Graphics Glitch - DUO

[mamejay]

Hey Guys,

I finally decided to try and tackle a junk DUO I purchased some time back.
Basically the console works perfectly except for the graphics.
i get constant graphics glitches on any screen.  This happens even on first start up and the Super CD rom menu so I suspect it is something to do with the video ram.
Can anyone help me with this ?  i am pretty good at soldering so i could replace most of the chips on the motherboard.  Its just trying to identify which one is the correct one to work on.
i would prefer to fix it than junk it as I hate to see retro console get trashed.
Here are some pics of the issue and the motherboard.  Any help will be greatly appreciated

[mamejay]

I think the chip that is causing the issues is a Sanyo lc3564pm-10L.
This is the VRAM for the PC Engine.
Now all I need to to try and source out is a working chip from somewhere to test this.  Can anyone on the forum help?

[ApolloBoy]

I'd get on replacing those caps ASAP while you're at it. Doing so might even fix the video issue you're having.

[thesteve]

thats not the Vram for the system
look at IC904 and IC905 both V-RAM

[mamejay]

Thanks for that Steve.  So do you suspect it could be IC904 and IC9045 VRAM causing the issues?

[Duo_R]

Does the TG16 have the same part?

[mamejay]

I have no idea.  I would suspect yes but really cannot answer that one

[Duo_R]

What is the markings on the chip where Steve mentioned. I can check my junk TG unit if it has it

[mamejay]

Will need to check when i get home.  i found this thread
https://www.pcengine-fx.com/forums/index.php?topic=9946.0
On the schematic the IC904 and 905 are labeled with 20256 which I have found a few on the eBay.  Will confirm 100% but I want to make sure that this could possibly be the problem I have

[thesteve]

take  a logic probe across them and make sure all the data and addressing lines are active first.
a bad connection/trace to them will have the same results

[MottZilla]

I agree with thesteve. The RAM chips may not be bad. You might just have a bad/dry solder joint or maybe a damaged trace. You could try reflowing/resoldering the RAM chips and see if that fixes the issue. Judging by the partial graphics correct display, maybe one or more of the address lines on the RAM chip is dry/poor contact. I'd suggest locating the RAM chip (make sure it is the right chip) and get a pinout and figure out which pins are address lines. Resolder those. If you fix it I would certainly like to hear what you had to do to get it working.

[thesteve]

remember its 2 chips addressed the same (all pins except data D0-D7 tied to both chips the same)
each 8BIT wide (combined for 16Bit graphics)

[Keith Courage]

Has anyone ever come across a system where one of the two chips were actually bad?

[mamejay]

Looks like mine are stuffed. 
Found the culprits on the underside of the motherboard.  Here are some pic of them.  i do not have a logic probe but I traced every single pin on them and they all seem fine.
Touching the pins cause the image to change somewhat but is still not good so I suspect these are fried.  Anyone got a couple of these from a junk board i am can grab ??

[Duo_R]

I can donate from a junked tg-16 unit shoot me a pm

[thesteve]

often if touching the pin changes the pic its a bad trace, but an open chip could do the same
what pins made it change?
if it was an address line, its not getting connection to the vid chip

[mamejay]

Finally got around to getting some memory chips.  Sourced them from Germany

Started with the first one.  one leg was stuck and ripped the pad up so had to trace back to a point to solder a wire to it.


Decided to try it before changing the other one.  Success!!!


Thanks to all for the advice guys.  Another DUO saved from the scrap heap.

[thesteve]

awsome

[Keith Courage]

May I ask where you bough the chips from?I cannot seem to find them from anywhere that doesn't require an order of like 50 or more.

[mamejay]

May I ask where you bough the chips from?I cannot seem to find them from anywhere that doesn't require an order of like 50 or more.

http://www.ebay.com.au/itm/170895817675?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649
This is the guy i brought them off

[HercTNT]

love a happy ending, nice work!!

[Sensato Black Lion]

I've got a Duo with a similar problem here.

Some screens look perfectly fine, some are incredibly glitched. The glitch display pattern is different from one screen to another but pretty much the same pattern for a said screen each time I encounter it.

I do not know if the problem is only related the the Hucards since the system is unable to read discs at this time.

I have changed every caps, I've swapped IC 904, 905 and 906 with known working ones... no results, I get the same glitches.

Any idea what I should check next ? I'm thinking about reflowing the 6270's pins since it seems to communicate directly with IC 904, 905 and 906.


Sample pictures:

[mamejay]

Yeap that looks definetly like the memory modules issue to me.  I would be replacing those.

[Sensato Black Lion]

That's what I thought too. The only thing is, I've already replaced them and it didn't solve a thing.

These screenshots were taken after I replaced the memory chips. There are exactly the same as they were with the original memory chips.

[Sensato Black Lion]

Well well, after countless hours or probing with an oscilloscope and replacing ICs, I was about to give up and put this system aside for good. That is when, out of nowhere, my eye spotted some corrosion UNDER the Hucard slot. I don't know why I lifted the slot to look under it but, there it was, a LITTLE corrosion spot on a via. This is what it looked like after I cleaned it (can't really see well...):



It appeared rather innocent but just in case, I cleaned it and did a continuity test to see if everything was ok. Turns out that little corrosion had cut the trace. I couldn't believe it! Could that be it? I redid the trace like this:



Tried the system one last time, and it worked! At last! I must have spent a dozen hours trying to fix this.

It was definitely related to the VRAM chips but for some reason, when comparing the signals on the chips with a working Duo, I couldn't see any major difference.

That via was nowhere near a leaky cap, I'm really wondering just how it got eaten by corrosion like that.

Thanks to everyone who helped!

[mamejay]

Another one brought back from the scrap heap.  Well done on the find!!

[Fidde_se]

This is because of moisture in the air, when the pcb manufacturer makes the circuit board out of laminate (glassfiber) and copper they are blowing liquid solder over the whole board so that the copper don't corrode, sometimes this doesn't get everywhere and the copper is exposed, especially in the vias, usually a green varnish is covering the parts that are suppose to not get soldered, time and the elements will corrode exposed copper in time

[Fidde_se]

I just thought I should give you all a crash coarse in VIA fixin.

From the picture you seemed to fix it by just finding where it leads and then taking a simple wire, while it works terrific, when doing other stuff while it's open you might get stuck in the cable and rip something apart and the pro way is also so much nicer, anyway this is how, no astronaut stuff but here it goes.


Heres a VIA thats not working


Here it is again, a little closer =)


Take a scalpel and scrape some of the pads surrounding it, do it on both sides.


Take a single wire, theres lots of different diameters for vias, take one that fits.


Bend the wire over the scraped part, on both sides.


Solder it, both sides, DONE! check for continuity, it should now work.
Note that it may only work on PCBs 2000 and older as newer ones can have
like 16 layers and be connected at several places inside the via, but for PCE
stuff no worries. Take a cottonstick and some alcohol and take away some
of that flux.


Here is just another PIC with a ballpen to get the feeling of how small
they can be, even if this was a fairly large via.