Caius

Sep 172018
 

Another shoot’em up on the bench and still a PCB received from Portugal.This is the time of Air Buster  – Trouble Speciality Raid Unit-  released by Kaneko in 1990 .

Board booted to a static grey screen :

Touching the board while powered on I found the IC @D21 was really hot :

The IC is a Nec uPD41101C, a 910-word x 8-bit high-speed line buffer :

The data outputs pins were all stuck low:

I removed the chip and replaced it with a compatible uPD42101 :

The board sprang to life with the power on self test which reported all RAM/ROM devices as good:

But it kept to reset and restart again the POST in an endless loop.Randomly the board gave an error related to the SUB CPU which was generating a not maskable interrupt (NMI)

There are three PLDs on board and the one marked ‘PR-501A’ is involved in SUB CPU circuit :

Probing it revealed some of the outputs were stuck low, they showed few Ohms of resistance to GROUND compared to typical values:

I pulled the IC and read it in my programmer, although the device was protected this would have give me an idea about its functionality.I got  a read error which confirmed that it was really bad :

Luckily we have dump of this PAL in our database so I burned the JED in a GAL16V8 and the board finally booted into game.But colors were wrong and sound absent :

The lack of audio was due to a missing OKI MSM6295 and YM3014 DAC:

As for colors issue, piggybacking the two 2k x 8bit palette RAMs lead to some improvement :

So , since the RAMs were also from Sanyo manufacturer (not really reliable in my experience) , I pulled and replaced them although the chips were tested good out-of-circuit.This fixed the issue and board completely.

 Posted by at 10:53 pm
Aug 232018
 

Another faulty board from the “portughese” box, a Flying Shark (by Toaplan/Taito)

It booted up, game was fully playable with sound too but colors were completely wrong:

By shortcircuiting some address/data pins and observing the changes on the screen I was able to figure out the palette RAMs and consequently all the remaining colors circuitry:

The data bits from the two 6116 RAMs (2k x 8-bit devices) feed into two 74LS245 and being latched by two 74LS273 whose outputs are tied to three separate R2R resistor ladders (one for each RGB color) for the analog conversion.I found nothing abnormal until I probed the  74LS273 @27N, a couple of outputs were stuck low:

This was confirmed also by logic analyzing:

I removed the IC and tested it out-of-circuit,  it totally failed:

A good IC restored correct colors :

Board 100% fixed.End of job.

 

 Posted by at 11:17 pm

Twin Hawk repair log

 PCB Repair Logs  Comments Off on Twin Hawk repair log
Aug 202018
 

Got on the bench some days ago this Twin Hawk PCB (another board coming from Portugal)

Board booted up and game was playable but with bad graphics :

All the GFX data are stored in four 4Mbit MASK ROMs:

I dumped them in my programmer and the ones @3K and 3J resulted empty :

As proof of this I loaded the dumps into MAME and I was abble to reproduce exactly the same issue :

I replaced the two faulty MASK ROMs with 27c400 EPROM devices

This fixed graphics and board completely.

 Posted by at 4:38 pm

Vigilante repair log and Irem ‘RGLD8M472J221J’ – ‘RGSD10L471G’ reproduction

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Aug 152018
 

Got this Vigilante PCB (by Irem) in a trade some years ago:

Board was dead, stuck on a purple static screen:

As usually I started my troubleshooting with a visual inspection and noticed severe corrosion in some areas, especially around the 3.579545 MHz oscillator:

Analyzying the main Z80 CPU revealed no clock on pin 6.This signal was present on oscillator output but then was lost when routed to the inverter (a 74LS04 @IC30) :

Replacing the TTL restored clock on Z80 but board was still dead.Probing the /RESET pin 26 with an logic analyzer showed an unhealthy signal, there was no proper transition from LOW to HIGH but only some oscillations :

/RESET signal is generated by the MB3771 voltage monitor @IC31.I promptly replaced it:

In this way proper signal was restored:

But main Z80 was still inactive, data/address busses were silent as well as control lines.This lead me to think the CPU was faulty so I removed it:

Testing it in another board confirmed it was really bad.With a good CPU the board finally booted up but the sprites were mostly absent, I could see only some parts of them randomly flying over the screen:

 

After some time spent to check different things I pinpointed the fault in a bad interconnect ribbon cable.For safety I replaced both of them:

Board fully working again and a quite enjoyable game added to my collection.

The repair was accomplished but, as always I do, I visually inspected the board looking for some candidate parts for a reproduction and I spotted two possible ones.The first is marked ‘8M472J221J’  (‘RGLD8M472J221J’ on manual parts list)

It’s nothing more than a custom resistor network used for inputs, you can find it also on other Irem hardware like M72, M92 and M107.Here’s snippet from R-Type schematics:

I reproduced it this way  :

The latter, marked ’10L471G’ (‘RGSD10L471G’ on manual parts list)  is interesting since it’s a R2R  resistor ladder used to convert to analog the 5 palette digital bits outputted by the surface mounted ‘KNA91H014’ custom.You will find always three of them (one for each RGB color) coupled to one custom.

Its implementation on schematics:

From my experience and other references too the original part is not really reliable (it will crack or burn) so I reproduced it as well:

Testing both reproductions on the repaired Vigilante PCB:

 Posted by at 11:15 pm

Out Zone repair log #6

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Aug 152018
 

Toaplan forever!

Yes, yet another PCB on the bench from this legendary manufacturer, an Out Zone (Europe version) in very good condition:

But faulty, all I got was a black/white wavy striped screen on boot:

As we know, this is the clear sign that systen is not properly initialized and not running so further investigation was needed.Main CPU is a 68000 clocked at 10MHz, it was receiving a good /RESET signal but on its CLOCK pin I could measure a wrong oscillation:

This is how it should look (scope screenshot taken from a same working board)

The 10MHz clock signal is generated by an oscillator whose output is tied directly to pin 15 of 68000 CPU:

I replaced it :

It was reallly bad so board booted up now, game was fully playable with sound but sprites had vertical lines through:

By shorting some pins I was able to locate the involved circuit, this is sprite line buffer which is made of four 2k x 8-bit static RAMs whose data are latched by four 74LS373:

Piggybacking the RAMs didn’t lead to any improvement or worsening so I went to check the circuit against schematics and found a missing connection between pin 10 (data line D1) of the 2k x 8-bit SRAM @12-13L and pin 18 of a 74LS373 @9L :

A “quick and dirty” jumper wire did the trick:

Board 100% fixed.And the Toplan legend goes on…

 Posted by at 10:18 am