68kMLA Classic Interface
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| help for a frustrating recap of a Classic | |
| Posted by: Classicman on 2022-11-04 06:07:17 Hi dear friends! I have fully recapped my Classic, only the huge cap near the transformer is the original one (mine AB is 220 volts type). I have also changed the TDA4605, the mosfet IRFBC40A and the optocoupler CNY17G-3. After the fully recap my Classic don't start: any chime but only a shaking image with irregular vertical white bars and small black dots. After various attempts the Classic started with a shaking image that slowly stabilized. I tried other times to turn it off and on again and it started without having problems and without the shaky video anymore. I had on the HD connector 5.10 volts and 12.22 volts. Delighted with my success and after a night of welding I went to sleep... This morning happy with my repair I go to start my Classic and it starts with the chime but only a very very bright horizontal line appears on the screen. DAMN! DAMN! Believe me to say that I am disappointed and exhausted for this thing is nothing! When I recapped the AB of the Classic I redid most of the welds and so renewed them (all video connectors and so many others). Damn, now I have to check/change also the TEA2037A? or something else to check?? Please friends, please... HELP ME! Thank you all in advance. | |
Posted by: bibilit on 2022-11-04 07:12:095.10 volts and 12.22 volts.Way too much (4.85 > 5.00 / 11.9 > 12.00 is enough ) very very bright horizontal lineCan be the opposite, not enough voltage. Have you replaced Dp3 and Dp4 ? a shaking image is a very well known issue with leaky diodes. I will test again with a multimeter at the Molex plug. TEA2037A never seen any bad. | |
| Posted by: Classicman on 2022-11-04 07:41:31 Thanks a lot bibilit!! Way too much (4.85 > 5.00 / 11.9 > 12.00 is enough )Ok I'll set the trimmer to proper voltages. Have you replaced Dp3 and Dp4 ? a shaking image is a very well known issue with leaky diodes.No they are the originals. You say that by changing these two "faulty" diodes I could solve the problem of the very bright horizontal line? I remind you that now I have no image on the screen. | |
Posted by: bibilit on 2022-11-04 08:17:34No they are the originals. You say that by changing these two "faulty" diodes I could solve the problem of the very bright horizontal line?replaced by the ton, unexpensive and commonly faulty, not easy to spot as they look and test good otherwise. Not saying is your issue, but they are next to nothing, so worth a try anyway. | |
| Posted by: Classicman on 2022-11-04 09:06:31 Ok thanks, I'll try to change the two diodes. Please, I'm not an expert of electronic component, can you indicate me the value of the two diodes? so I can ask precisely to my electronic reseller. Considering that I am dismantling the AB of my Classic for the umpteenth time, would you advise me to change other components? for example the large 220uf 400wv cap? the other mosfets? I tried to restart the Classic in the afternoon and this time it did not emit the chime immediately but after about 50 seconds. The voltages have gradually risen and stabilized over time. | |
| Posted by: bibilit on 2022-11-04 11:24:58 1N4148 (same for both Dp3 and 4) no, the large cap is ok. never seen one bad, nothing else in my opinion. | |
| Posted by: Classicman on 2022-11-05 07:49:28 dear @bibilit, thanks again but I have changed the two diodes DP3/DP4 and the TEA2037A (my reseller had one) and nothing... The Classic boot with the chime but the problem remains the same: screen displays single horizontal bright line. Could it be some cap that I replaced that has failed (no one looks swollen on sight) or maybe of low quality? A cap that interests the video part? | |
| Posted by: bibilit on 2022-11-05 08:21:35 Looks like a vertical collapse to me. The chip is a TDA1170, but can be something in the neighborhood. Have you tried adjusting the pots on the back of the AB ? | |
Posted by: Classicman on 2022-11-05 08:29:53The chip is a TDA1170, but can be something in the neighborhood.I remind you that until now I have replaced: TDA4605, the mosfet IRFBC40A and the optocoupler CNY17G-3 and the TEA2037A. I have tried to adjusting only the horizional pots (first and last pots). | |
| Posted by: Classicman on 2022-11-10 23:06:38 Bad news: I have also changed the mosfet IRF740, the schottky MBR1045 and the v. regulator LM317T on the AB. For the sake of scruple I preferred to replace all the power supply caps with excellent caps... Nothing to do, it doesn't work !! same problem but the bright horizontal line has changed slightly and now no startup chime...!! Now I have 4.80 V and 12.20 V. The resistors on the board all look ok, the weldings all look ok and tested with my multimeter. Now I really don't know what to do, everything I could replace has been replaced. | |
| Posted by: TheMightyMadman on 2022-11-11 14:31:36 Hi, I had a Mac Classic II (same analogue board) suffering from vertical collapse, even after a full recap. I also replaced a lot of other parts (mostly actives), including the TEA2037 IC. The analogue board had suffered from extremely bad electrolyte leakage. Electrolyte had damaged RF2 (2kOhm 1W power resistor) which powers the TEA2037. It looked quite corroded, and read very weirdly and intermittently both in- and out-of-circuit. I replaced it, and I had vertical deflection again. I'm still trying to resolve other issues with that specific analogue board, but I hope this helps. Many thanks, Adam | |
| Posted by: Classicman on 2022-11-12 00:46:04 Thanks @TheMightyMadman !!! your suggestion of this resistor (RF2) is really PRECIOUS! please, can you indicate also the tolerance of RF2? From my little electronic experience I know that for some electronic circuits (precisely on some portions) it is a very important value. Beyond the question of the vertical deflection now I do not explain why I no longer have the startup chime...! In my "desperate" attempts, for the sake of scruple I recapped the whole board with excellent red WURTH caps. Since I will be removing the AB of my Classic for the umpteenth time, can you please tell me if I need to replace something else? On the web I have read that many have replaced other small transistors and caps: "CP21", "CP23" and "CP39". Please, do you know the values of these three caps? Thanks again friend! | |
Posted by: TheMightyMadman on 2022-11-12 01:00:48Thanks @TheMightyMadman !!! your suggestion of this resistor (RF2) is really PRECIOUS! please, can you indicate also the tolerance of RF2?No problem đ If I recall correctly, RF2 was a 5% tolerance part, but I fitted a 1% tolerance part just in case. As for the lack of a startup bong, based on the gaps in your horizontal line, it looks like youâre getting a checkerboard screen. There could be a problem with your digital board, but itâs also worth checking that your analogue board is still providing suitable and stable 5Vdc (4.8V-5.0V). You can find Mac Classic schematics here: 
Macintosh 68k Schematics : Bomarc Services : Free Download, Borrow, and Streaming : Internet ArchiveMacintosh 68k Schematics========================Bomarc Services reverse engineered several Apple Macintosh computers in the1990s and sold the schematics to...
archive.org
Many thanks, Adam | |
| Posted by: dochilli on 2022-11-12 04:09:40 Your 5V is too low with 4,8 V. Did you turn pp1? You need 4.85 for a chime! If you turn pp1 then you have to look at the 12V too! | |
| Posted by: Classicman on 2022-11-12 06:50:02 Thanks a lot @TheMightyMadman !! Iâll change the resistor and let you know!! @dochilli Thanks for your important suggestion! Tha strange thing is that my Classic was starting with chime with the âeconomicalâ caps and the voltages adjusted by me was 5.00 V and 12.19 V. Now with the high quality caps (Wurth), new mosfet IRF740B, new schottky MBR1045 and new v. regulator LM317T no sign of chimeâŚ! As you right say, I have tried to adjuste the âPP1â but with new caps my best value at maximum clockwise rotation was 4.82 V and 12.86 V. And the another strange thing is that the volts increase gradually. The anomalous behavior of the new caps reminds me of when the classic had installed the first nichicon caps ... Iâm going crazyâŚ! | |
| Posted by: Classicman on 2022-11-15 14:07:26 Hi dear friends! after many attempts, anger, desperation and replacement of electronic components, my Classic is back to life!!! I replaced the 2 KOHM 1W âRF2â resistor that feeds the TEA2037 (miraculous suggestion from friend @TheMightyMadman) and then I gave the whole analog board a new good âbathâ (this time extremely degreasing) in isopropyl alcohol (suggestion from italian friend @demonlg). The Mac started immediately without any hesitation after the replacement. All works perfectly! As the friend @demonlg rightly hypothesized, the burnt resistor inhibited also the correct functioning of the power supply (generating unbalanced voltages). In fact, now the trimmer "PP1" works regularly and I managed with the help of the multimeter to perfectly adjust the 5V and 12V. To say that I am happy and satisfied is an understatement! Thanks to friend @bibilit and all the friends who participated in this conversation, but special thanks goes to @TheMightyMadman for his âmiraculousâ tip. Unity is strength! I hope this "desperate" post of mine will help some Mac enthusiasts who will run into the same nasty problem in the future. âWe greet youâ cordially, Me and my fantastic Mac Classic. | |
Posted by: TheMightyMadman on 2022-11-15 14:57:29Hi dear friends!That's great! Very nice work indeed. I'm very glad that my tip helped out. | |
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