68kMLA Classic Interface

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-12, +5, +3?
Posted by: mst3k on 2023-12-11 14:34:06 Greetings, I've messed up somewhere on this analog board recap... I'm getting a perfect -12v on the -12v rail, a perfect 5v on the 5v rail, and a way not very perfect 3v on the 12v rail. Mac Classic does not chime, but screen comes up to a very nice, bright square image filled with random squiggles. No checkerboard or straight bands/lines. Mac Classic logic board works fine in my other machine. I checked the polarity of the caps and all are correct. I'm pulling the analog board and examining my (admittedly weak) soldering for errors. Any other obvious place to look for such an odd voltage loss on just one rail? Thanks in advance for any guidance.
Posted by: GRudolf94 on 2023-12-11 15:04:05 If your analog board is P/N 820-0525-A, check DP7, LP5, CP8 and CP12. If anything I'd bet on DP7.
Posted by: mst3k on 2023-12-11 15:19:43 Thanks. I will take a look and report back. Board number is 630-0525. Screen image attached
Posted by: GRudolf94 on 2023-12-11 15:20:39 630-0525 is the PCB itself, 820- is the whole assembly, so we're on the right track 🙂 Certainly an odd pattern...
Posted by: mst3k on 2023-12-12 06:06:11 Where would I look for a new replacement part number for the diode(?) at DP7?
Posted by: Phipli on 2023-12-12 06:12:00 Where would I look for a new replacement part number for the diode(?) at DP7? You're best to click the reply button under the post you're replying to, or "@" @GRudolf94 so that they are notified that you're talking to them, otherwise they are likely to miss your message.
Posted by: mst3k on 2023-12-12 06:35:33 630-0525 is the PCB itself, 820- is the whole assembly, so we're on the right track 🙂 Certainly an odd pattern... Where would I look for a new replacement part number for the diode(?) at DP7?
Posted by: mst3k on 2023-12-12 06:36:04 You're best to click the reply button under the post you're replying to, or "@" @GRudolf94 so that they are notified that you're talking to them, otherwise they are likely to miss your message. Thanks.
Posted by: GRudolf94 on 2023-12-12 09:36:46 Where would I look for a new replacement part number for the diode(?) at DP7? https://www.mouser.com/ProductDetail/onsemi-Fairchild/EGP30D?qs=sEr9WmQGXZabTAKVOEB3mQ%3D%3D An exact match - whatever your favorite source for an EGP30D is also works.
Posted by: mst3k on 2023-12-12 11:19:18 https://www.mouser.com/ProductDetail/onsemi-Fairchild/EGP30D?qs=sEr9WmQGXZabTAKVOEB3mQ%3D%3D An exact match - whatever your favorite source for an EGP30D is also works. Thank you so much for the help! DP7 is heavily corroded, as is the identical part just below it. I intend to replace both.
Posted by: GRudolf94 on 2023-12-12 12:39:35 Thank you so much for the help! DP7 is heavily corroded, as is the identical part just below it. I intend to replace both. If you mean DP5, they're *NOT* identical parts (DP7 is the only EGP30D in the machine). DP5 is a BYT03-300 according to Bomarc.
Posted by: mst3k on 2023-12-12 19:57:31 If you mean DP5, they're *NOT* identical parts (DP7 is the only EGP30D in the machine). DP5 is a BYT03-300 according to Bomarc. Thanks for the heads up. The byt03-300 is proving hard to find. I’ve located some byt03-400’s in Europe but nothing locally, so it looks like I’ll take my chances with replacing the egp30d at d7 and leave the byt03-300 at d5 for now. Thanks again for the help and I’ll let you know if it all works.
Posted by: mst3k on 2023-12-13 08:36:09 Thanks for the heads up. The byt03-300 is proving hard to find. I’ve located some byt03-400’s in Europe but nothing locally, so it looks like I’ll take my chances with replacing the egp30d at d7 and leave the byt03-300 at d5 for now. Thanks again for the help and I’ll let you know if it all works. I did find this equivalent at Mouser for the BYT03-300. 400v reverse voltage max versus 300v for the BYT03-300. https://www.mouser.com/ProductDetail/511-STTH3R04RL
Posted by: GRudolf94 on 2023-12-13 10:11:51 Sure, that'll work 🙂
Posted by: mst3k on 2023-12-19 11:42:21 OK. Tested and checked all the caps for polarity (again) and resoldered a couple that felt loose. Replaced the EGP30D rectifier diode at DP7. I also removed the wiring harness from the board and cleaned, installed new ferrules on every wire and re-soldered into place. Lastly, I found the ground wire from the CRT neck board hanging by about 3 threads of the wire so I pulled it, put on a new ferrule and re-soldered it back to the CRT neck board. Now, I have NO idea which one of things did the trick, but I am now getting good voltages, a nice strong BONG! on startup and a clear and bright image on the CRT. Thanks for the help GRudolf94!
Posted by: GRudolf94 on 2023-12-20 13:34:56 Glad to hear you got it running! Nice catch on the grounding.
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