Prehistoric Isle in 1930 repair log

Recently I found in my pile of PCBs an original Prehistoric Isle in 1930, I remember I bought it as faulty on eBay some years ago but never had the chance to look at it for repair.Recently I decided it was time to finally do it.Board was in good condition :

As always I do before powering the boards up for first time I made some checks with a multimeter and found that there was almost a short between GROUND and +5V, only 9.7 Ohm of resistance exactly :

 The PCB is actually a two stack one with a top CPU board and bottom VIDEO one hence I separated them and figured out the short was in the bottom one :

As said, this was not what is called a 'dead short' with zero Ohm of resistance across GROUND and +5V , therefore it was impossible to locate it with normal equipment, whatever you are gonna to use (multimeter, ohmmeter, ESR meter) you would obtain always the same value of resistance all over the board.

At this point I had two options : desoldering all the components one by one until catching the almost shorted one(s) or resort to some special equipment that was able to locate them with a certain precision.After some reseraches I came across this tool, the 'LeakSeeker 89' from EDS Inc. :

 

It's essentially an autoranging fault locator (with a range from zero to 300 ohms) that can locate not just shorted, but 'leaky' parts as well.You can read more about here :

LeekSeeker 89

Here's how it works .

 

It was just what it suited me so I ordered a kit which came in the mail after some time.The assembing was pretty easy and quick :

After some practice with this equipment I was able to narrow the field of search of the short in this particular area of the board :

I started to removed some ICs in this area :

I found a couple of  faulty 74HCTLS157, both were from Samsung manufacturer.This lead me think they perhaps belonged to the same bad lot so I pulled the others in the involved area.Some were good but other three were bad for a total of five ICs of same type :

All were 'leaky' with low resistance between power supply pins, the 'LeekSeeker 89' made his job, it was really worth the buy, money well spent!

I installed good chips and powered up the board for first time.To my surprise it booted into game 

I played some games with no other issue found.So this was another succesfull repair and a great shoot 'em up added to my collection.

Toaplan 'GXL-02' reproduction

 The 'GXL-02' is a custom chip (in DIP42 package) found on most Toaplan arcade PCBs but you'll never encounter it with this part name but instead diffrently marked.Here's an overview:

"FDA MN53007T0A" on Hell Fire, Out Zone, Fire Shark :

‘TOAPLAN-02 M70H005" on Truxton/Tatsujin :

‘T.T-2’ on Twin Cobra/Kyuukyoku Tiger :

‘WT2’ on Wardner/Pyros :

 ‘L-02’ on Sky Shark/Hi Sho Zame :

 

 ‘12.02’ on Rally Bike/Dash Yarou :

But, despite different labels, the silkscreening under the chip says always 'GXL-02' .

Unlike the 'GXL-01' (already reproduced by me  Toaplan 'GXL-01' reproduction ) that generates backgrounds, the' GXL-02' handles sprites acting like a big counter.As usual, for my reproduction I took into account some bootlegs in order to study how the functions of this custom chip had been reversed by bootlegers.Unfortunately none of them was a complete TTLs implementtion of original chip but they gave me for sure hints on how the chip worked.

After some time spent adapting the designs from different bootlegs replacement boards I was able achieve a TTL prototype working for all games, here's testing on an Hell Fire PCB  :

The next step was pretty easy since I had a valid design in my hands so I engineered a proper board that was as much compact as possible without the use of modern programmable logics (CPLD or FPGA) but simple surface mounted TTL gates.The result was pretty good :

 The replacement worked fine on all PCBs I could try :

Hell Fire .

Out Zone :

 

Rally Bike :

 

Wardner/Pyros :

Rainbow Islands (original and Extra version) double repair log

Some days ago I received for repair a couple of Rainbow Islands PCBs, they were the original and Extra version.The game needs no introduction, it's the Taito masterpiece (released in 1987) that built a generation for video games.

The Extra version PCB was the first one I troubleshooted :

The board played absolutely fine but the sound was faint and corrupted :

Inspecting the board I noticed an electrolytic capacitor was installed in series between one output of the 'TL074' OP-AMP and the input of the potentiometer :

This was clearly a poor attempt to workaround the fault, the board indeed became totally mute if I removed this kludge.Using my audio probe I figured out the sound was correctly generated by the analog output of the 'YM3012' DAC:

But then it got worse before reaching the main amplfier.In the middle there are two OP-AMPs (a quad 'TL074' and a dual 'uPC4556') responsible of pre-amplifying and mixing the signals :

The 'TL074' is a well known to be an unreliable part, I have encountered tons of bad ones in all these years of repairs so I went straight to remove and replace it :

This improved things a little, now I could hear clear sound but it was still faint also at highest volume sting.So the other OP-AMP was most likely faulty too :

Removed and replaced with an equivalent part:

 

This brought back a lound and clear sound and fixed board completely :

 

The Rainbow Islands original version PCB was in good shape :

But it booted to an 'OBJECT RAM ERROR" message :

The object/sprite RAMs are four 2K x 8-bit devices (addressed by the near PGA sprite generator 'PC090' which process their data)

Probing the RAMs with a scope revealed unhealthy data signals on the one @IC50, here is below a comparison with a good signal where you can see the correct data transaction on the left snaphot :

I pulled the chip, it failed the out of circuit testing :

Fitted a socket and a good RAM chip :

Board booted into game and it was fully playable with no other issue.

End of (double) repair job.

Super Space Invaders '91 repair log

Recently I found in a lot of untested/faulty PCBs I bought an interesting and unusual  Super Space Invaders '91 (also known as Majestic Twelve: The Space Invaders Part IV).Game has been released by Taito in 1990, it's basically an enhanced version of the classic Space Invaders game and runs on F2 hardware :

When I powered the board up it surprisingly booted into game but graphics was glitched  :

I was under impression the garbage floating over the screen was actually sprites leftovers and I was right because it changed its pattern when I shorted pins of the Toshiba TC51832 pseudo-static RAMs (pin to pin compatible with 62256) which are part of the sprites circuit along with a couple of QFP custom ASICs that generates address to the RAMs and process their data :

The first suspects were these TC51832 pseudo-static RAMs because I often encountered bad ones during my repairs.So I went to probe them with my scope and I found a couple of them @IC29 and IC33 with unhealthy data signals.You can see in below picture a comparison with an good signal on the left, the data transaction appears to be complete and valid compared to the bad one on the right :

 So I removed the two RAM chips, they both failed the out-of-circuit testing :

 I installed sockets and spares: :

Board 100% fixed!

Another successful repair and a good game added to my collection.

Big Bang - Power Shooting repair log

Received for repair a Big Bang PCB (also known as Thunder Dragon ), a vertically scrolling shoot'em up released from NMK in 1993.The board was actually a factory conversion from Macross II :

It played absolutely fine but sound was missing at all:

A background buzzing noise suggested that analog circuit was doing its job hence the fault was located in the digital section.Most of its components (Z80/EPROM, YM2203 and samples MASK ROMs) were already socketed :

They were all good except the Z80 CPU which I tested as bad in another board.I swapped a good one but this didn't restore sound.So I fired up my Fluke 9010A troubleshooter and hooked up the Z80 POD to the board :

According to MAME sound memory map the RAM lies from 0xc000 to 0xdfff of Z80 address space :

map(0x8000, 0xbfff).bankr("audiobank"); /* banked ROM */

map(0xa000, 0xa000).nopr(); /* IRQ ack? watchdog? */

map(0xc000, 0xdfff).ram();

map(0xe001, 0xe001).w(FUNC(nmk16_state::macross2_sound_bank_w));

map(0xf000, 0xf000).r(m_soundlatch, FUNC(generic_latch_8_device::read)).w("soundlatch2", FUNC(generic_latch_8_device::write)); /* from 68000 */

map(0x0000, 0x7fff).rom();

I launched a RAM test at this specific offset, it failed meaning the chip was likely bad :

 

 

 I removed the RAM chip and one pin remained attached to the board :

I  installed a socket and new RAM :

Sound was restored but playing some games I noticed an entire music track was not correct because samples were played wrongly or randomly :

The audio samples (stored in two 16Mbit MASK ROMs) are played by two OKI MSM6295 ADPCM IC :

"Listening" with an audio probe to the analog output of them I was able to figure out which one was playing wrong samples.I promtly removed it with hot air : 

Then soldered in a spare :

 

It did the trick,  music was fully restored and board 100% working again.Another repair successfully accomplished.

DC2N5-LC : a 'low-cost' powerful Commodore Datassette emulator ...and much more!

Code name : 'DC2N5-LC':

It's the last version (the fifth) of a standalone device designed by Luigi Di Fraia whose purpose is to make digital backups of Commodore tapes. It can also be used as a replacement for the Commodore Datassette 'CN2' allowing you to playback tape image files.The suffix 'LC' stands for 'low-cost' but don't get fool by it because it has nothing to envy to the other commercial devices.It features a connector for the datassette port , a slot for a microSD card, where tape image files can be stored, and a cardedge connector, where a Datassette can be connected, to allow tape content imaging.

As said, it can playback .TAP files (V0, V1 and V2 supported) but what makes this device so special is the .IDX file support that allows you to playback multiload/multiside games & programs (a counter and counter reset buttom have been implemented as well).For a complete overview of its features you can visit the site of the developer :

DC2N version 5 Low Cost

This is really a must for every Commodore nostalgic so, please, support the amazing work made by Luigi Di Fraia and grab a unit! (as I happily did...)

Time Soldiers repair log and 'ALPHA-INPUT84/87' reproduction

Bought on eBay from the States a Time Soldiers PCB which arrived to me some days ago (by the way, thanks to 'coolmod' for his mediation and forwarding) :

Auction described the board as “TESTED NOT WORKING-GARBAGE ON SCREEN-MISSING CHIP”

The missing chip was actually one of four program ROMs:

I programmed a blank EPROM and finally powered the board up.All I got was a partially dimmed screen in which I could recognize the power-on TEST which probaby failed since the board kept resetting:

As usually I do, I started my troubleshooting with a visual inspection.Board was in very good shape but flipping it I noticed scratches on solder side, some quite deep :

A couple of traces looked really severed under the microscope and my multimeter in continuity test confirmed it :

 

Patching the broken traces fixed board completely :

 

The repair was pretty easy, just a quick fix hence it gave me time to look for something on the board to reproduce.The long custom SIL marked 'ALPHA-INPUT84' was a good candidate :

 

  

This custom IC handles inputs and Time Soldiers, being a rotary joystick game, uses four of them (two for 8-way joysticks of both players and the other two the 12-way rotary joysticks).Later Alpha-Denshi games like Gold Medalist use a part marked 'ALPHA-INPUT87' that externally is identical but with slight internal differences :

 

 

The two parts are not interchangeable but I was able to merge both designs in a single reproduction part: :

 

Testing on PCB was successful on Time Soldiers PCB, all directions of both players worked with 8-way joystick (could not test rotary because I don't have them)

 

Another PCB and custom IC preserved!