SEGA System 16 repair log #2

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May 052017
 

I received this system 16 motherboard from an arcadeitalia member called Consoleman.

The motherboard was missing completely the sound and didn’t sync with the monitor.

After checking with my sound probe that the sound was really missing by probing the pre amps, I started to check the Z80 which was confirmed good and RAM which was faulty.

Changing the ram restored the sound completely.

The Sync problem was not easy to check because the TTL which outputs the signal to the finger board is under the romboard.

In anycase I managed to test it from the solder side and I noticed the composite signal was about 14khz, that is why was not syncing.

I proceeded to desolder it

and tested out of circuit with my programmer as good….

Instead of soldering the same part, I decided to find another 74Ls125 among my junk boards.

It was not common but in the end I found it on another board and soldered it.

Motherboard was 100% fixed.

 

Probably my programmer hadn’t enough sensibility to declare the part as bad.  First time it happens.

Mar 162017
 

I bought this game for my collection declared perfectly working.

Flashgal runs on the very same pcb of other two games developed by Kyugo which are 99 Last War and Legend.

On the bottom pcb it is silkscreened CVG-48C which is very similar to the codes used by ORCA on their hardware

 

Anyway  it seems that in these days I am very unlucky because the game once fired up had a sync problem

 

Tracing back from the SYNC pin I came to a 74LS08@2P

With the frequency counter I got very unstable readings so I proceeded to piggyback it with a good 74LS08 which restored the sync.

After changing it, it tested good out of circuit, yet with the new TTL in place I got a stable sync….

The screen looked very reddish so there was obiviously a palette problem

I checked the palette rams and they were all good so I dumped the colour proms until I found the RED component one @1J which didn’t match anything in MAME.

The prom is an 82s129 and it is almost impossible to find an empty replacement these days.

Therefore I went again for the Bprom to Gal replacement already used last year in another repair log:

Mad Gear repair log AKA reproduction of a bprom to GAL

Since the prom was the very same type I replaced for Mad Gear, I converted the prom file to the PLD equations using Elgen tool U2pa and I reused the same pin configurations which fits nicely without any major hardware mod except a jumper wire to connect GND to the correct pin on the GAL

I tested the the game but I noticed something strange:

The colours were right but there were glitches on the left and right borders and also a flickering red component across all the screen (which cannot be seen on the static image offcourse)

At first I thought it was an access time problem with my GAL , therefore I burned the fastest one I had  but no luck.

I tested directly the signals of the PROMS and  found out that the CE pins were not tight to GND but were controlled by another ttl.

Soon it became clear why it didn’t work: the GAL was sending datas out of sync  therefore producing artifacts.

Since I am not a programmer I asked to Elgen and Caius (who asked to Porchy) an advice how to replicate on the PLD  a tristate behaviour.

All of them were very kind and in few minutes they informed me how tristate works on PLD equations

I added the needed modifications to my PLD equations by declaring each data pin  enabled when pins CE1 and CE2 were low.

 

This completely fixed the colours on the pcb with no artifacts:

 

Again a big thanks to Elgen from http://elgensrepairs.blogspot.it/ for the invaluable tool  and to Caius and Porchy for being always very helful

 

Shinobi repair log

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Feb 092017
 

This pcb from my collection developed lately a sound problem only with the jump sound which is a sample

 

This particular sample was distorted, the others were all OK

The clock and the 7759 sound chip which is responsible for the samples I was sure they were OK otherwise

all the samples would have the issues

I dumped the sound program rom 11372 and the samples roms 11288 and 11289 expecting that one of them could be corrupted

but surprisingly everything was fine.

If you have noticed, I still the original NEC module with a suicide battery from 1987. The battery is still working after all these years but unfortunately cannot be changed since the module is one piece injected with epoxy.

I started to think that the module could have the decryption key stored in ram somewhat corrupted after all these years and this could affect of course the sounds

Therefore I proceeded to use a z80 with the decrypted program rom taken from a mame set which use the same 5358 rom board (it is currently set 5)

The jump sound was correct with decrypted rom, confirming my theory that the key in the module is corrupted.

Mar 292016
 

Sometimes it is very annoying when you have placed your arcade monitor vertically and certain games are shown upside down and  even worse they have no option to flip the screen.

It happens very often with older games but sometimes even with newer games from the 90s.

In these cases you have 2 options: reverse phisically your monitor or reverse the yoke  on the pcb of the monitor (not very safe).

Now I show you a new option available only when the games were also prepared for cocktail tables (that means they have already the circuit to flip the screen for the second player)

In this first “flip screen” hack I will take Zaxxon / Future Spy hardware as an example

 

Future Spy_2

These games have no flip screen option but they have the preparation for cocktail tables play.

The board is very big but luckily schematics are available and they are very clear to understand.

The hack is very simple: input nr.4 of 74ls367@U64  is normally LOW for first player game and HI for second player game.

We have only to force pin 4 to HI (+5V) using a wire so that the screen is always flipped as it was playing the second player even in upright mode.

Zaxxo_flip screen hack

 

FS

 

Very simple and effective!

Quartet repair log

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Mar 042016
 

Here is a recently repaired Quartet running on Sega pre-system 16 hardware.

quartet1

The 3 replaced chips are highlighted in red. I will explain every step of the repair process below.

1) The game was booting to a garbled screen:

quartet2

The 74LS04 located at 1H (an inverter, manufactured by Fujitsu) had good clock signals on entry pins 11 and 13 but had floating signals on outputs pins 10 and 12. Replacing the chip changed the result on boot…

2) I then got a screen with garbled graphics but with a countdown appearing at the center.

quartet3

This is normal on pre-system 16 games. Normally, after reaching 0, the game is booting. Here, I got a black screen after the countdown.

I noticed that the reset pin on the 68000 had a square looking wave signal, which was pretty weird (it should be a linear signal at +5V). The chip that generates the reset signal is a MB3771 located at 1G (again, manufactured by Fujitsu). Probing its pins made the game sometimes booting. I replaced it by a new 3771 chip and the game was now starting normally.

quartet4

3) The game was working and playing fine except for the FM sound that was inoperative (only the voices could be hear). It was due to the YM3012 located at 1B (a DAC). It had good looking signals on its inputs but nothing seemed to get out of its outputs. After replacing it I got the FM sound back.

Now the game is fully working.