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…
The Toaplan “bonanza” goes on with this Truxton PCB on the bench:
Board was booting into game but sprites were totally scrambled:
This seems to be a common issue on this hardware.Sprites generation circuitry is quite wide with many components involved (an ASIC, counters, four MASK ROM, RAMs, a couple of Bipolar PROMs, etc..).I made a quick check and found nothing abnormal until I probed a 74S20 (Dual 4-input NAND Gate) @5D :
Both outputs were floating:
This was confirmed by logic analyzing :
Chip obviously failed the out-of-circuit testing:
I pulled and it and replaced it with a 74F20 (‘F’ logic sub-family of 74 TTL series has more or less same delay propagation time of the ‘S’ one ) :
Sprites restored and board 100% fixed.
Just a quick note : although the marking of the faulty TTL was partially deleted, I could recognize its brand.It was the only Fujitsu TTL on the board!
Recently I had two faulty Sunset Riders (by Konami) PCBs on the bench for repair.The first board came from Portugal:
Board was watchdogging in an endless loop, sign that no valid code was executing by main CPU:
Doing the usual check on CPU/RAM/ROM circuit revealed that a data line of a WORK RAM was stuck low:
Pin was almost shorted to ground:
This is shared with main CPU (pin 1, data line D4) and other devices too.Using a short locator I measured resistance to GROUND of all common points:
The lowest resistance was on pin 7 of a 74LS253 @14B:
The IC failed the out-of-circuit testing:
The board booted up but failed the POST showing a bad device @15B on an upside down screen:
The device concerned is the ER5911 serial EEPROM :
Someone previously replaced and socketed it but managed to rip the rivet of the pad of its pin 4 (which is the data bit output) which lost connection with the rest of board :
Once restored the connection I had to re-initialize the EEPROM:
After this the board successfully booted into game with no further issue.First board fixed.
The second board was in a lot of faulty PCBs I bought:
It booted up but jailbars were present all over the screen:
Lines are a clear sign that something is wrong with the graphics data (in this case the tilemap).I launched a MASK ROM check which found a bad device @16K:
I was about to replace the device when I gave a look on its soldeside and found a deep scratch :
Under a microscope two traces appeared to be severed, they were indeed two data lines of the MASK ROM (pin 13 ‘D0’ and pin 28 ‘D7’).A quick check with a multimeter in continuity confirmed it.
I restored connections with some AWG30 wire:
No more complain of MASK ROM check :
Board 100% fixed and double repair accomplished.
