PC Engine GT (expert) repair help needed! [Lots of repair info here too!]

[Segasonicfan]

So my friend gave me his childhood PCE GT to fix.  Lots of sentimental value.  and man is it busted.  At some point the 5v rail I believe shorted and it fried the controller board IC.  ordered a new controller board, but I don't know what else it took with it.

I get a yellow and reddish screen, split in 2, with noise lines in the middle (attached).

- Fully recapped
- Fully cleaned in Ultrasonic and manually with 99.9% IPA
- No audio (I can hear speaker hum, and volume works.  I'm confident audio amp output stage is OK.
- +5V rail is good (4.85v)
- +2.5v rail is good (for audio)
- 24mhz clock is good on both ASICs (followed guide from TG-16 service manual)
- H sync and V sync outputting correctly
- Backlight circuitry all tested good, 30V and 24v OK.  All oscope probes of transformer check out too.  Screen seems to be fine.

What I did in addition to above:
- swapped in full Hudson ASIC chipset from working CoreGrafx (white)
- probe (diode) tested every on the Hucard slot.  Compared with working unit, and values seem to be as expected / close (bus pins within range of each other, no obvious shorts).
- swapped in one of the VRAMs from the CoreGrafx as well

I'm at such a loss with this one...the board looks *so* clean.  anyone know of other stuff to look at?  I've poured so much time into this one it hurts.

I'd be so indebted to a helpful benefactor.  Willing to kiss feet / send free things.
<3

Thanks in advance!

[Segasonicfan]

Here's a video of the problem too, in case that helps:
https://imgur.com/a/OSysS6m

[Segasonicfan]

so I had a working one to compare, but the owner doesn't want to recap it yet, despite the speaker failing and leakage everywhere.  I had to give it back because I didn't want it to kick the bucket in my hands :/

What I need is a better service manual, that gives more debugging info than "replace all ICs if clocks are bad." (basically what TG-16 service manual says).

I wish I knew:

• - what is the boot up order? Which pins go high / low on startup (OE, WR, Read, etc)?
• - does the metal shielding need to be in place for a proper boot?
• - what pins / ICs are known to go bad with a +5V short?
• - is there a source for the knobs (volume / brightness) to replace?  some of the plastic got melted on mine with chip removals  note to others - desolder those, dont even try to take out the main CPU (80 chip) with them in there.
• - can a white Core Grafx PCE chipset be swapped in?  Or are there revisions to the huson chips and/or VRAM / work RAM?  (I actually swapped the *entire* chipset and results were the same).  I'm sorta guessing there's a slight variation.

Essentially what I have here is what appears to be a fully working GT (audio, screen, contrast, high voltage stuff), but a dead PCE chipset.  Would love help from someone who knows that architecture more.
I found this epic write up: (https://www.copetti.org/writings/consoles/pc-engine/) but it's not really repair / failure focused.

What I really don't understand is why the symptoms are the same no matter what I try.  No garbled graphics or sound.  I fear these 80's ASICs dont have the diode protections of 90s logic used in Sega consoles that I'm more familiar with, and just bite the dust at the first sign of a +5V short (such was the case with the controller IC, that's for sure).

I can verify from a working system that it will boot without controller PCB connected (just make sure pins 12 and 8 are bridged on the controller FFC plug if you want backlight on)

Some essential repair resources:

• - Retrosix Repair Wiki (board scans and info galore.  Really great stuff): https://www.retrosix.wiki/board-scans-pc-engine-gt
• - GameSX epic redrawn (legible) schematics for PCE.  and the TG-16 service manual: https://gamesx.com/wiki/doku.php?id=schematics:console_related_schematics
  also located here: https://wiki.console5.com/wiki/File:PC_Engine_Schematics_r1.2.pdf

Main IC pinouts:

• - https://wiki.console5.com/wiki/Hu6260
• - https://wiki.console5.com/wiki/Hu6270
• - https://wiki.console5.com/wiki/Hu6280
• - PCE system architecture info: https://www.copetti.org/writings/consoles/pc-engine/

Hopefully all my debugging and suffering can benefit some other poor soul out there.
Attatched pics of my board, literally one of the cleanest you'll ever see.  Sigh.

Repair log:

• - Removed / replace all ICs (even RAM) with those from white CoreGrafx PCE and replaced.  All pins checked under microscope.  No difference in symptoms
• - Removed all electrolytic caps and rechecked all values and pads with DMM on cap setting
• - Cleaned under all ASICs and RAM and caps, no trace damage I could find
• - Ultrasonic cleaned the entire PCB in friends $1000k+ cleaner.  Cleaned uup after with 99.9% IPA after
• - Began removing chipset again to fix my white CoreGrafx PCE now, while doing that, melted some of the plastic on knobs   they still turn, though.

My postmortum on this guy:

• - Was given to me for a recap, which was successful on first try
• - Was given back years later for speaker not working. Likely broken headphone jack (testing seems to confirm this).  Wired up audio signals using retrosix guide and speaker "hiss" comes back
• - +5v rail was shorted somehow in this test process.  Controller IC was getting *very* hot until it was removed.  Controller PCB replacement ordered
• - Current symptom ever since then

To help others, I am attaching here:

• - My board scans
• - Blank board scans (courtesy of retrosix)
• - TG-16 service manual
• - ASIC chip pinouts
• - Diode probe compare excel I made (comparing good unit FFC 40 pin cable with my bad unit)

[Segasonicfan]

Ran out of space in the last post for board scans (attached here).  Courtesty retrosix: https://www.retrosix.wiki/board-scans-pc-engine-gt
(note, their scan viewer seems down right now)