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 fry) so I reproduced it as well:

Testing both reproductions on the repaired Vigilante PCB:

 Posted by at 11:15 pm
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
Aug 122018
 

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!

 Posted by at 9:54 pm
Aug 072018
 

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.

 Posted by at 6:08 pm
Aug 032018
 

Yet another Truxton II PCB (overseas release of Tatsuin Oh) on the bench and always from Portugal :

The board was stuck on boot on a colored striped static screen :

Main 68000 CPU was not running, both data/address bus were inactive.For first I checked the ‘GP9001’ custom GFX controller  (QFP 208 pin ) which is a common issue on hardware that use it:

I found some lifted pins:

Reflowing them didn’t lead to any improvement.The board uses the ‘infamous’ custom ‘HK-1000’ custom (the early fragile ceramic revision) which handes inputs :

A closer inspection revealed the IC was damaged, some pins were broken at package insertion so beyond repair:

As said, the ‘HK-1000’ handles inputs but a faulty one can prevent the game to boot because some CPU address lines are used to generate the enable signals for it.I removed it:

Without the custom the board successfully booted up but obviously game was not playable due not working controls:

So I installed two strips of 2.54mm female machined pin headers in order to host a reproduction of mine:

The last issue I had to fix was some rustling background noise:

With the help of my audio probe I quickly figured out the sound was clean before reaching the 2.2K sound potentiometer :

I replaced it and this restored a clear sound.As ‘icing on the cake’ I removed an ugly hack to use a quartz instead of an oscillator and installed  the proper part:

Yes, yet another Truxton II PCB fixed!

 

 Posted by at 10:46 pm