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
Sep 042018
 

Bought this game for personal collection with a problem on the RED colour which was completely missing.

Checking the schematics there are some 74541 chips which are sending the colour bits to a resistor network for Dac conversion. The one on the red colour @UC4 was correctly working so I checked the 74ls07@UA4  involved in the blanking circuit.

Pin 2 was always below 0.5V  and muted the total red colour

 

After changing it I restored the red colour

Sep 042018
 

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

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