norm, how can they be adjusted? Would I need to change resistors?

[lcaillo]

If resistors or off value or discolored, replace them with exactly the same type and value, including tolerance.

[ender23]

I wish I would have read this a while ago, as I just finally got a copy of the service manual for this TV (couldn't find it a few years ago). I already disabled my digital input a while ago when working on the TV, wondered for a while why it quit working. I just got done replacing the convergence boards and the picture looks good again.

It would be nice to have the digital input working again though. Do you know of a way to restore the digital input functionality after you already triggered the booby-trap? If it erases part of the EEPROM, I'm guessing not... But if it just switches a register in the EEPROM or something, this might be reversible. But figured I'd ask. I was thinking of trying to short connectors one and two and unplug the tv for a while and see if that does the trick..

Any response appreciated,

Thanks!

-John

[ljryjj]

http://www.hometheatershack.com/foru...dvi-video.html