Got another Rastan for a repair.
This one had some vertical black lines and some text was wrong
Piggybacking sram IC2 and IC4 with a good ram brought some better changings so I decided to desolder them and test out of circuit
Both were reported bad
Game was 100% fixed
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.
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.
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:
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…