While unrelated to this problem, replacing C18 and C77 on the Z-SUS board is good practise. They measure 0,5-1,5 Ohm of ESR on all my boards. Of course, that won't remedy an exploded IPM . . . 😉
The short version:
- Replace all 33 µF SMD electrolytic capacitors on the Y-SUS board. Start with C72.
- Replace C21, C24 and C72 with bigger capacitors, 100-330 µF.
- Adjust potentiometer VR1, labelled Setup, until the dots disappear.
- Make sure to fine-tune VR1, as over-compensating will cause dark dots to appear instead.
- Adjust the VS voltage on the power supply. Lower VS reduces the number of dots but increases overall picture noise. 180V seems to work well.
- Adjust brightness and contrast in the menu to compensate for any changes in panel behaviour.
- VR2, Setdn, seems to make very little difference unless turned extremely, in which case it causes serious image artefacts. It could be worth adjusting if adjusting VR1 fails to improve on the problem.
- The through-hole electrolytics are of poor quality, check and replace if necessary. In particular C71 and C77.
- Component locations here.
The long version:
Everybody who has dealt with televisions based upon the LG PDP42V5 and PDP42V6 plasma TV platforms will probably know about the problem of random coloured dots appearing in dark areas of the screen when the picture is moving. It is an extremely common problem that seems to affect most (if not all) 42V5 and 42V6-based plasma screens. Despite it being such a common issue, little information regarding it is available on the Internet.
There is an LG service/assembly manual floating around on the net (link). It is fairly useful in diagnosing these screens, but it’s written in shoddy English and it addresses the “coloured dots” problem (plasma mal-discharge) by replacing the Y-SUS and Z-SUS boards in complete. I felt this solution was unsatisfactory, as new boards often are hard to come by, expensive, or both. The manual does, however, hint at adjusting the “Setup” potentiometer to adjust plasma discharge, an adjustment that can make all the difference. More on that further down.
Before touching the boards past re-soldering and replacing random electrolytic capacitors, I decided to check the Internet for hints. I stumbled upon this page, which addresses the issues on both 42V5 and 42V6 screens, but fails to acknowledge the similarities between the two; setting the VS voltage down also works for 42V5-based systems, for instance.
During the experiments, I used parts from three different televisions, all PDP42V5-based.
One Andersson X1, which had the “coloured dots” problem very, very bad before the panel failed due to hitting the floor at speed.
One LG MZ-42PZ44, which I received with a failed Z-SUS board. Its Y-SUS board proved to have the “coloured dots” problem upon replacing the Z-SUS board, although not as bad as the Andersson’s Y-SUS board. The original Y-SUS board in the LG is of a newer revision than the ones found in the other televisions.
One Grundig Tharus 110 PW 110-9410 TOP, which I received with a failed power supply. It did not have any faults beyond the power supply failure. It did, however, immediately develop the “coloured dots” problem once I installed the Andersson X1’s Y-SUS board into it. Upon re-installing the original board, the problem disappeared.
The test rig:
I used the confirmed bad Y-SUS board from the Andersson, installed in the LG television. Upon making the adjustments, the problem was almost completely remedied. I have not made any changes whatsoever to the original Y-SUS board from the LG.
I’ll start by posting some before and after pictures of the TV that I experimented on.
TV on with original Y-SUS board. Results were similar with the un-adjusted Andersson board.
TV on with the adjusted Andersson Y-SUS board. Aside from the problem disappearing, the contrast ratio improved immensely.
Showing a picture of wood with the original board.
Showing the same picture with the adjusted board.
So, those are the results. Onto the fix.
Adjustments made to the Y-SUS board
The Andersson Y-SUS board. Red arrows mark 33µF capacitors. Blue arrows mark the Setup/Setdn potentiometers.
As you can see, all of the 33 µF/25 V, surface-mounted capacitors are arrowed. When I measured the capacitance/ESR of them, not a single one was above 25µF. I incrementally by-passed them with 10 µF and 22 µF electrolytics, and every time I added a capacitor, the problem became less severe. However, one made a considerably bigger difference than the others, and that was C72. It’s the one below the transformer, to the left.
After replacing all the 22 µF capacitors, the problem was not completely gone, however. While considerably better than before (the TV went from “unwatchable” to “tolerable but poor”), I tried adjusting VR1. Turning it clockwise would produce a darker black level and less coloured dots. Counter-clockwise did the opposite and introduced black dots.
After setting VR1 to a position where some (very few) coloured dots were present, I set VS on the power supply down to a lower value. The problem can more or less be remedied by only adjusting VS to a very low value (170-175V), but the results are poor in comparison to also repairing the Y-SUS board; the lower VS is, the noisier the picture is. Contrast ratio seems to also be negatively affected by setting VS down. Chances are also that the problem will return when C72 deteriorates further and the panel gets more worn.
I found that a VS of about 180-183V to be optimal in this TV, but it probably varies from case to case.
Upon by-passing the capacitors as in the picture, adjusting VR1, and setting the VS voltage down to around 180V, the MZ-42PZ44 shows no signs of ever being broken. The adjustments did, however, make the image somewhat darker, and I had to turn the brightness up a tad in the menu to compensate for it. It is a very minor complaint, however; the picture is still very bright.
I have tested the TV for a few hours, and have not spotted any signs of the issues returning, nor have I spotted any other abnormal behaviour.
I also found that the big 680 µF/250 V capacitors on one of the Y-SUS boards had very high ESR compared to the others. They may be worth checking. Note that they’re connected in parallel.
To the left is a measurement of a fresh cap pair, to the right, a failed pair.
I have applied the VR1/C72/VS-adjust fix to a PDP42V6-based TV as well, with good results. Replacing capacitors past C72 did not make any significant improvements to it. I might make a follow-up article covering PDP42V6-based televisions soon. The procedure is identical on them.