Wrong/lacking colours on screen

[Macross_VF1]

God damnit, something is broken, third time I write this. Anyway, last week I managed to supply my 6128 plus with too much voltage and I asked for some help about it in a separate thread. At the time the only damage seemed to be the Basic cartridge and one RAM chip. However, now that I have a fully wired RGB cable for it I've managed to find a new problem.


When I connect it to a CRT the colours are wrong, mostly black as you can see. If I connect resistors on the R, G, and B lines, some of the colour return but not completely (forgot to take a picture of that though). This lack of colour didn't appear before the voltage accident. What is the problem and can it be fixed?

[Macross_VF1]

This site really doesn't like pictures it would seem.

[Macross_VF1]

Have to add them one at a time in separate posts for it to work.

[Bryce]

Not good. How come you only noticed it niow? Does it display properly on any other monitor?

Either way, here's how the colours work and what you need to test:
Each colour is created with 4 bits from the ASIC. Red is on pins 113 -> 116, Green is 109 -> 112 and Blue is 104 -> 107. Set the background to fully red, fully green and fully blue one after another and check that all pins associated with the chosen colour are putting out 5V. If all are working you're winning, anything after this is easily replacable.

Next in line is the DAC (Digital to analogue converter) IC115. This converts the 3 4-bit colours to analogue values. You should be able to measure these with a scope on pins 17 (Red), 16 (Green) and 15 (Blue). If these are missing or too low, either the DAC has failed or one of the discrete components between the DAC and the output socket is pulling the signal low. You'll have to then start isolating the parts to determine which one is bad.

Bryce.

[pelrun]

However, now that I have a fully wired RGB cable for it I've managed to find a new problem.

You say "now", does that mean you were using something different beforehand?

[pelrun]

Also, dark images from consoles when using RGB/SCART inputs is very often a symptom of the console trying to drive current into a DC-coupled input when it's not designed for it. RGB/SCART cables often come with series capacitors on each signal line to correct this, but often they're left out.


https://db-electronics.ca/2016/06/30/dbgrafx-booster-not-every-scart-cable-is-created-equal/

[Macross_VF1]

   I've figured out the problem. I'm a stupid git and this is my story.

Not good. How come you only noticed it niow? Does it display properly on any other monitor?

You say "now", does that mean you were using something different beforehand?

When the accident happened I had recently tested the system with what basically amounts to a home made test harness. I have a lot of old consoles and computers (over 175) and I'm currently trying to modernize and update as many as possible, first and foremost getting better cables and/or modding for RGB. After the voltage overload and subsequent removal of the fried RAM chip, I tested the system with what was supposed to become the RGB Scart cable, however it was only half finished, with only the RGB blanking signal, ground, sync and Red lines connected. However it gave a good picture albeit only red so I figured it was okay. Where was the the test harness, I hear you ask? Well, that I had already re-arranged for another system.

So I finished the cable some days later and the problems I have described showed up. Today I fixed up the test harness in the proper arrangement for a 6128 plus and started measuring voltage on each line. I immediately saw that the RGB blanking signal was way lower than it was supposed to, only about 0.4V. And that was when it hit me. I had for some reason connected pin 3 as the blanking signal but that outputs luminance! When I originally tested the system with the harness I didn't connect pin 3 but bypassed it and used a convenient +5V point I found on the motherboard.

TL;DR? I'm lazy and forgetful and should probably don't be let loose on sensitive electronics. 

Now I did ask this in the other thread but I don't think I got a proper answer, can I replace the dead RAM chip with a D41464C-10? The original chip is a KM41464AP-12.

[gerald]

can I replace the dead RAM chip with a D41464C-10? The original chip is a KM41464AP-12.

Yes.
D41464C, KM41464AP, TMS4464, all were used in the Plus.
HY53C464 should do as well.

[Macross_VF1]

I just checked up on the chips, the 10 and 12 at the end seems to indicate access time. Won't it cause trouble if I mix chips with different access times or will they all adapt to the lowest common denominator?

[gerald]

I just checked up on the chips, the 10 and 12 at the end seems to indicate access time. Won't it cause trouble if I mix chips with different access times or will they all adapt to the lowest common denominator?

The required access time is 200ns, 10/12 mean 100ns/120ns which is more than fast enough