A fix for LG PA75U "purple image" problem (and PA70U & PA77U?) - AVS Forum | Home Theater Discussions And Reviews
Forum Jump: 
 4Likes
  • 2 Post By pjp
  • 1 Post By descalabro
  • 1 Post By pjp
 
Thread Tools
post #1 of 4 Old 07-20-2019, 01:23 PM - Thread Starter
pjp
AVS Forum Special Member
 
pjp's Avatar
 
Join Date: Feb 2004
Location: NJ/NY
Posts: 1,626
Mentioned: 8 Post(s)
Tagged: 0 Thread(s)
Quoted: 795 Post(s)
Liked: 456
A fix for LG pa75u "purple image" problem (and maybe pa70u & pa77u)

If your LG pa7xu projector has a purple image, it is almost certainly because pin 6 on the "green" circuit is burned out. This appears to be an LG design issue because that pin on the connector can't handle the current load. A new connector does not seem to be available from LG, and I strongly suspect that replacing that connector with a new identical connector would be a temporary fix anyway given the high failure rate for this LG model.

The "short" answer is simply this -> Use a jumper wire to bypass pin 6 in the connector and solder it directly to the rear pin of the black connection block.

However, because the wires are small, the connector is fairly weak (especially after having a pin burned), and the solder point is tiny, I've added detailed instructions below.

The much longer version -> Steps for what should be a permanent fix:

Please note that everything below applies specifically to the pa75u model -- I suspect it is likely the same for the 70u and 77u models, but that is just a guess.

1) Remove the 4 phillips screws and then remove cover;

2) Look at the "Green" 12-pin wire connector along the back edge of the circuit board (you should see a burned pin in the middle of the connector);

3) Remove the white side of the connector by gently lifting the tabs on both sides while lightly pulling the white connector away from the connection block;

4) Turn the white connector over and be certain that it is pin 6 that is burned (look at the "1" and "12" numbers on the circuit board as reference points to see which side of the white connector is pin 1)

5) The problem is a design error where the connector can't handle the current load on pin 6 (the schematic at the bottom of this post shows that pins 6,7,8 on the black connector block are all connected to ground). Replacement connectors do not appear to be available from LG and would likely fail again anyway because it's a design issue. The only permanent solution seems to be add a jumper wire around the failed connector directly to pin 6 on the black connector block (where it is connected to the circuit board).

6) Cut the black wire on pin 6 flush with the white connector. Although the pins on the white Molex connector can usually be individually removed, don't try to remove it because the plastic is already melted and weakened, so the connector might break when you try to remove that burnt pin. It is better to just cut that wire flush with the white connector.

7) Strip the last 1/4" of the wire you just cut

8) Get a 3" length of smaller-gauge stranded wire (preferably OFC and not CCA), probably around 18 or 20 AWG, then strip off 1/4" on one end and 1/8" on the other. "Tin" both sides of that wire by applying a little flux to the exposed stranded wire and melt solder over it. Try to leave a tiny bit of excess solder on the 1/8" end to make it a lot easier to later solder it onto the black connector block.

9) Use an ohmmeter to verify that pins 5 and 6 (on the back of the black connector going into the circuit board) are not connected (you should seek a non-zero ohm reading slowly increasing). You can also verify that pin 6 is connected to 7 and 8 (the other ground pins) and reads 0 ohms.

10) Apply a tiny bit of flux to the exposed pin 6 terminal on the back of the black connector block (where it is soldered into the circuit board). Do not get any flux on the surrounding pins (5 or 7) because it's a very tight soldering area and you don't want it to connect to the surrounding pins (especially pin 5).

11) Use a small clamp to hold the heavily tinned 1/8" end of the wire against the pin 6 contact on the back of the connector going into the circuit board. Select the smallest soldering iron tip you have and let the iron fully heat up. The 1/8" end of the new jumper wire was intentionally over-tinned in step 8 so you don't have to apply any more solder in this step -- just place the tip of the heated soldering iron on the clamped wire against the black connector contact just long enough for it to fully flow to the underlying contact (but not so long that you melt the existing joint connecting that terminal to the circuit board). You can use a small flat screwdriver to hold the freshly soldered wire against the contact as you remove the soldering iron so it doesn't lift while the solder is still molten. This is a small solder point -- to maximize the strength of the joint, be sure to let it cool completely before removing the clamp.

12) Verify the connection is very solid by pulling lightly on it. Then repeat step 9 to confirm that pin 6 is not connected to pin 5 and confirm that 6 remains connected to 7 and 8 (<- this confirms the pre-existing solder joint under the circuit board was not harmed).

13) Take the longer 1/4" tinned side of the new jumper wire and braid it by hand with the wire cut from the white connector. Use a very small wire nut (typically gray) to hold them together. This connection could alternatively be soldered and covered with heat shrink, but given that there is very little wire to work with, a wire nut is probably better if you ever need to do anything later with that wire.

14) Carefully bench test the projector with the cover off to verify the image is no longer purple. Let it run for several minutes and verify that the connector no longer runs hot. Don't let it run for too long without the cover because the fan airflow doesn't work as designed with the cover off (other things might overheat if left on too long).

15) When everything checks out, use narrow cuts of electrical tape to keep the jumper wire routed around the black connector side, and to tape the wire nut down to the lower section of circuit board -- tucking in the wires will ensure that the projector cover fits correctly.

16) Carefully refit the cover, test again to confirm, then mount the projector.

The above hopefully will be a permanent fix because pin 6 now has a robust enough connection to ground that it can handle the current load, and the white connector is offloaded from the pin 6 heat so that pins 7 and 5 are no longer exposed to the adjacent heat of that pin.



NOTE: If you don't have this problem yet:, there are some proactive steps you can take to help prevent it from happening:

1) Set "Energy Savings" to either "Medium" or "Maximum" -- this will dim the bulb, which will reduce the current load through the green connector. If you have a darkish room, the picture can still be great in lower bulb modes (bumping gamma up from 2.2 to 2.4 and tweaking brightness allowed the picture to look as good on "medium" as it did with max bulb brightness when the room was fairly dark). In a semi-lighted room, it may not look as good with energy savings enabled.

2) It's not a bad idea to change the fan settings to "high altitude mode" so that the internal fans run more aggressively. Although I don't think this is really a "fan" issue (no fan is going to prevent a current overload from burning a pin), having things cooler inside this particular projector is probably safer.

3) The fan of this LG projector is normally very quiet (that's the main reason I bought this model). If your fan is loud (as mine was before the failure), you may be on the brink of having this issue. If you have Energy Savings set to "minimum", changing that to "medium" or "maximum" may permanently prevent it. I say that because, even after mine was burned so that the image was purple with Energy Savings set to "minimum", it still worked fine for a while with Energy Savings bumped up to "medium" and "maximum". In other words, if you do this before the Energy Savings "minimum" setting no longer works, it may not happen at all.

Circuit Diagram:


Burned Pin 6 on "Green" Circuit
descalabro and HaniDread like this.

Last edited by pjp; 07-21-2019 at 11:39 AM.
pjp is offline  
Sponsored Links
Advertisement
 
post #2 of 4 Old 07-21-2019, 01:52 AM
AVS Forum Special Member
 
descalabro's Avatar
 
Join Date: Aug 2017
Posts: 1,186
Mentioned: 14 Post(s)
Tagged: 0 Thread(s)
Quoted: 468 Post(s)
Liked: 178
Although I don't own this projector I really appreciate every time someone takes the hard work of finding a solution to a problem and sharing it with others.

Cheers!
pjp likes this.

4,3 x 2,8m semi-blacked out media room | 2010 MacMini > Pioneer VSX-520-K > JVC RS50 (X7) > 107", 1.1 gain, white screen | Planar PD7150
descalabro is offline  
post #3 of 4 Old 07-21-2019, 11:33 AM - Thread Starter
pjp
AVS Forum Special Member
 
pjp's Avatar
 
Join Date: Feb 2004
Location: NJ/NY
Posts: 1,626
Mentioned: 8 Post(s)
Tagged: 0 Thread(s)
Quoted: 795 Post(s)
Liked: 456
Quote:
Originally Posted by descalabro View Post
Although I don't own this projector I really appreciate every time someone takes the hard work of finding a solution to a problem and sharing it with others.

Cheers!
Thanks! I posted the steps quickly and somewhat sloppily yesterday while it was still fresh in my head. I updated post #1 a lot this morning to provide more info and to offer suggestions (at the bottom of post #1 ) that might help proactively prevent this problem from occurring for those who haven't experienced it yet.
descalabro likes this.
pjp is offline  
Sponsored Links
Advertisement
 
post #4 of 4 Old 12-01-2019, 09:03 AM
Member
 
freekmccoy's Avatar
 
Join Date: Oct 2010
Location: OHIO
Posts: 64
Mentioned: 0 Post(s)
Tagged: 0 Thread(s)
Quoted: 9 Post(s)
Liked: 12
Recently picked up a used LG PA75u that has 3500 hours..It's very dim and I've tried to adjust color on it to no avail.I took it apart to clean it and see if anything stood out as being wrong with it,as I don't have any idea on how to work on electronics. I did notice (in pictures) but can't seem to find any other mentions of the red plug being burnt..any ideas?

freekmccoy is offline  
Sponsored Links
Advertisement
 
Reply Digital Projectors - Under $3,000 USD MSRP

Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page


Forum Jump: 

Posting Rules  
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off