Vintage Console Repair Help [Solved]

Legacy & Vintage
1

So I recently acquired a Fairchild Channel F, and it kinda works but has some issue with the video. Pictures are provided below showing my console output (RF being fed into my TVs coax) vs the expected output (from mess emulator).

Helpful links:

My thoughts so far

Since the issue is with the colors, I looked into that. There are four colors for the console stored as 2-bit values in VRAM (00=Green, 01=Red, 10=Blue, 11=Background/Off), and each one is acting differently.

  • Blue has no issues
  • Red is faded/off on every odd column
  • Green will sometimes appear as Blue on odd columns

This lead me to believe there was an issue with VRAM. More specifically, it looked like the MSB of the color is becoming high on odd columns. Like so:

  • Blue(10) → Blue(10)
  • Red(01) → Off(11)
  • Green(00) → Blue(10)

So next I looked at the schematics. The 2-bit VRAM is actually made of four MK4027 DRAM chips, where:

  • chip D7 = VRAM bit 1 odd column
  • chip D8 = VRAM bit 1 even column
  • chip D9 = VRAM bit 0 odd column
  • chip D10 = VRAM bit 0 even column

And so I believe chip D7 is the culprit. This would explain what I’m seeing, but I have no experience repairing electronics, so I could be completely wrong. What are your thoughts on the matter?

Pictures

comp1
comp18320×3120 18.3 MB

Start menu of Cartridge 2. These white bars don’t appear in game or anywhere else, so I’ll ignore them for now. Notice every Red odd column is off

comp2
comp29320×3120 15.7 MB

Game 1 of cartridge 2. Notice, every Red odd column is off, that nearly every Green odd column is blue, and that Blue is unaffected

comp3
comp39320×3120 13.2 MB

Start menu of built in games. Notice, every Red odd column is off, and that Blue is unaffected

comp5
comp58681×2562 11.6 MB

Built in game 1. Notice, every Red odd column is off, and that Blue is unaffected

comp6
comp68930×2902 13.5 MB

Built in game 1. Notice, every Red odd column is off, and that Blue is unaffected

1 Like
2

looks like your signal block in the console has bad components.
likely and aging capacitor.
its years old then i would recommend re-capping the entire motherboard.
and re-soldering all the joints on the rest of the components as they will have dried out over time.
that alone should fix the colour bleed and dirty analogue signal that looks to be the issue in the pics in the left column.

another suggestion for the colour shift, it might be artifacting due to being in ntsc or pal mode on a pal or ntsc screen.
it was deliberately done to get colours outside the hardware’s colour ranges.
but is showing up here as smearing due to your screens higher resolution.

3

I suppose it has been ~45 years, haha. Luckily there’s only 6 electrolytic capacitors. How does this sound as a game plan then:

  • Replace the electrolytic capacitors
  • re-soldering all the joints
  • Replace VRAM chips
  • replace the 7812 12v regulator (saw it mentioned)

The Fairchild has a USA (which is where I am) plug wired directly into it, so it’s probably not a PAL/NTSC thing? Still, I’ll look into it

1 Like
4

This is sick watched

Look up Adrian Black hes prolly hot somethin

1 Like
5

This sort of thing is often down to a bad power supply. If you have an oscilloscope you can look for ripple, but even without that you can simply replace the regulators and all the capacitors.

For future reference video issues like jailbars and scanlines getting dimmer based on content are often down to power problems.

1 Like
6

Ben Heck just posted a video repairing one of these. Maybe you can glean some nugget from it…

1 Like
7

Damn, love that guy. Thanks everyone for your suggestions and help! Looking around on some old apple related forums, I saw folks suggest the MK4095/4027/4015 as a possible replacement. I bought some Mostek MK4027N-1s off of eBay and the issue was resolved!

I pulled out the din pin from the ram chips so I could force them to +5V, and thus get a constant voltage on the Dout pin. The 330470 ICs output 10.35V - 10.65V, while the new MK4027N-1 outputs 10.93V, which is probably close enough.

1 Like