Pc Engine Duo Repair Log

[LostMonkey]

I recently picked up a couple of broken Duos to work on in my spare time.

One of them has several missing chips = just leaving it alone for now.

The other is complete, appears to have had chips remove/replaced/reflowed  - not really sure, they just look like they have some extra solder on the legs.

I have gone through and removed any obvious solder bridges on the chips before ever plugging it in.

When it was plugged in, there was no audio/video activity and no response from the CD drive when "Run" was pressed.  It was on for about a minute and I noticed that the left hand (when looking at the front) 7805 was super hot.  Probing around with the multimeter, I found that after that 7805, ground and output (5v) are shorted.

I found a couple of caps that appeared to be shorted, but they were just spots where ground was on one side and 5v on the other - the caps were fine.

I have since removed all of the caps and still have the short. 

Anyone know of any components on the Duo that may be prone to shorting like this?  I am having a hard time finding the culprit.   

The power to the CD side doesn't appear to be shorted.  I do have the other Duo to test continuity etc - so I have been slowly muddling through the two boards looking for oddities, but any advice would be appreciated.

[LostMonkey]

Ok - well since I removed about 50 caps and it didn't change anything, I decided to go over the legs of all the chips with the braid, flux and iron just in case there was a short there some where.

....and success!  The short is gone. 

Now I get to put those caps back...

[LostMonkey]

Got the caps back in internal system rom will not boot.

HuCards (select few) will boot, but with completely corrupted graphics.  Sound seems normal.  This might be what the previous owner was trying to fix - hence the two Duos and one with all the chips missing...

I cannot get an Arcade card to boot in it either.

[LostMonkey]

So at this point, I removed the system rom chip and discovered that 6 of the pads had been lifted or were missing altogether.  I am currently in the process of rerouting those pins...

In the meantime, I have a video of the output. I am not sure if this is bad ram, bad Hu6270 or possibly more bad soldering on the chips that were removed and replaced by the previous owner.  I did remove and resolder one of the ram chips and did get a noticeable improvement (the video below is the improved version).

Note that some text is legible, but most sprites are scrambled.

[LostMonkey]

Well, it has been a while.  Got back to this system this past weekend (finally).

I decided to take out all the bodge wiring that I did back in 2022 (on the bios chip) and instead "fix" the pads underneath the chip.

In all there were 6 pads either completely gone or lifted.  These have now been replaced with solid core copper wire from the vias on the underside of the chip. Looking at it from above you wouldn't know they were repaired until you looked very closely and I like this much better.  It also appears to be 100% connected, all pins have been tested for connectivity with their documented destinations.  Fixing the pads also corrected another continuity problem that previously required a wire running over to the D91317GD.

After all this, the output was still pretty much exactly what was posted in the video above.

• No Bios loading
• HuCards loading with very garbled grafx
• Sound working fine
• System cards would not work.  If I could get a System card to boot, it would make a weird sound and then get stuck on "Just a moment"

So, I started mapping out all the IC's starting with the 6270.  I found in this process that all the IC's appear to be connected properly, but there was one leg lifted on a RAM chip.  I fixed this and now HuCards work normally.

Notes:

Bios does not load - voltage does change properly depending on the presence of a HuCard.

Cannot properly run from a System card. I have both a Super System Card and an Arcade Card.  I also tried running the bios roms from a Turbo Everdrive - no dice.

HuCards work great.

I replaced the backup SRAM.  System behaved exactly the same when the chip was removed completely and after the new one was installed.

When the BIOS chip  (IC515) was removed, the system also behaved exactly the same.

If I try to load or save a game which supports the "file cabinet", I get the same error you would on a system without backup RAM.  'no cabinet present'.

I have also gone over the whole board with a microscope and cleaned up and filled in any bad looking vias.  I have been filling them with solder from both sides and chasing out any corrosion.

At this point I am at a loss with what to try next.   I have an extra D91317GD on the other Duo (only main chip left on it), so I was thinking about swapping it in, but I am concerned that it may be a signal to the Bios chip that may be the problem.

 

[Keith Courage]

Common areas for bad traces to the CD bios is under or just behind the HU card slot and on the top of the board from where the actual bios chip sits. Usually it's easiest to just bride a wire from whatever trace is bad all the way from the Hu card pins to the bios chip itself instead of checking every single via or trace in between.

[LostMonkey]

Thanks.  I?ll try that next.

How common is the bios failing? 

[Keith Courage]

Rare, but it can happen. However, if just the bios is bad, you can test the system with a CD system card to see if the CD drive still starts up. It should start with a CD card inserted if it's just a faulty bios chip.

If the CD drive will not start up as well as the CD bios not showing, well then there's another problem causing both the bios and CD drive not to work. Both likely related to the same issue. Usually a bad trace somewhere but can also be caused by a faulty NEC D91317GD .

I just remembered that sometimes a faulty Save memory Sram chip or bad traces to it can cause the CD bios not to boot as well. So the Sanyo LC3517BML chip on top of the board above the Bios chip.

[LostMonkey]

Since my last post, I have checked all the traces between the bios and the NEC D91317GD.  Everything seems to be connected.

I removed the NEC D91317GD and checked all the vias under it.  I tried the system without the NEC D91317GD installed and it worked the same:

Hucards work, Everdrive works, system cards do not.     If I try loading an official system card from the Everdrive, the system makes a "raspberry" noise and screen goes blank. 

I replaced the NEC D91317GD with another one from a 2nd Duo and everything is still the same.  No bios, cannot boot a system card.

[Keith Courage]

Try changing the Save memory chip and check the via hole connections just below it. Sanyo LC3517BML chip on top of the board above the Bios chip. I worked on a DUO recently where it couldn't load the CD bios even with a CD system card. Changed out the Save ram chip and repaired a bad trace to the chip and the Super CD bios for the DUO started working again.