Bought this classic as not tested but luckily it was working except the missile sound was completely missing.
Looking at the schematics I found this
In the game there are only two missile sounds.
Shot A is your ship missile sound, while Shot B is the one from some enemies
IC37 is an LM3900 op amp which is very unreliable and infact after changing it , the missile sound was restored
The game was working good except the sprites were all missing some lines
The board is using some unreliable drams 41464 from Fujitsu similar to the ones used on Konami Haunted Castle or CPS1 A board.
I bought some units from Aliexpress , put some handmade socktes and changed both to be 100% sure
Turned out only one of the two was faulty, in anycase the graphics were 100% fixed
I bought a Joust boardset declared 100% working.
When I tested it all the coin inputs didn’t work
I ran the test and infact they were all activated
I was not worried because I was sure it was the 6821P PIA on the romboard.
After replacing it didn’t fix the problem.
Looking at the schematic there are also some 4066 CMOS chips involved.
I desoldered the one used by the three coins inputs and after testing it on my programmer it was declared good.
I decided to add a socket and test another one on board
Luckily the problem was fixed and it turns out my programmer is not reliable in testing CMOS devices.
After reparing my Defender ( see this log: https://www.jammarcade.net/defender-repair-log/ ) I noticed that the screen had some horizontal lines of different brightness.
At first I didn’t give much attention but then I decided to look at this issue deeper.
Moving the screen left and right with the monitor commands I noticed these defects:
Having the experience of a similar problem on my Burnin Rubber ( https://www.jammarcade.net/burnin-rubber-repair-log/ ), to me it was clear there was a video blanking problem
With the schematics in hand I found this circuit:
Signals from 7420@5C were good so I started probing the 7400@4B
All signals were good except that pin 10 was floating.
Looking at the above mentioned schematics, pin 10 is tied to the nearby pin 11!
I checked the board and found out that actually the pcb has no trace connected the 2 pins togheter!
No broken traces, it is actually simply not there!
I connected the two pins and…..
Game fixed!
Now I am wondering if my CPU board revision has a production error.
Searching on internet I haven’t found a similar problem therefore I would like to have a feedback from other owners of Defender if their pcbs has the trace or not.
Received this Japanese version of Liquid Kids for a repair running on a different F2 hardware.
The game was playing fine but hadn’t any sounds.
Amplifier and preamps were good so I started to probe the sound circuit logic.
The Z80 sound cpu ram had some weak signals
After replacing it , the sound was restored.