JVC 56WP30 - replaced convergence chips, now works but pic quality not good

[rev engineer]

I recently got a hold of a JVC 56WP30 that had a no-start problem - typical thumping shutdown sound that has been described before. I replaced both STK-394 ICs and 1 associated resistor. The set now powers up and I was able to realign the convergence (outside of service mode), but the picture quality is not that good. The convergence crosshair in the center looks fairly sharp, but the outer ones look a little fuzzy. Not quite the sharp hd picture that I was expecting. I tried adjusting the focus, but anything other than the factory setting looks worse. Any ideas anyone? Could this have been happened from transporting the set? Picture quality from an hd digital tuner via dvi is about the same quality as a regular sd signal. I ran my PC into to make sure it wasn't my hd tuner and found the same results.

Thanks for any help.

[rev engineer]

I worked with the set some more and got a much better picture using the component video inputs. Looks like the DVI input was not providing a quality picture and now that input appears to have stopped working completely. Some of the broadcast hd channels look pretty good, but I'm still concerned about the convergence crosshairs looking fuzzy. Can someone confirm that they are all supposed to look sharp?

Also found another problem with this set. The green channel has a small dot in the center of the screen thats visible when the screen is light, but not visible when the screen is dark. It doesn't move with convergence adjustments. I'm guessing it's CRT related. Has anyone encountered a problem like this?

[lcaillo]

If you removed the back cover with the set plugged in there is agood chance you have disabled the DVI input. Some of the JVC sets have a light sensor that causes the set to disable the digital input if the back is opened without first unplugging the set. This might be a good note to put in a sticky to warn others. I am not sure whether this model is one of them or not, but I have seen the warning in the service manuals and had other techs run into this unbelievable scenario. When it happens, there is no way to recover the digital functionality without major cost.

The dot in the middle of the screen is likely a phosphor burn from scan failure. If you look in the lens with a flashlight with the set off and can see a dark spot, then you have a damaged phosphor and there is no fix other than a new CRT.

[rev engineer]

Thank you very much for the response.

I think you're right on disabling the DVI.. A saw the note in the service manual that mentioned shorting pin1 and 2 on the SB connector before disassembling. I think the SB connector is located on the DVI board. I ignored it because I had already taken the unit apart and it hadn't stopped working. The note doesn't mention this anything about disabling DVI, but states the the set may stop working and may render "inactivation of the set." (The translation language in the manual leaves a lot to be desired!) I had been using the DVI input with the rear covered pulled without a problem. I think the light sensor you're talking about is associated with pulling the mainboard unit away from set - not necessarily from just taking the rear cover off. I can only assume this from the comments you made and the fact that DVI was working with the rear cover off - probably before I pulled out the mainboard, but I can't exactly remember. Real nice for JVC to implement such a useful feature. I assume this sets some bit in memory and forces the DVI to stop responding. I don't see how any electrical damage can occur unless they are worried about the electrical sensitivity of the i2c bus or maybe the sensitivity of the DVI inputs with the unit being powered with the mainboards pulled out. Strange and stupid... :headb:

Any more info on what it takes to fix it? Actual hardware damage or some memory setting? I might not care too much since the picture looked terrible when I was using the DVI connection. Now that I'm getting a decent hd picture using the component inputs, I'm no longer convinced that the focus is far off. But, I would still like someone to comment on the "fuzziness" of the convergence cross-hairs. Maybe it's normal for this set to not have very sharp crosshairs at the corners of the convergence screen.

I did find a tiny dark spot on the crt. I thought that maybe I got some dust in there from blowing out the chassis with compressed air before I started working on it. Is there any interface between the concave glass and the crt, or is this all one piece? It looks like it is all one piece, but again I'm hoping that maybe a spec of dust got stuck in there when I was blowing compressed air inside the set.

Thanks again for your help!

[lcaillo]

There is coolant between the concave lens and the CRT and dust cannot get in there. There may be a gap where it can get between the adjustable lens and the concave part that is sealing the coolant in.

The last I heard, JVC was making folks replace the entire circuit board to repair the DVI input. You are right, it does not do damage to the hardware, but likely changes a value n an EEPROM or progammable controller. I am not a JVC ASC, so I don't have that much experience with them. The ones that I have seen have had a light sensor on top of the board that won't get enough light to matter until you open the back in a well lit room or add some light to work on the set. Then you are screwed. A bit of electrical tape solves the problem. but the set needs to be off to take the back off safely.

This is just another of the long line of stupid and/or arrogant things that JVC has done, IMO. The good news is that most of the newer sets only require opening the front to service the convergence circuits.

[rev engineer]

Thanks again for the help.

I was able to shine a flashlight from the neck of the CRT and up through the screen. I was able to see some light come through that small dot/speck. So, there is defiantly a small piece of phosphor missing. Kinda strange that a single dot got burned. Wouldn't this more likely a an entire line would get burned?

Others take notice - be careful when powering up a JVC set with the rear cover off!

[Master Drew]

Hi Guys,

Recently got a JVC 56wp30 for free from a friend because it was wonky. It powered up, showed the channel number 03 (I noted poor color conversion) and then it powered down. It's done the hearbeat thumping thing since. I opened up the back and may have powered it up while plugged in. I don't know if I disabled the DVI or not. Can anyone find a picture of the DVI sensor so I can see what to tape over if I didn't ruin it? I can only find one disable link claiming to have a pic. No luck finding it. this would be a big help. I can probably replace the stk's myself. But I don't know what to tape over when I have the back off.

Thanks!