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Dec 21, 2020
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Montevideo, Uruguay
Hi guys, I had a CPS3 board with the blue color missing. Basically everything looked either red, green or yellow.
Color bar test screen confirms blue color is completely absent.

Began troubleshooting the issue by measuring with an oscilloscope the blue color signal from its JAMMA edge pin, there was no signal at all.
Going deeper into the board, measured the pins of those 3 transistors that clearly would be each one for each color. There were healthy signals on the ones for Red and Green but nothing on the one for Blue.

So next step was to check the MC44200FU IC, which is a "Triple 8-bit Video DAC"

Measured its outputs (analog):
  • Pin 44 (Red): good
  • Pin 40 (Green): good
  • Pin 36 (Blue): nothing.
So, went to measure its digital inputs, Red and Green bits had activity of course, Blue did not at all, at any of its 5 bits (pins 21 to 25).

Note: although the DAC is 8-bit, it can be seen on the way it's wired that CPS3 uses only 5 bits per color, thus it could be determined that it can produce 32768 possible colors. (32 * 32 * 32)

So it seems issue is beyond the DAC, there are some 3383 ICs labelled CU383 (full name: SN74CBT3383), that are "10-Bit FET Bus Exchange Switches". Located the one where blue signals went in (marked as 8 D) and very carefully measured its pins trying not to short anything. This IC also has inputs and outputs, I could check that while it wasn't outputting nothing at all (unlike it's relatives), it was receiving good inputs! (didn't check where they come from, maybe color RAM?)

Since it had good inputs and no output, (and also good "enable" and switching "BX" signals present), I determined it wasn't working properly and decided to replace it.

Fortunately I have a heavily rusted spare CPS3 mobo where I could take a possibly good CU383 IC for this repair instead of having to order a new one and wait for it to arrive.
So, turned on the hot air station and started to work on this.

Labelled all its pins on a photo to measure them better afterwards. D ones are the inputs, B ones are the outputs. Brown and Black ones seem either NC or connected to GND.

Here comes the boring part:
After replacing it, it still didn't work!||, but after measuring its signals this time 1 blue output bit was present, probably it was the least significant bit because was basically invisible on my test TV. So it 1/5 of the IC worked. Because of this I had the theory I could have replaced it with a faulty IC so desoldered yet another one from the spare parts PCB and soldered it back on (took hours). That was the issue, it was faulty indeed, I guess its the risk of using parts from spare boards that were probably left at the outdoor for a long time.

It worked!, it was a bit of a pain soldering it back in, but that was it. Also checked color bars, blue is entirely present.:thumbsup: