Author Topic: PC Engine GT - No Video or Backlight  (Read 178 times)

WaldoX

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PC Engine GT - No Video or Backlight
« on: October 05, 2021, 12:40:51 PM »
Hello everyone, I am obviously new here, but this seems like the perfect community to try and get some assistance.  I have done a lot of googling already, but have not come across a thread with my exact issue. 

I recently purchased a PC Engine GT from Japan.  When I first received it and tested on batteries, there were no signs of life at all.  Now, I wish I had left it running for a minute to see if any sound started (I have one game to test with), but I didn't.  I flipped the power swtich, saw no signs of life, and turned it back off.  I wish I had played with the volume and brightness wheels, but I can't say that I did.  I know, I did a terrible job of inspecting it's operation, but I expected to have to recap it anyway, so I think that's why I glossed over it.  My bad!  One of the battery terminals had quite a bit of corrosion, so I also ordered an AC Adapter and tried with that once I got it, but same results (and same crappy testing on my part).

Ok, so I did a full recap last weekend, and when I first tested the system after finishing, it appeared to still be dead, which was quite soul crushing, but luckily I left it running, and after 20 or 30 seconds music started playing!  Nice and loud too!  It also seems like the game responded to me pressing start.  Sadly though, the screen doesn't do anything at all.  Not a flicker.  No video, no backlight.

Seems like maybe it's not getting power?  I have a multi-meter, but I'm quite new to all of this and would love a little help in knowing where to test for voltage to the screen, and any other advice you folks may have!

P.S. While most of the old caps had corrosion around their legs, none of them had spilled their contents and damaged anything around them.  I got the old caps off, and cleaned up the pads and all looks good to me.

Thanks again!


WaldoX

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Re: PC Engine GT - No Video or Backlight
« Reply #1 on: October 07, 2021, 11:26:44 PM »
Hello again, I had a chance to do some more testing, and it appears I'm not getting voltage at the red connector for the screen.  I found a thread at badcaps.net that suggested some voltages to check:

Quote
Also, try for the following voltages at these Test points:
TP500 30v
TP501 -24v
TP502 5v (ish, you will see it from 4.8 to 5v depending on factory)

The 2x Variable Resistors adjust the supply rails:
VR500 LCD Voltage Adjust (-24v)
VR501 System 5v Adjust

Here's what I got:
  • TP500  30.1
  • TP501  -24.3
  • TP502  4.91
  • VR500  -23.1
  • VR501  3.1

None of those seem off by too much except for VR501, but I don't know enough about any of this to know if that's a problem.  HOWEVER, like I said, I also tried to probe the power connector for the screen directly, and cannot get any sort of voltage reading out of it! 

I also don't get anything from TP901 which is next to the connector, but I also don't know what I should get there. 

Click here for a closeup of the area around the connector.

Notice what might be cap fluid lurking around?  Do you think there could be damage under those grey blobs that is preventing those pins from getting power?

Any advice is appreciated!  Thanks!

Keith Courage

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Re: PC Engine GT - No Video or Backlight
« Reply #2 on: October 08, 2021, 01:33:42 AM »
Check the traces+connections for both 4.7UF capacitors and the two 100UF caps near them. Make sure the same voltages are going through the via holes since these are what normally short out from old cap leakage.

WaldoX

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Re: PC Engine GT - No Video or Backlight
« Reply #3 on: October 08, 2021, 06:31:12 PM »
Check the traces+connections for both 4.7UF capacitors and the two 100UF caps near them. Make sure the same voltages are going through the via holes since these are what normally short out from old cap leakage.

Hello and thanks for the reply!  I have carefully inspected the areas you mentioned and don't see any broken traces or connections.  I have tested voltages at the capacitors you mentioned, and I tested many many vias in the area.  Here are some pictures showing my results.  I think there's an obvious problem at CC503 with it reading 0 volts?  The vias right next to it are reading well, so I think I had better pull that cap and see what it going on under there.  Any other advice?




WaldoX

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Re: PC Engine GT - No Video or Backlight
« Reply #4 on: October 08, 2021, 06:33:12 PM »
And here are the other 2 pics:


WaldoX

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Re: PC Engine GT - No Video or Backlight
« Reply #5 on: October 08, 2021, 07:15:20 PM »
I may have found the problem!!!! I haven't had a chance to remove that cap yet, but I realized I had taken some pictures during the recap, so I went back and found a picture of the pads for CC503. There is CLEARY corrosion on the bottom 1/3, and that seems to be exactly where it connects to the trace. I've very new to all of this, and probably thought since 2/3 of the pad looked good, all would be well. Lesson learned!



The pads looks scratched up from me trying to remove some of the corrosion with my tweazers and some deoxit. I suspect if I can't remove more of that corrosion I'll need to either scrape away some of the solder mask at the edge of the trace and connect to that? Or can I run a jumper over to the via? Thoughts?

Thanks!

MasterofBiscuits

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Re: PC Engine GT - No Video or Backlight
« Reply #6 on: October 09, 2021, 02:37:10 PM »
That should easy enough to sort, on the + side you can run a connection to the via that the pad runs to, and on the - side there is a big fat pad you can scrape back. I had a missing + pad on this cap on a previous machine and I found it neater to use a leaded electrolytic cap so I could put the wire into the via hole.

I'm fairly sure this cap is part of the backlight circuit so I think this may well be your problem.

WaldoX

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Re: PC Engine GT - No Video or Backlight
« Reply #7 on: October 09, 2021, 08:07:22 PM »
 ](*,)  Ugh, I knew I shouldn't have been celebrating so soon, but damnit it felt good.  Turn out, it was just my ignorance leading me to incorrect assumptions.

I was looking for -24V on that cap, but I don't really know what "negative voltage" is, so I didn't realize I needed to check for the voltage on the negative side of the cap.  It was there all along.  In fact, when I removed the cap, I found fairly pristine pads underneath; turns out I took that picture before I went back and cleaned up the pads.  Here's what they actually look like:



I've looked at this board up and down a magnifying glass and I still have no idea why there's no power here:



Do you think I need to scrape away the grey goo, and is that safe?

Thanks

:(

MasterofBiscuits

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Re: PC Engine GT - No Video or Backlight
« Reply #8 on: October 12, 2021, 12:40:50 PM »
That sucks.

I have a freshly recapped machine here that is fully working so I checked the voltages you have above. Most are the same, but a few are different.

- Positive side of CC503 (22uf cap) should show 24.2v
- The vias that you note are fluctuating should show 7.3v

I would test the transistors around the red backlight connector and also check and double-check the traces and vias in that area as I think your problem lies there.

The good news is if it's just the backlight, you can work around this by replacing the LCD with a modern one.

WaldoX

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Re: PC Engine GT - No Video or Backlight
« Reply #9 on: October 12, 2021, 04:11:41 PM »
That sucks.

I have a freshly recapped machine here that is fully working so I checked the voltages you have above. Most are the same, but a few are different.

- Positive side of CC503 (22uf cap) should show 24.2v
- The vias that you note are fluctuating should show 7.3v

I would test the transistors around the red backlight connector and also check and double-check the traces and vias in that area as I think your problem lies there.

The good news is if it's just the backlight, you can work around this by replacing the LCD with a modern one.

Wow, thank you so much for doing that!  I have actually been contemplating doing the LCD mod as a work-around to my issue, but just want to confirm if I can, that my issue is just the backlight and not the video signal.  Do you know of any way to check that?

MasterofBiscuits

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Re: PC Engine GT - No Video or Backlight
« Reply #10 on: October 13, 2021, 06:42:24 AM »
Since you confirmed no voltage on the backlight power connector I think it's safe to say it's just the backlight (it would be pretty unusual for the video to go too) - you could try shining a torch at the LCD while it's connected to see if you can make out any pixels but this is kinda tough to see in my experience.

WaldoX

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Re: PC Engine GT - No Video or Backlight
« Reply #11 on: October 13, 2021, 09:30:46 AM »
Since you confirmed no voltage on the backlight power connector I think it's safe to say it's just the backlight (it would be pretty unusual for the video to go too) - you could try shining a torch at the LCD while it's connected to see if you can make out any pixels but this is kinda tough to see in my experience.

Hello and thanks for the reply.  I thought I had confirmed no power, but turns out I'm just an ignorant monkey poking at things I don't understand.  Here's what someone at badcaps.net had to say:

Quote
... but just FYI, that red connector is probably for supplying power to the CCFL backlights of the LCD. IDK what voltage and current those would require with this being a handheld device, but even small CCFLs in 13-15" LCD monitors still run at 500-700V AC. So I expect the small CCFLs in this screen to still be running at relatively high voltage and probably a few mA of current or less (with total power possibly under or around a Watt.)

For this reason, don't expect to be able to measure any voltage on the red connector - not with a regular multimeter anyways. The voltage will likely be high voltage and high frequency AC. It can even damage some cheap multimeters.

Now, I don't know if they're correct, but it's much more likely that they are than me.  :-(

MasterofBiscuits

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Re: PC Engine GT - No Video or Backlight
« Reply #12 on: October 13, 2021, 11:54:33 AM »
That is true, CCFLs use high voltage circuits - however, the voltage on the positive terminal on that red connector should show a nominal voltage when running (4-6v - I just tested it). So you can check that. I assume the voltage is stepped up within the LCD unit itself (if you open it up, you'll see a circuit board with a high voltage warning on it).

Edit: Does the screen look like this when you turn it on? https://i.imgur.com/zeY7VFn.jpg
« Last Edit: October 13, 2021, 12:22:25 PM by MasterofBiscuits »

WaldoX

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Re: PC Engine GT - No Video or Backlight
« Reply #13 on: October 13, 2021, 04:02:40 PM »
That is true, CCFLs use high voltage circuits - however, the voltage on the positive terminal on that red connector should show a nominal voltage when running (4-6v - I just tested it). So you can check that. I assume the voltage is stepped up within the LCD unit itself (if you open it up, you'll see a circuit board with a high voltage warning on it).

Edit: Does the screen look like this when you turn it on?

Thank you for checking the voltage on the red connector, that gives me something to go on.  My screen doesn't come on AT ALL.  No flicker, nothing.  After I recapped it, I thought the entire unit was completely dead, but luckily I pressed Run and heard the game music start!

MasterofBiscuits

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Re: PC Engine GT - No Video or Backlight
« Reply #14 on: October 14, 2021, 08:25:33 AM »
You may have a video problem as well then - the above unit is what a dead backlight looks like, the LCD has it's own 'glow' which will change if you play with the brightness wheel - or maybe the LCD unit itself has failed.