The project recently has undergone a redesign and now it's mature for a release.I changed the layout of the main board and added a control sub-board.Now everything can lie inside the console :
Like the previous version, the emulator is powered by the open-source Flash Floppy firmware and it loads disk images from an USB storage (thumb drive or HDD).
'The Thing' can be installed normally on Marty consoles modded with DocBrown, the optical drive emulator designed by Deunan :
Received for repair a V-Five PCB, a vertical shooter arcade video game originally developed and published by Toaplan and also know as 'V-V / Grind Stormer'.The board was in very good condition :
But it booted all the time to a 'PALLET RAM ERROR' message on screen (I think 'pallet' is a japanese misspelling of 'palette')
The palette RAMs are two 2K x 8-bit devices @U4-U5 (Sharp LH5116 the exact part used)
Probing them with a logic probe revealed some stuck outputs of the device @U5 :
I promptly pulled the chip and tested it out circuit.It failed :
With a good RAM chip fitted the board finally booted into game:
I wanted to play some games for testing further the board but I could not coin up with player 1.Pin 16 parts side of JAMMA connector was stuck low :
I traced the input back to a custom resistor/capacitor array marked 'RC8528' :
Measuring the resistance of the internal resistors (which act as pull-up) I found one with a value lower (588 Ohm) than others (4.7K Ohm)
The part was bad internally but I had no spares of it.I remembered some years ago I reproduced a similar custom resistor/capacitor array used on NMK PCBs (marked as 'NMK-006') so, after figured out the correct resistors values, I assembled one and installed it :
But I was still unable to coin up.Still going backwards there is a 74LS240 @U29, input pin 8 was almost shorted to GROUND :
Pulled the device and tested it out-of-circuit, it failed in the exact logic gate unit :
Fitted a good chip and I was finally able to coin up.All seemed working fine but going futher into the game I noticed some sprites were glitched :
All the graphics are generated by the custom ASIC ' GP9001' which produces the addresses for six Fujitsu MB81C4256 (256K x 4-bit) dynamic RAMs and read back their data :
I found the datasheet of this DRAM and looked at pinout :
Probing the data outputs revealed unhealthy signals of the device @U17, you can see them on the right of the below picture (compared with good signals on left)
The device failed when tested out-of-circuit:
A fresh RAM chip was installed on machine tooled socket to finish the job :
Who among you does not know and has never played Bubble Bobble in his arcade life? Personally I love this game and I was looking for an original board since ages but never been lucky to get one so I always ended up settling for bootlegs.But recently I spotted on eBay a legit PCB from Taito.The description said the board was in good condition but not working due to missing security chip.Despite this I made some bids and won the auction (final price was not really cheap though...).When I received the board there were no surprises to me, the chip @IC17 on top board was missing hence the PCB was dead :
This chip is, indeed, the custom MCU with Bub silkscreening, here's a picture of it found on the net:
Some years ago this chip has been decapped and internal ROM extracted improving the MAME emulation that used a simulation of the chip behaviour until that moment.You can read more about on the blog of Nicola Salmoria (the original developer of MAME)
Thanks to this brilliant work we know now that the Taito 'PS4' custom IC is a Motorola 6801U4 MCU in disguise with some additional features.Since the internal ROM was dumped the only chance to repair my board was to get a chip and program it with MAME dump.Sadly I found no MC6801U4 around but only MC68701U4 which is pin to pin compatible.The chip arrived and thanks to one of my programmers I was able to program it with the file in the MAME ROM set :
With MCU installed the PCB came back to life, the game was fully playable with sound but some parts of graphics were wrong or missing :
All the graphics are generated by the bottom video board :
Data are stored in 12 27256 EPROMs :
I dumped all of them and did a compare with MAME, two files were unknown as they didn't match any existing ROM sets :
Trying the bad dumps in MAME exactly reproduced the board issue :
Reprogramming two blank EPROMs fixed the graphics.Playing some games I noticed the sound was quiet also on highest volume setting , I fixed this issue by fully recapping the sound section :
The repair was now complete, board was 100% working but I thought that solution to replace the 'PS4' custom MCU with a MC6801U/MC68701U4 was not accessible to everyone because very few programmer supports this obsolete device.So, I looked for another solution and found that some bootlegs use a little sub-board with a different MCU and some glue logics as replacement of the 'PS4' custom IC.I managed to find such bootleg and did a modern reproduction of this replacement, it worked fine when I tested it :
Another successful repair and another custom IC preserved!
Got for repair a 1943 : Midway Kaisen PCB (the japanese release of 1943 - The Battle of Midway).The board, a two stack one, was in good condition :
But according to the owner the PCB lost its sound after he connected it backwards.I had confirm of the issue as soon as I powered the board up :
Using my audio probe I was able to figure out that the lack of sound had a digital nature.Indeed, probing the Z80 audio CPU revealed the /INT line (pin 16) was asserted all the time :
This means a maskable interrupt to the CPU was triggered by an external
I/O device.This causes execution to jump to a specific interrupt vector
(which is some code at a fixed location) so the CPU was not properly running.
The Z80 CPU was socketed by factory so this was a good chance to fire up my Fluke 9010A troubleshooter and perform some tests.Looking at the sound memory map in MAME source the Z80 RAM lies from 0xc000 to 0xc7ff of the address space of the CPU :
On the board the device is, indeed, a 2K x 8-bit static RAM, specifically a Toshiba TMM2015 which is well known prone to failure :
I set up the Fluke 9010A for a RAM long test which failed at initial address :
This meant the RAM chip was likely bad so I removed it :
The pulled chip failed, indeed, the out-of-circuit testing :
I fitted a socket and a good RAM chip :
This restored the sound and fixed the board completely.Another successful repair.
Recently I've been asked to look at the Taito 'PC050CM' in order to study a replacement.It's another custom IC you can find on arcade PCBs from '80 like Rastan, Rainbow Islands, Operation Wolf, Chuka Taisen, Dr. Toppel Tankenkai, Kuri Kinton, Continental Circus and others.The chip has a SIL28 ceramic package coated by epoxy :
Regarding its functionality, the 'PC050CM' handles coin inputs and counters/lockouts as Rastan schematics shows :
If you encounter a bad 'PC050CM' the board won't boot and you will get an error message on screen:
I took on the task to study a replacement for this chip.First of all I tried to "decap" it but it was useless because there are only bare silicon dies under the coating.So, I used a different approach and tried to reproduce the functions building my guess on previous Taito hardware.I made a design based off the 'PC030CM' which is a similar Taito custom IC which I aready reproduced in the past :
The board was in very good overall shape but, when I powered it up, I got only a solid black screen,no video output.Checking with a logic probe the RGB colors revealed they were inactive :
The 68000 main CPU was running so I made some check with a video probe (a simple device that can display data on screen by using one of the RGB colors, in my case I used the GREEN one).I went to touch with the probe the GFX EPROMs and I got parts of graphics displayed on screen :
In this way I was able to figure that out the board was actually alive but playing 'blind' (with no sound, too).The final stage of the RGB colors generation is accomplished by some DACs in form of custom SILs marked '054573' (plus the '054574' that acts as filter/mixer).They converts into analog the digital signals of the '054338' custom ASIC (in QFP160 package) that receives, in turn, the digital bits from the three palette RAMs.
The outputs of the '054338' to the custom SILs were obviously inactive but also many of the inputs were in same state.Looking at Lethal Enforcers II schematics (which run on System GX hardware but shares same design) I figured out the '054338' receives some color bits from the '055555' custom (in QFP208 package) :
On schematics these signals are labeled 'PCOL' :
Probing them revealed they were all stuck so likely the chip was faulty.Having some spare I decided to replace it :
The spare was soldered and area was cleaned from flux :
At power up I was finally greeted by the POST screen, the '055555' was really bad :
Board successfully booted, game was fully playable but, as previously said, the sound was missing at all :
Obviously my first suspicions fell on the '054986A' hybrid audio module which was in rough state :
I could have tried to replace the electrolytic capacitors on top but I settled things once and for all and opted to replace it with a reproduction of mine.The original module came off quite easily using hot air :
The reproduction was installed on a pair of header pin strips :
This gave sound back to this cool game.Board 100% fixed and another repair successfully accomplished.
