Oct 222019
 

Another board from Mr Muddymusic and this ones a pain.
It was reported that this one has a short and sure enough when I checked resistance between GND and VCC I got pretty much a short

Using a Polar Toneohm micro-ohm meter I quickly found that the capacitor next to the edge connector had a lower reistance than at any other place on the board. Removing it cleared the short.

With that clear I could fire the board up and it was dead.
Checking the 68000 CPU I found I had a dead clock input.
The clock comes from the lower PCB and using the schematics show it coming in through a nearby 244.

Replacing it gave me my clock back but now we have some watchdogging.
Again I probed the CPU but all seemed fine so moving out I started checking the line drivers and buffers on the address and data pins until I came to a 74LS244 @ 15F with dead outputs.

Replacing this gave me a kind of booting game

Watching MAME run through the game it seemed to me that this was getting to the screen where it displays the POST results and then hangs. I confirmed in MAME that if a test fails then the board will hang at this screen.
I pulled all the ROM’s and they all checked out good so I turned my attention to the RAM.
The main RAM appeared to be good so I moved on to the other RAM that gets checked at startup.
The schematics show a 138 used for selecting different RAM. I found pin 15 was very weak

The enable was now fixed but it didn’t really change anything

As all the chars were a mess I decided to start looking at the character RAM section.
After a bit of probing I found that all the address lines to the 4416 DRAM’s were stuck high a lot of the time. I traced these back to another 244 over in the top corner of the CPU board.

Finally I had a booting game.

Instantly I see the colours are off and the sound doesn’t work. Starting a game also revealed the game started itself on coin up and I was unable to control anything. The ship also fired on its own.
Time to boot into test mode and see what was going on.
Test mode is invoked by switching DIP 3 on DIP bank 3 but the game just booted as normal.
I confirmed the switch was actually working using a multimeter but somewhere along the line it was going missing.

I traced it back using schematics to the 253 @ location 3F.

As the outputs of this chip are on the databus I struggled to see if it had real outputs or not so I just opted to replace it and it paid off, I could now access test mode

A couple of buttons seem to be stuck. Long story short all of the PS2401 octocouplers had some issue on one way or another so replaced them all with ones I found on a few scrap boards

Next, the dodgy colours.
Looking at the schematics there are a few LS09’s the output into the passive components that give the RGB outputs.
The 09 at location 3L had all its outputs stuck high.

I didn’t have any of these spare so I removed the offending chip, which failed an out of circuit test, and moved on to the sound fault.


The sound is stuck in this constant loop. I started probing the Z80 in the sound section and all the pins appeared to be doing what they should.
I moved on to the EPROM and while all the usual pins seemed to be doing their thing, the VCC pin wasn’t what it should have been.
Checking the pin of the socket showed +5v

but checking the pin of the actual EPROM in the socket showed I had logic low on there.

Wedging the logic probe tip between the socket and EPROM brought the sound back so I replaced the socket and the sound is fixed.

The 74LS09 replacement came the day after that and replaced it to fix this up

 Posted by at 4:52 pm
Oct 152019
 

Got K1ngarth3r’s Mega-Tech PCB here.

On these boards there is a daughterboard present which houses the Z80 used for the menu.

This daughterboard is used for book keeping and isn’t really required so to save space I fitted the Z80 directly into the slot the daughterboard previously occupied.

The fault was the board didn’t see slots 4, 6 or 7 as shown in the picture he sent

Initial inspection showed the EPROM for the menu was fitted upside down but it wasn’t dead and the menu wouldn’t work at all if this were the actual fault.

Using the partial schematics I made a few years ago I could quickly pinpoint an area to start checking from.

Using my latest favorite tool, Slice, I could easily see that for some reason the 3 aforementioned cartridge slots were never being addressed.

This is a bit weird as on initial look at the disassembled code it seemed that it just counted through the slots.
A bit more code scrutinizing revealed that the CXD1095 chips are read back between slot addressing.
Using the Fluke 9010 I could now see that at certain output configuration on the CXD1095’s would lock up the chip and skip whatever it was doing.
I wasn’t really surprised at this because every single Mega-Tech PCB I’ve looked at has had issues with these chips.
I ordered a couple of these chips up and waited….. and waited……… and waited until I got hit with a lovely little letter from Royal Mail telling me I owed them money for customs fee’s.
I paid this and a couple of days later I received my items.
I removed the old and fitted the new

Pleased to say this board is now working again

 Posted by at 8:40 pm

Double Dragon repair log #10

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Sep 292019
 

Another PCB from Olly. Not sure if this is a bootleg or not but it is certainly of lesser quality than the other Double Dragon boards I’ve had.
Anyway, on boot up we get this mess but at least it boots

I pulled all the ROM’s on the video board to start with and found the 4 tile ROM’s dead.

I replaced these with 27C512 EEPROM’s which then gave me this

Much better but all the sprites are missing.
Looking at the schematics I could follow them through testing from the ROM’s onwards.

I ended up at a 7425 where all the inputs were active but the output on pin 8 was dead

I replaced this and tested again. All the sprites were back on the title screen but the in game sprites had coloured blocks around them.

Seeing as though I had 4 dead ROM’s I decided it wasn’t too ridiculous a thought to think that some of sprite ROM’s were dead too.
After confirming that the sprite ROM’s were being selected correctly I pulled them all and checked them.
I found 1 dead ROM here

Again I replaced it and tested

All graphics now fixed, the sound however was a different story.

The sound effects were all messed up but I could hear the music was fine in the background.
I pulled the two PCM ROM’s and tested them. Both failed.
Replaced these too and tested

All fixed.
7 dead ROM’s and 1 TTL. Not sure whether the ROM’s are just getting too old now or if they were taken out by a voltage problem.

 Posted by at 5:42 pm

Xexex repair log #3

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Sep 222019
 

Got this board from Olly.

Board looks very clean and plays but has graphical issues

Initial visual inspection showed nothing. All voltages seemed fine too.
Next I started gently pressing down on the custom chips which made a lot of changes when I pressed on 053250

Found a bunch of pins on that were lifted.
I reflowed these and checked the rest of the chip which all seemed fine.
Next boot up test gave me a lot better results but there were jailbars down the whole screen sometimes.

Continuing pressing the custom chips I could make everything change again by pressing on custom 054157.

Found most of this row lifted

and few a few here

Reflowed those little guys and all is fixed in Xexex land.
Its a really nice game that I’ve never played before.

 Posted by at 4:40 pm
Sep 212019
 

Received from Austria this Heavy Barrel PCB, a run and gun arcade game released by Data East in 1987 :

Board was in really good shape and game was fully playable too but lacked of sound :

Using an audio probe revealed no sound came out from analog circuit hence the fault was of digital nature.Audio system is ruled by a 6502 CPU located on top board :

Its pinout :

Checking it with a logic probe revealed that the /NMI line (pin 6) was asserted as well as two address lines were stuck low (A12 and A14)

A non-maskable interrupt triggered is usually due to some hardware failure hence for first I dumped the ROM containing the audio code (located on bottom board), it turned out to be good.Then I focused on the RAM accessed by the 6502, a 8K x 8-bit device (6116 compatible)

As soon as I piggybacked it with a good chip I got some random sounds played.The scope revealed weak signals on some data lines of the RAM (a good signal on the left of below picture for comparison)

I removed the IC :

And sure enough, it failed the out-of-circuit testing :

New chip fitted :

Sound was thus restored and another board fixed.

 Posted by at 10:31 pm