PCB Repair Logs – Page 48

SEGA System 16 repair log #1

Feb 092017

My Alien Syndrome pcb one day decided to stop working and it showed a black screen.

I changed the battery 9 years ago and I thought it was suicided.

Just to be sure I tested it romboard and FD1094 module on another motherboard and it worked without problems.

I took a Tetris romboard then and tested on Alien Syndrome motherboard and this time the game showed a stuck reddish diagnostic screen which unfortunately I didn’t take a picture of.

Tested the inputs to see if there was a shorted one but there were all OK.

The program was running otherwise I wouldn’t get a stuck diagnostic screen therefore looking at the schematics I started to probe the 74LS253 (all Fujitsu parts) to see if the comunication with the cpu good.

The logic probe didn’t show stuck signals but I didn’t trust it and proceeded with piggybacking all of them with a good 253.

The game booted when I piggybacked the 74ls253@C15 but with wrong colours.

I desoldered it and tested it on my programmer and it was confirmed bad

After changing it I was welcomed with a running game but as said wrong colours

I ran the diagnostic memory test and the ram @J10 was given as bad.

Unluckily the ram was under the romboard therefore difficult to test in circuit

I decided to have another look at the schematics:

As you can see we have 2x 74ls244 which address both rams. The RAM given as bad was only J10 therefore it couldn’t be the 244.

After the 16 bits data lines we have 2x 74ls245 , and the first 8 bits from ram J10 are connected to the 74ls245 @H10.

Without even probing anything it was clear for me which chip to change blind

We had a Toshiba 2015 ram and a Fujitsu 74ls245.

Obiviously I decided to change the fujistu part

The guess was right because the game was fixed 100%

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.

Rastan repair log #4

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Feb 082017

I had on the bench this Rastan PCB:

Board suffered from severe graphical issues, sprites were absent and jailbars all over the screen.Sound was corrupted too:

I had the feeling that the backgrounds were actually good although the jailbars since the lines were changing as sprites moved so I went to check this part of circuit.Objects are generated by the custom ‘PC090’ which takes data from some 1Mbit MASK ROM and write them to four 2018 static RAMs:

Probing the four RAM revelead weak signals on data lines of the one @IC5:

Piggybacking a good RAM restored all graphics:

The chip failed when tested out of circuit:

But after I installed a new RAM on board, sprites were not good as when I made piggybacking, just blank with no colors :

Probing around I found a 74LS245 @IC85 which didn’t do its job :

This is used to exchange data between two bus called ‘IO BUS’ and ‘MD BUS’ on schematics:

Chip failed the out-of-circuit test:

Graphics were OK now so I moved on to trobleshoot the sound issue.I found three missing electrolytic capacitors in sound section : @C23 (1000uF 16V), C26 (10uF 16V), C28 (100uF 16V):

and replaced a bad one with increased ESR @C27 (100uF 16V) :

But this was not enough to restore a good sound.So I decided to use my new audio probe:

An audio probe is essentially a portable amplifier useful to test parts of an analog sound circuit by ‘hearing’ them and hence figure out what is actually faulty.Mine was designed by Phil Murr from UKVAC forums, credits to him:

https://www.ukvac.com/forum/topic355350.html

Using it revealed that sound was still good on input pin 6 of the TL074 OP-AMP @IC106 (used on final stage to mix FM music with PCM samples) but then was outputted distorted from pin 7 :

Replacing the OP-AMP cured the issue.Mission accomplished.

Rolling Thunder repair log #2

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Feb 042017

While I was playing this game from my collection ,I discovered it was missing all the samples (gun shots, walk and so on).

The game is a Namco system86 with an additional daughter board used for the samples

Before beginning the troubleshooting I searched on Internet for a similar problem and found a repair log

where all the caps on the daughter board weere changed , restoring the samples.

I am not a fan of changing caps randomly so thanks to Atari I checked the schematics and

found that the on the daughterboard there is a little circuit which produce 24V out of the 5V for the op amp.

Around that circuit there was a hissing noise therfore I checked if the 24 volts were present which was confirmed.

I proceeded with my sound probe to see if from the op amp I could hear the samples but when I turned on the game,

it continued to reset itself….the watchdog was active.

At this point I was a little puzzled and tried to disconnect the daughterboard which granted me with the game that booted

but with missing background and crashed as soon as the attract mode began.

I took some customs from various Namco games and checked if the game booted but no luck.

Also I hadn’t another Rolling Thunder to check the two customs on the daughter board therefore I put the pcb apart for sometime.

After some weeks i resumed the pcb and without much hope I checked various TTLs until I found a 74LS244@A1 on the daughterboard

which had pin 12 not oscillating properly while the input was healthy (Fujistu part….)

I piggybacked it with a good one and the game booted briefly correctly.

After changing it, I returned to the original state with the gameplay without samples.

I tested the output of the op amp TL074 with the sound probe and I could hear the samples.

Ttherefore there could only be one cause which was the capacitor @C1 placed between the output and the daughterboard connector, which infact was tested as open with an ESR meter

Changing it restored all the samples.

Lightning Fighters repair log #3

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Feb 042017

Received a Lightning Fighters for a repair.Board was in bad shape, some ICs were missing and solderside had a lot of scratches:

In the details, it missed two 4Mbit MASK ROMs used for sprites (but rounded machine-tooled strips were installed in place of them), the YM251 and MB3722 amplifier and its heatsink.Besides, the YM3012 DAC was partially desoldered and damaged:

At power up I was greeted by the typical flashing blank screen, board was constantly resetting due active watchdog circuit:

This meant that main CPU program code couldn’t be properly executed due a probable problem in 68000 busses.With the help of schematics I went to check this circuit and found a missing connection between data line D4 (pin 18) of the program ROM @E15 and corresponding data line (pin 1) of 68000:

Tracing back both pins I found a broken trace on solderside :

I restored the connection and board successfully booted with obviously no sprites and sound due missing components.I decided to fix the graphics for first so I burned two Macronix MX27C4100 EPROMs (same pinout of 27C400) to replace the two missing 4Mbit MASK ROMs @K2 and K8.In this way sprites reappeared but they were glitchy:

MASK ROMs check reported a bad device @K2 (and another one @C5 but this was due the missing YM2151 as I discovered later)

Again I checked connections of the device @K2 against schematics and found intermittent contact between pin 21-31 and VCC (pin 31 of a 27C400 EPROM must tied HIGH to enable the Word-wide organization of data).Who put hands on this board removing the two MASK ROMs and installing round machine-tooled managed to rip off the rivets which are inside the holes, this explains the poor contact of the two pins.I reinforced the connections with some AWG30 wiring them between the solderside of the pins and alternative VCC points :

Sprites were good now but still not perfect, some of them (like explosions) had jailbars:

I reflowed the ‘053244’ custom ASIC sprites generator:

Jailabars were gone:

All graphics were pefect now, confirmed also by the MASK ROMs check:

So only the sound needed to be restored.l fitted all the missing componens (the YM2151 and MB3722 amplifier with its heasink), removed the partially desoldered YM3012 replacing it and at same patched some broken traces on its solderside:

MASK ROMs check reported now success also on the device @C5 (which contains PCM samples)

Once entered in game I had confirm that sound was fully working.Another PCB saved from certain death!

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