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caius

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Got for repair a faulty original 'P-47 - The Freedom Fighter' PCB, an horizontal shooter originally developed by NMK and published by Jaleco in 1988 running on Mega System 1 hardware :

PCB.jpg


The board was dead, no sign of life, only a steady black screen.First of all I checked with a logic probe if the RGB colors were active, they were all stuck :

RGB_stuck.jpg


I traced the signals back to the cutom SIL marked 'JK-02' :

JK-02_original.jpg


This part is basically an RGB DAC (like the Taito 'TC0070RGB' for example), doing some check with a multimeter I found its pin 15 was almost shorted to GROUND :

JK-02_RESET_signal_shorted.jpg


Pin 15 is the RESET input signal for the internal circuitry.I traced it back to pin 23 of a near surface mounted IC (a custom gate array from NEC) :

NEC_uPD65006.JPG


I lifted that pin and the short was still present on the 'JK-02', this confimed it was really bad.So I replaced it with a same part taken from a donor board (but with the intention of reproducing it at a later time).This gave me back the RGB output but all I got was a static garbage screen :

garbage_screen.jpg


Checking the 68000 main CPU revealed no proper /RESET signal, there was no transition on boot from a LOW to HIGH logic state :

RESET_main_CPU.jpg


The signal is generated by a typical circuit based on the 'PST518' voltage monitor :

RESET_circuit.jpg


Then it's inverted by a Schmitt trigger (74LS14) but my logic probe detected no signal on this IC :

20201110_195800.jpg


A closer insection revealed a broken trace from the near 10uF electrolytic capacitor to the 74LS14.The role of this capacitor in this reset circuit is primary because, when the power supply voltage drops below a certain threshold, it suddenly discharges generating thus the required reset signal.I restored the connection and finally the board booted into game but sprites were almost missing, they were displayed as a few lines :

View: https://youtu.be/bMFMKKy0rxw


Studying the hardware a little I figured out the part of the circuit that acts as sprites line buffer, there are four 62256 (32K x 8-bit) static RAMs:

sprite_line_buffer.JPG


I probed these RAMs with my scope, all of them had unheathy signals on data lines :

bad-data-lines.jpg


I removed the first two chips :

first-two-62256-removed.jpg


They failed the out-of-circuit so I replaced them.This improved things, sprites were more visible but not yet perfect.So I pulled the other two RAM chips, they were bad as well :

62256-reworking.jpg


Replacing them restored sprites completely.Playing some games revealed a new issue, controls were not working, both players' airplanes moved downward by themselves and crashed :

View: https://youtu.be/EpHlroJ0tKg


This is a common problem on Jaleco Mega System 1 hardware because inputs are handled by some custom SIL marked 'JK-03' whose legs most of times bend and lose connection with the internal circuitry.This was also my case, they were not in great shape :

JK-03-bad-2.jpg


I was able to salvage two of them but the third one was damaged beyond repair so I replaced it with a reproduction of mine :

JK-03-replaced.jpg


I was about to archive this repair as successful but playing the game I noticed some sound samples (like explosions, power-ups) were missing :

View: https://youtu.be/FAN0oKbMNGs


The samples are played by two OKI MSM6295 located on ROM board :

OKI-MSM6295.jpg


"Listening" with an audio probe the analog outputs of the two MSM6295 revealed the missing sounds were present and correctly played by one of them but then lost somewhere.Tracing the output back lead me to the cause :

missing-10u-F.jpg
free image upload

A 10uF electrolytic capacitor (that routes the analog signal from the OKI MSM6295 to the motherboard) was ripped.Fitted the missing part restored full sound and fixed board completely.Another repair successfuly accomplished.

View: https://youtu.be/ewGr-wAW0R0


Now the reproduction section of this repair.


As said above, the 'JK-'02' custom SIL was bad and, after replacing it with a same part, I later reproduced it :

JK-02-reproduction-mod.jpg


Testing of the 'JK-02' reproduction :

View: https://youtu.be/uDIjcnSDoRM


The Jaleco Mega System 1 hardware uses a third custom SIL marked 'JK-01' which is involved in graphics generation:

JK-01-original.jpg


I thought it was good for hardware preservation to reproduce this as well so I made it:

JK-01-reproduction.jpg


Testing of the 'JK-01' reproduction :

https://www.youtube.com/watch?v=il3sqORgF-s

Now the set is complete and we have a replacement for all these fragile Jaleco 'JK-0x' custom SIL.
 

djsheep

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Those modules near the JAMMA edge are super easy to bend, happened to me and thankfully still working even though it looks like the pins have bent right out the module on one them?
 

caius

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Those modules near the JAMMA edge are super easy to bend, happened to me and thankfully still working even though it looks like the pins have bent right out the module on one them?
Yes, this is a common issue on Mega System 1 hardware.The 'JK-03' bend very easily but keep in mind that only a row of legs is connected to the internal circuit, the other one is just for support.
 

djsheep

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I’m super lucky it bent the right way then :). I’m not bending it back.

Good work! And nice log :)
 

djsheep

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@AlxUnderBase did you try bumping your 5v up a bit? It’s a fairly power hungry board?
 

Hammy

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Quite an easy one compared to those caius has been looking at.
Lets just say, one man's fails are the masters winners :)
 
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