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Macintosh 128k~Plus Analog Board, Safety Capacitors C33, C37, C38
Posted by: JDW on 2020-01-24 18:00:09 I tend to swap out the electrolytics in a recap and leave the film or metalized paper caps alone if they are good. Some among us have experienced blown filter caps near the power input on the original Macintosh analog boards, specifically these Rifa caps: C33: 4700pf 250VAC Minibox (sometimes found to be 2200pf?) C37: 4700pf 250VAC Minibox (sometimes found to be 2200pf?) C38: 0.1uF 250V Minibox The stock Rifa caps all appear to be metalized paper capacitors. A directed replacement for C33 and C37 would be the KEMET PME271YA4470MR30, also metalized paper. But would a Polypropylene version safety capacitor like the WIMA MKY22W14703F00KSSD be a better choice? There is also the KEMET F462AG472J1L2A polypropylene cap, but it is not classified as safety and therefore I do not know if it's failure mode would be open circuit when it blows. Similar thinking should apply to replacements for C38. Please let me know your thoughts. Thank you.
Posted by: JDW on 2020-02-02 16:16:25 75 views of my opening post, but the only reply is mine. :-( Since everyone appears afraid to reply, let it be known I am open to "speculation" too. @Bolle?
Posted by: mpfjr on 2020-02-02 22:06:09 75 views of my opening post, but the only reply is mine. :-( Since everyone appears afraid to reply, let it be known I am open to "speculation" too. @Bolle? Console5 now has them in stock. You can buy an analog board cap kit.
Posted by: Bolle on 2020-02-03 01:03:02 Since everyone appears afraid to reply, let it be known I am open to "speculation" too. @Bolle? No idea. If I was to replace them I would go for what was in there originally. I yet have filter caps to get blown on any of my machines. I'd probably try and see what happens when I just leave them out.
Posted by: bibilit on 2020-02-03 01:30:42 I have had a lot of RIFA caps failures, most of them in the Apple II series PSUs, but also in the Macintosh 128/52 and Plus units. When they failed, it's a real mess, so i replaced all of them as a matter of fact. Indeed, i have replaced more RIFA filters than electrolytics in the Macintosh Analog Boards.
Posted by: JDW on 2020-02-03 05:01:35 I have had a lot of RIFA caps failures... [SIZE=1.4rem]i replaced all of them as a matter of fact.[/SIZE] Replaced them with what, specifically? For that is the very topic of this thread.
Posted by: marcelv on 2020-02-03 06:13:00 C33 and C37 replace with 4n7 300Vac
Posted by: beachycove on 2020-02-03 06:45:57 Console5 now has them in stock. You can buy an analog board cap kit. Thanks for this! Too bad they don't sell a ready-made set for analogue board/ps repair in an SE/30 too.
Posted by: mpfjr on 2020-02-03 08:56:46 Thanks for this! Too bad they don't sell a ready-made set for analogue board/ps repair in an SE/30 too. Probably because they haven't looked at one and made a list yet. If you take your phone and take really close up photos of every capacitor with its board assignment number such as C1, C2 etc and then one photo of the top and bottom of the entire board and send it to them, they will make a kit. That is how I got them to make one for the plus, 128 and 512.
Posted by: MOS8_030 on 2020-02-03 10:34:24 I have had a lot of RIFA caps failures, most of them in the Apple II series PSUs, but also in the Macintosh 128/512 and Plus units. When they failed, it's a real mess, so i replaced all of them as a matter of fact. Indeed, i have replaced more RIFA filters than electrolytics in the Macintosh Analog Boards. I had a RIFA cap explode in the PS in my Profile HD. Also had one go in a 512K also. Yep, replace them while you're in there.
Posted by: bibilit on 2020-02-03 12:38:49 I don’t use the paper ones, the new ones in your post are a better choice.
Posted by: JDW on 2020-02-03 22:34:08 C33 and C37 replace with 4n7 300Vac View attachment 31297 Polypropylene (PP)? Can you please post a link to that exact part on Mouser? If not, what is the exact part number? I want to see the datasheet for it. Thanks.
Posted by: Tekk on 2020-02-05 16:05:51 I just had C38 explode on a Plus analog board, and I replaced all three safety capacitors with the following: C33, C37: https://www.mouser.com/ProductDetail/80-KJN472MQ55FBAAA C38: https://www.mouser.com/ProductDetail/80-PME271M610MR30 Works great and fits the footprint on the PCB perfectly.
Posted by: superjer2000 on 2020-02-05 22:02:53 I just replace them with the paper KEMETs again. I figure if they last for another 30 years, that's just fine.
Posted by: Iesca on 2021-09-05 15:40:32 Sorry to resurrect an old post, but I have had a couple of analogue boards with the 2200pF Rifas at C33 and C37. Are we in collective agreement then that the 4700pF replacements are preferred? It's what Larry Pina lists in his component list for those locations, but he makes no mention of the 2200pF components. I sort of understand that there is some wiggle room with those particular caps since they're safety caps, but I'm not an engineer, so the specifics of why particular capacitance values are selected is beyond me, and 2200 vs. 4700 seems a world apart to me.
Posted by: marcelv on 2021-09-07 01:44:45 The 4700pF I use are for the international 220V version. Don't know if there is a difference for the 110V version.
Posted by: mg.man on 2021-09-07 02:30:34 The "US" (or non-International) A/B had a different safety cap setup. According to this - https://macgui.com/news/article.php?t=509 - the RIFA in those is a 0.1uF... Of course, YMMV!!
Posted by: Iesca on 2021-09-07 12:00:39 C38 is always .1µF (100nF) I believe, but C33 and C37 are 2200pF on mine (US). I do see in DogCow's post that their C33 and C37 are indeed 4700pF...
Posted by: 68kPlus on 2022-11-18 23:56:18 The C33 and C36 on the Plus analog board for international are 4700pF 250V caps. The C37 is larger but also 4700pF 250V I believe
Posted by: AwkwardPotato on 2022-11-21 23:33:23 Sorry to resurrect an old post, but I have had a couple of analogue boards with the 2200pF Rifas at C33 and C37. Are we in collective agreement then that the 4700pF replacements are preferred? It's what Larry Pina lists in his component list for those locations, but he makes no mention of the 2200pF components. I sort of understand that there is some wiggle room with those particular caps since they're safety caps, but I'm not an engineer, so the specifics of why particular capacitance values are selected is beyond me, and 2200 vs. 4700 seems a world apart to me. There is also the KEMET F462AG472J1L2A polypropylene cap, but it is not classified as safety and therefore I do not know if it's failure mode would be open circuit when it blows. Wanted to answer these in case others land on this thread in future Google searches. The short answer: Replace the safety capacitors with parts of the same capacitance and safety class as originally installed. Or, longer: the safety capacitors, along with a choke, form the EMI filter at the AC input of the power supply. The exact values of the Y-capacitors are chosen in a compromise between the performance of that filter, and their leakage current to ground (remember, the Y's are connected between each of the line/neutral, and ground). The magnitude of the impedance of a capacitor, in ohms, is inversely proportional to its capacitance and the frequency of the signal passing through it. In other words, increasing the capacitance of the Y-capacitors will decrease their impedance at line frequency (50/60Hz depending on where you live) and increase the leakage current as a result. This leakage current is tightly regulated by safety standards. According to IEC 60950, it must never exceed 3.5mA in this type of equipment. This is required in the event that the ground at the AC inlet isn't connected (e.g. due to faulty wiring). In that case your body could establish the ground connection, and an otherwise dangerous current would flow through you. Either 2200pF or 4700pF Y-capacitors should result in sufficiently low leakage currents even in worst case scenarios (there is some additional leakage past the Y-caps due to parasitic elements). All that's to say: while moving up from 2200 to 4700pF is safe, your best bet is to stick with whatever was in there originally (thinking especially of other models of computer that may use wildly different values for the Y-caps than the early Compact Macs). These are absolutely not parts you want to experiment with, as you may put yourself or future owners of your computer in serious danger.
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