A quick repair for this Sky Kid (Namco) I received some days ago :
Board was in near mint condition but it didn’t boot sitting on a semi-static garbage screen (which was looping sign that watchdog was active)
Most of the devices were socketed so for first I ruled out the many custom ICs present, they were all working.Then I tested the RAMs out of circuit.Two Mitsubishi M58725P (2k x 8-bit static RAM) failed (at different address location)
Replaced them with compatible 6116 and board successfully booted with no further issue.Job done.
A friend got a Mahou Daisakusen (japanese version of Sorcer Striker) board with faulty graphics.
It was playing fine with sound but there was an obvious color palette problem.
I started looking at signals around the color outputs of the PCB and quickly found weak signals on the 74HCT273 at location U12. Piggybacking the chip with a working 74LS273 fixed the colors immediately.
I replaced the chip… Board fixed.
Here is a picture of the PCB with the defective chip highlighted in red:
Received some Irem 92 boardsets mostly for replacing preventively the infamous ELNA brown electrolytic capacitors but two of them were faulty too.
The first was a Cross Blades (japanese version of Blade Master!), board had green ELNA capacitors so no strict need to replace them.
But it suffered from sprites isssue, they were missing some parts:
Pressing the board in the area of ‘NANAO GA22’ custom ASIC restored the sprites :
A closer inspection of this custom revealed three liften pins :
Reflowed them fixed the issue :
The second board was Hook (World version) :
Game booted up but sound was missing, I could hear only a rustling noise.Besides, video had some interferences:
Usually the two issues are strictly related, the cause is some bad electrolytic capacitor in audio circuit which affects video output too.The board mounted ELNA brown capacitors which are well known to age poorly.Sure enough :
The 470uF 25V @C202 was clearly leaking, this capacitor is used as filter for the +12V for the amplifier, this explained the issue.Measuring it out of circuit confirmed it was bad with an increased ESR of 150 Ohm :
I replaced this and all other electrolytic capacitors, this fixed board completely:
Here’s for reference the capacitors list with value and location :
Game was resetting over and over right after the first stating screen without even the possibility to see the diagnostic screen “RAM ROM OK”
The code was running and the watchdog was triggered by some event which I eventually found in a faulty 2114 SRAM @E15 on the video board
Marked in red on the actual pcb:
After changing it, the boot process went on until it displayed RAM and ROM OK but it reset again just before screen with the white grid.
At this time I was complete blind , missing any programming skills I couldn’t check what the code missed to go on.
I had a suspect on the sound part and after checking the clocks on the 8039 MCU I noticed it was completely missing.
The oscillator was working correctly and from the schematics I could see that the clock for the 8039 and the AY is generated using some 4bit binary counters 74ls293, very difficult to find on other boards.
Probing pin 9 of 74ls293@A12 it was stuck high while on pin 10 I had the clock.
I proceeded to test it out of circuit but was tested good.
Not sure about the reliability of the tester, I found a Starforce boot which use one 74ls293 and I installed it in a socket on megazone.
Still reset and no clock from pin 9
The output of the IC@A12 goes to pin 8 of a 74ls240, therefore I decided to test pin 8 of IC @D14 against +5V and it gave me a short.
After changing it the game finally booted without any other faults
This is a quick double repair log of Liquid Kids, a cute platform game release by Taito in 1990 on “F2 System” hardware.
The first PCB:
It played fine but sprites were blocky:
This Taito board like many other uses 4Mbit MASK ROMs to store GFX data, they are well known to be prone to failure.When I went to read the one @IC54 containing sprites data my programmer complained about pin 32:
The device was internally damaged and its dump was bad so I replaced it with a pin to pin compatible 27C400 EPROM, this fixed the issue and board completely.
The second PCB:
It was dead giving just a black screen on power up.Probing the main 68000 CPU revealed the /RESET and /HALT lines were both asserted all the time.The /RESET signal (and /HALT which is derivated from the first) is generated by the usual circuit made of a voltage monitor (in this case an MB3771) and some external components but it’s not directly tied to 68000 but it goes to the custom IC marked ‘TC0220IOC’ (which handles I/O too) which outputs it on its pin 8 :
Lifting this pin confirmed the custom was generating a stuck /RESET signal.Since by-passing the IC is not possible (device is addressable by main CPU) the only choice was replacing it, this is for sure another part very prone to failure, I can say from my experience that when a board that uses it is not booting then this custom is most likely bad :
