Tsubasa97 Posted October 5, 2020 Share Posted October 5, 2020 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! Quote Link to comment Share on other sites More sharing options...
CoeurYggdrasil Posted October 5, 2020 Share Posted October 5, 2020 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. Quote Link to comment Share on other sites More sharing options...
Tsubasa97 Posted October 5, 2020 Author Share Posted October 5, 2020 The 19v input on the regulator is fine, so it's probably just busted if it's stuck open on the output if you're getting nothing. Hopefully swapping it out (because these things do go bad over time) brings yours back to life! Quote Link to comment Share on other sites More sharing options...
e5frog Posted October 11, 2020 Share Posted October 11, 2020 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. Quote Link to comment Share on other sites More sharing options...
Tsubasa97 Posted October 12, 2020 Author Share Posted October 12, 2020 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... Quote Link to comment Share on other sites More sharing options...
e5frog Posted October 12, 2020 Share Posted October 12, 2020 That's odd... Which model do you have? Did you reseat/clean chip pins? The RAM is a common thing to break, usually it just results in stripe:y scrambled screen. It's not impossible completely dead RAM causes such an image. Quote Link to comment Share on other sites More sharing options...
Tsubasa97 Posted October 12, 2020 Author Share Posted October 12, 2020 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! Quote Link to comment Share on other sites More sharing options...
e5frog Posted October 13, 2020 Share Posted October 13, 2020 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. http://channelf.se/veswiki/images/0/04/FVE_schematic_sheet_2_of_3.png 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) etc 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... 1 Quote Link to comment Share on other sites More sharing options...
Tsubasa97 Posted October 15, 2020 Author Share Posted October 15, 2020 No change after a good application of Deoxit. Guess I'll have to put the logic probe to use and start tracing these connections... GOOD TIMES! I was getting low voltage out of the bridge rectifier that supplies the 5V regulator. Might start with replacing that? Quote Link to comment Share on other sites More sharing options...
Tsubasa97 Posted October 16, 2020 Author Share Posted October 16, 2020 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! Quote Link to comment Share on other sites More sharing options...
e5frog Posted October 16, 2020 Share Posted October 16, 2020 Come on... can't be that hard... https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=7402+nor&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=7402 A 74ACT02 will probably be a good fit as well, unless it's too fast. https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2334524.m570.l1313&_nkw=74ACT02+nor&_sacat=0&LH_TitleDesc=0&_osacat=0&_odkw=7402+nor Quote Link to comment Share on other sites More sharing options...
Tsubasa97 Posted October 16, 2020 Author Share Posted October 16, 2020 Hah hah hah. I was hoping to get one from digi-key or mouser, but they didn't have any in stock (or only in quantities of hundreds). I ended up getting a 4 pack from one of the first few sellers on eBay. Hopefully they work, as you never know with these guys... Quote Link to comment Share on other sites More sharing options...
e5frog Posted October 17, 2020 Share Posted October 17, 2020 (edited) 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 October 17, 2020 by e5frog Quote Link to comment Share on other sites More sharing options...
Tsubasa97 Posted October 22, 2020 Author Share Posted October 22, 2020 (edited) 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. 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! Edited October 22, 2020 by Tsubasa97 TYPO 1 Quote Link to comment Share on other sites More sharing options...
+fdr4prez Posted October 22, 2020 Share Posted October 22, 2020 Congrats!!! Quote Link to comment Share on other sites More sharing options...
e5frog Posted October 22, 2020 Share Posted October 22, 2020 Nice work! Quote Link to comment Share on other sites More sharing options...
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