Attempting to create a Turbo Express/PC Engine GT repair megathread.

[DrunkenMonk]

So this thread is going to be just essentially a little more specifically related to the video output and the backlight circuit, I intend to flesh this out as I get more time and after testing more things, reading more etc.

We all know this information is already out there in some form or another but I was planning on consolidating it in here for future would-be repair journeys.



Everything below assumes you have already recapped perfectly and your caps 100% are not the problem



BACKLIGHT DIM/Flickering

Ordinarily this is just recap, see if it improves/fixes the issue and if not diagnosing it can be a little bit of a rabbit hole.

Firstly check the negative leg of CC501, see if this is grounded, high resistance or no circuit.

If it's grounded or there's resistance, try checking continuity of D502 (bottom right leg and top leg) if you do, then D502 is defective and needs to be replaced.
A replacement for this is 1SS184 (B3) (A comparable "triode" can also be pulled from a sega game gear, which are also susceptible to damage from leakage  ).

If the above checks out, take a close observation at Q500, usually you can see a little point where the IC is cracked or a small area looks shiny because of high temp. It can also short out, so make sure to check it isn't all shorted to itself.

A replacement for Q500 is 2SD1005 BV/BU



NO BACKLIGHT(Assuming you have power through the F500 fuse)

This is usually either going to related to the large T900, Q900 or Q901.

You can check if the primary is open and the ground is making a connection on both sides, if all the checks come out alright, set your eyes on Q900 and Q901.

A replacement for the T900 can actually be pulled from the Sega Game Gear, they're ever so slightly different - but it's a great easy, cheap salvage part.

Check Q900 and Q901 very closely for any signs of cracking/multitone/shiny parts and check if they've shorted out. If anything looks suspicious, change them out!
Replacement part for both Q900 and Q901 is 2SD1898.

completely corroded D502 and Q500 in some instances can also cause an issue with no backlight, see replacements in previous section.



Video Artifacts, glitching and missing/jumbled sprites

Depending on the type of issue, it can give you a better idea as to what's going on.

9 times out of 10 this always seems to be trace damage or leg damage, but there's ways to identify which chip could be bad and save a little time.

If you see completely jumbled sprites, some normal and some totally garbled:
This can be an indication of a bad RAM connection to HU6270, usually checking traces very closely and legs to make sure none of the nasty electrolytic fluid has eaten something important is enough to see your own problem, but if not you can check the legs of ram manually with a multimeter set to diode/continuity mode and find if there's any breaks or high resistance where there shouldn't be any (Will map this out later to make diagnosis faster). If all of the connections seem fine and you can't find anything after dutiful inspection (seriously, inspecting the board with a microscope would be best), you may need to replace your ram, you can pull donors from a standard model PC Engine.


If you see sprites in blocks that would ordinarily seem normal but they're repeated or just slightly incorrect:
This is an indication that HU6270 isn't mapping all of the sprites correctly, carefully inspect legs 59-76 and make sure they're perfectly intact and making a good connection, if they all seem fine, check ram traces (though this is less likely due to the repeating textures usually happening due to one or more mapper missing)
it's very unusual to have to replace HU6270, but if it's extensively damaged and nothing else works, the replacement can be pulled from a PC Engine  .


This is it for now, but hopefully this gets a lot more expanded and I can work on the text formatting to improve readability.