Jump to content

Fairchild Channel F Green Screen


Recommended Posts

I've been working on resurrecting a Channel F I recently acquired that was in pretty rough shape.  Replaced the axial capacitors, voltage regulators, and bridge rectifiers to get an image on screen, but now I'm faced with what would be a great background for streaming (and not gaming).  Does anyone have thoughts on what might be bad?  The service manual suggests a problem with the 12 volt line, but the points on the schematic I tested show that it seems to be good...


Thank you!

Unintentional Chroma Key.jpg

Link to comment
Share on other sites

Interesting, I was just about to post a similar question.  ?  My Channel F was working fine until I ran it for about 20min straight, at which point it displayed a green screen, and it now does that every time I turn it on.  I saw the same note about the 12v line in the service manual, and I found that at the UA78M12 voltage regulator, the input was 19v (too high?), but the output was close to 0v.  So I'm assuming this means the voltage regulator needs to be replaced.  My experience with this kind of thing is somewhat limited, but I'll update here if I find out anything more once I get a replacement.

Link to comment
Share on other sites

Yes, green screen  means check 12V.

If regulator outputs 12V correctly, check -5V on RAM and 12V on each 40pin chip - on the actual chip pins. 

Those chips are often oxidized black (silver content). It can help polishing that off as it may cause poor connection to the socket. Even with 12V in, you may not get the outputs due to that. 

Link to comment
Share on other sites

Thank you for the tips!  Took the meter and the 5V, -5V, and 12V all check out at the pin for the RAM chips.  Also 5V and 12V check out on the CPU/PSUs.  If it helps, the ROMs contained in the PSUs still seem to work, as I can get the built-in game to start "ping-ponging" with button 1, then button 4 presses...

Link to comment
Share on other sites

It's an original version with a run date of October 1976 (man, that's a long time ago now...).


I gave the pins on the PSUs a good scrub down.  I'll try giving them an overnight deoxit bath and see if that helps.  Ordered some replacement RAM as well - one of these things has got to work!


Thank you!

Link to comment
Share on other sites

Luckily this model has a bunch of easily replaceable logic chips, however it can be tricky to locate a broken one (piggybacking with a working chip sometimes help finding the problem). 

If the VLSI-chip in a Channel F II break you're pretty much screwed. 


Not sure what would be the next thing to check. The chain of logic chips between RAM and video output seem logic, IMHO most likely a single broken chip than several. Depends on what has happened though.


74LS195 (D5) or Fairchild 93L00

74LS298 (C4)

7445/74145 (D2), for example if A0, A1, A2, A3 is locked to H, H, H, L it would give a low signal on FG GRN, H H L L for BG GRN

9322 (C2), which has the function of a 74157 (Fairchild's 93xx series), sometimes there's 74xx-numbered chips on the board instead of the 93xx-ones.

(D1), (C3)


You could measure your way back, maybe signals change before a circuit changes but not afterwards... 
Then there's the possibility that one of those large 40pin chips is defective, one thing at the time though. 


BTW just checked some chips on one of my NTSC boards - the 5V died, haven't figured out why yet, bridge, massive 5V regulator or the electrolytic cap most likely. I meant to check what happens if you remove all RAM, will fix it...  later...


  • Thanks 1
Link to comment
Share on other sites

Was finding no data out from RAM and traced back an address line that appears to be stuck on high back to what I think is a busted NOR gate here.  Replacement 7402 is on the way (it's getting tougher to find these random TTL logic chips!).  Stay tuned, and hopefully this thread is of some help to someone in the future!


Link to comment
Share on other sites

Yup, eBay is good for older components, there's going to be stock for a long time - some will be more expensive though. I was getting some SOIC 74LS156 earlier this year, those are not cents each but dollars. ;-)
Stealing from broken electronics is always an alternative I guess. 


Edited by e5frog
Link to comment
Share on other sites

Great news!  My logic probing/oscilloscoping proved correct!  Tested the old 7402 and the suspect logic gate tested bad, the new one tested good, a quick solder job and she's good as new!  Thank you so much for your help @e5frog, especially the most important reminder not to be a lazy ass and just trace back on the damn schematic.  :D  


For a quick summary in case someone else comes across this issue - a NOR gate went bad on a 7402 IC at location E7.  Simple replacement proved successful (I have a few extras - ping me if you need one).


The 5V bridge rectifier was also bad - replacing that with part VS-KBPC102 from digi-key fixed the 5V line


Thanks to everyone for your help, and happy gaming!


7402 New - Good.JPG

7402 Old - Bad.JPG

It Lives!.jpg

Edited by Tsubasa97
  • Like 1
Link to comment
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

  • Recently Browsing   0 members

    • No registered users viewing this page.
  • Create New...