Porchy

I have no background in electronics or programming. Everything I have learnt over the years has been through self teaching and the help from other kind people willing to lend a hand.

Jul 012018
 

Although this is a bootleg it follows the original PCB closely.
Couple of easy fixes on this one.
First a character issue and second a sound issue.


As you can see in the video above, the screen is filled with “A”s and none of the text is correct

There is a seperate EPROM used for characters on this game.

Pulling this and reading back with my programmer yielded a valid dump for Black Tiger.
Checking the adjacent associated 74LS273’s and 74LS245 showed I had at least one pin floating.
I traced this back to a nearby TMM2016 which had D7 floating.

Replacing this fixed the screen

The sound issue was even easier. The sound program EPROM was completely smashed. Replacing it fixed the issue.

Job done.

 Posted by at 11:49 am

Taito MB112S146 custom reproduction

 Reproductions  Comments Off on Taito MB112S146 custom reproduction
Jun 122018
 

Recently I have been working on reproducing the MB112S146 custom chip for my friend BanjoGuyOllie.
This module is found on Arkanoid, Darius and probably some other titles too.
This has now been done and is available in the downloads section. It required the adapter board from ColinD but I will soon compile for MikeJ’s module too.
So far its only tested on Arkanoid but probably will never get a chance to test on other hardware.

 Posted by at 7:39 pm

PLD Archive news

 General  Comments Off on PLD Archive news
Jun 102018
 

The PLD Archive is currently in the process of being moved to a new wiki.
There may be a delay is adding new files while the wiki get populated. This is currently over half way done but the process is long and tedious bear with us.
In addition to the normal PLD files there is also scope to add more detail and pictures so if anyone has any good quality PCB pictures or extra information like missing PLD IC location details or names then please let us know.

The old table will be kept up for now until we are sure all the detail has been moved over correctly.
You can take a look at the wiki at
http://www.jammarcade.net/wiki

 Posted by at 9:18 am
Jun 012018
 

I recently borrowed this Arkanoid PCB from NES4Life. I wanted it as part of a project to recreate the 28 pin custom chip that found on this, Darius and probably a couple of other Taito games of the same era.
He kindly loaned me this but knew it had issues so I agreed to look at this one rather than poke around on one of his workers.

First up the game would not boot, just watchdogged.
Using the schematics available in the download section I could easily track down a stuck bit on the data bus.

IC42 was to blame for this. Replacing it let the game boot but with all new issues.

You can see the game has booted straight into test mode and the colours are messed up.
One of the DIP switches is for test mode and the DIP’s are handled by the YM2149 chip. Looking at the databus when this chip is supposed to be active showed all data bits stuck.
Replacing this chip let the game boot.

Colours are still messed up so lets look into that. I chose to start at the 3 colour PROM’s

Probing the address pins revealed most were floating. Going backwards we come to a 74LS298 at location IC38. Replacing this then gave me this:


You can see now the screen is doubled up.
The graphics are contained within 3 EPROM’s. These EPROM’s are all addressed by 74LS298 chips before outputting their data to the custom bit shifters.
Checking these 298’s showed some stuck output pins on IC50.

Replacing this gave me a more normal screen but the bat sprite is being drawn half way up the screen and is doubled up.

Horizontal position has its own bus on the schematics

I confirmed the operation of the 74LS157’s first so moved onto the 74LS669’s. I’ve not dealt with these before and had none to hand either but I was fairly certain id narrowed it down to IC65 so I ordered some new ones up, desoldered the old one and tested it. Thankfully it failed.

Got the new chips a couple of days later and fitted.

Just the double bat issue to go.
The schematics also have a bus identified as OBJ which Im taking stands for Object. It made sense to me that the issue was somewhere along the same path as the previous positioning fault so armed with my best guess going from schematics I carried on following the path first of all checking the other 74LS669. This appeared to be OK and the RAM didn’t have any unusual looking signals on so I carried on. After these there is a 74LS374 on the OBJ bus. Probing the outputs of this found a single stuck bit. Replacing this gave me a seemingly fully operation game.
I cannot check the spinner inputs but all the sounds, DIP’s and regular inputs are working.

 Posted by at 7:44 pm

Sky Skipper repair log

 PCB Repair Logs  Comments Off on Sky Skipper repair log
May 052018
 

Recently I was sent a TNX1 Sky Skipper PCB for repair from my friend, Whitney.
It had developed a sprite fault which looked like this

As you can see in some places the sprites look fine but as it scrolls they start to disappear and flash etc.
I had already done quite a bit of work with these PCB’s and the regular Popeye PCB too so was fairly familiar with the whole setup.
I knew the main RAM also held the sprite data too which the DMA circuit then pushed out to sprite RAM during VBLANK.
With this in mind I was sure the main RAM was OK and enabling the test mode also backed this up.
I checked most of the DMA circuit out which is mainly just counters and they all seemed fine too..

I moved onto the RAM PCB next. I could rule out the 82S09 SRAM chips straight away as they are all socketed and I have spares.
The 5501 sprite buffer RAM was a little more difficult to check. While I was probing about these RAM’s with the scope I found the voltage wasnt stable and fluctuated every few seconds a little and also there was a big volt drop compared to the output of the PSU.
I confirmed that with this RAM PCB unplugged that the volt drop went away as did the fluctuation of voltage.
I hooked up this PCB to my bench PSU on its own and monitored the current draw and voltage. I later pinpointed these issues to the RAM that I had swapped in to test. They were older ceramic types and clearly they were drawing a lot more current than the original. Swapping them back cleared this issue but obviously was not going to solve my problem.

Moving on the whats seems to be getting called the “timing” PCB I started probing the counters and found a 74LS161 which had weak looking outputs.

*note this picture was taken after being changed hence its in a socket

I piggybacked a new 161 on top but with the output legs bent out so as not to make connection then probed again.

I didn’t really think it would clear the problem but as there was such a difference I swapped it out anyway

After checking the game I had this

Game is all fixed.
Going to do some minor servicing on this and then send it on its way home.

 Posted by at 2:59 pm