So My //c just up and quit on me a few weeks ago. Would have gotten to it sooner, but I had to replace my multimeter. However, now that I have one again, I was able to trace at least part of the issue to a partially blown fuse in the power supply. It's not completely blown, as I do get some continuity across it, but there's a lot of resistance and it was only putting out about 0.2 volts.
The good news is that I was able to get it open and I know where to get a replacement. I've temporarily jumped the fuse, and it was putting out about 17 volts. Then I accidentally shorted something, and now I'm getting about 1.5. It's not really a problem though, as the transformer seems fine, and there are only about six components to replace (fuse included). And worst-case scenario, even if I can't rebuild it, I can always replace it with a modern 15 volt supply, and splice in the correct connecter.
The bad news is that the fuse at F1 on the motherboard is also blown. And I mean completely. Unfortunately, the thing's a little too small for me to make out any markings, so I don't know what to get as a replacement. I would very much appreciate it if someone could tell me what I'm looking for. Also, if anyone out there has had a similar malfunction, are there any other problems I should be on the lookout for?
I've managed to test all of the components on the external PSU, and it's now "working". The jump wire I used wasn't making good contact on the PCB so I've temporarily soldered it in place and now get somewhere between 18 and 19 volts. Which I believe (and please correct me if I'm wrong) is within an acceptable range for an unregulated linear power supply of that era. Still, the thing did manage to take out two fuses, and the voltage kinda fluctuates so I might consider replacing the capacitor. It tested fine and it's not bulging. But I'm not sure I trust it. Either way, it needs a new fuse before I plug it into my //c!
On the other side of this, the fuse on the motherboard still needs to be replaced. There's no continuity on it, and while I may be crazy enough to jump a fuse on a PSU to test voltage, there's no way I'm risking my //c! So if someone could please tell me the proper replacement component, I would be very grateful.
I can check the component on the //c mainboard sometime this week, perhaps evenb tomorrow, as I have two or three spare boards. I cannot however tell you anything about the inside of the PSU as they are a sealed unit, and I have only one original remaining. There are good aftermarket replacements, even in fact, you can wire up external battery packs if you wish. We used to do that back in the day (at least, those of us with an LCD attachment, did.)
My advice to you at this point would be to replace the power brick with a modern replacement. That way you have a known-good power supply that doesn't drift or fluctuate or give you noisy voltage.
You can even adapt some older laptop power supplies to do the job.
Many have 16 volt power supplies and that's fine.
You can find dozens on eBay.
If the SAMS //c photofact is to be trusted (which is sometimes hit or miss), it says the fuse is 10A 32v fast acting.
OK! So the good news is that this is the correct replacement fuse. Thank you so much for that!!! The bad news is that it blew again. (As did the one in the brick, but I'll get to that later.) No big deal, I can always replace them, but I'd still like to figure out why they're blowing.So I managed to replace the fuse on the motherboard, tested continuity with my multimeter and everything looked good. I put a DIN connector on a modern 90 wat 15 volt (puts out 15.75 with no load) 6 amp laptop power supply, plugged it in, and then the cord to the PSU started to heat up, but no response from the board, and I was getting less than .03 volts on the 5-volt rail.So I turned back to my original PSU. I replaced the fuse in a little locking holder and tested it. It was putting out a little over 19 volts with about a 0.01-volt variance according to my multimeter, which according to the Apple //c Technical Manual is within spec. So I decided to give it a go. Both fuses burned out almost instantly. At this point, I believe that the internal voltage regulator is the problem. I don't see any physical signs of damage, but I can't find any issues beyond the blown (twice) fuse on the power lines before the regulator. I was wondering if anyone had a schematic of the regulator board or any ideas on how I can trace the problem?
Hah. I was going to mention in my reply the fuse would probably just blow again if you didn't find the fault. But I forgot.
Anyway, I believe Apple used a couple different versions of the internal regulator. I thought I saw a schematic of at least one of the versions in the //c photofact. Try This link to see if it matches yours. The early schematic is missing a page, but if you go down to page 36 they put the missing pages in. It also shows a visual picture of the supply to see if it matches.
Thanks for this! While sadly it didn't have my specific regulator in it (An Aztec AA 7343A), it did prove very helpful in identifying some of the components. I managed to trace at least one of the faults to the DUV26 Transister at Q2. The center pin was fatigued and not making full contact. When testing, it sent my multi-function tester into a self-test mode. Did the same thing to a friend's tester as well. So I'm figuring that it's the transistor, not the testers.
In any case, I've ordered a new transistor and will probably recap it just to be safe. Thanks so much. And if you have any more suggestions, they would be very much appreciated.
I've replaced the DUV26 Transister but so far no power. Couldn't be something simple. Any suggestions?
So I've continued my probing on the //c voltage regulator and have come across something odd. The HA17339 IC is grounded on pin 11. I looked up the datasheet and pinout, and from what I can see pin 12 should be the only ground. 11 does have continuity (with resistance) to 13 (+ on 11 to - on 13, but not the reverse), as well as to pin 10 (with + on 11 and - on 10, and again not in reverse), which seems to correspond to the datasheet. However, the sheet makes no mention of 11 to ground. Is this normal? Or should I be replacing this IC?
I think that is what Robespiere meant...
Fuses dont blow just randomly. It can happen with nothing wrong in the cicuitry behind it, like i had with a apple monitor the other day (knock on wood), if someone has been playing with the power on/off for instance and some odd current blows the thing, but generally, fuses dont blow randomly.
They are always set to a level where power consumption exceeds spec so much, it is endangering the system (will cause more defects) and safety. So you never short over a fuse. Just never.
Even more so if 2 fuses have blown, one in the psu and 1 to protect the low V circuit. you can bet all you have that something has shorted in the secondary circuit. So your quest should start there and not by replacing fuses or powering up, but by measuring resistance to find what part exactly is shorting out.
Fuses can be very unpredictable, or I should say, they are predictable in a less than obvious way.
Let's say that you have profiled (or merely simulated) the normal current consumption to be 1A. By specifying a 1A fuse, you believe that will protect from short circuits while never blowing unexpectedly. But it may not be the case!
If you look at a typical fuse data sheet, there is a characteristic curve for different overcurrent scenarios. At 2A, our hypothetical fuse may not clear a fault for hours! Even at 10A, it can be too slow to protect ICs from damage. How much fault current is the supply capable of delivering?
There is a reason it's so common to see that "the semiconductor sacrificed itself to protect the fuse".
In the other direction, there may be transient surges that should be absorbed by other protection devices such as zeners, but an unlucky fuse may simply end its life that day. There is indeed an element of randomness in fuse behavior, although some type of overcurrent is always the cause.
What made me say that the OP seemed naïve are sentences like,
This is a red flag. Any "partially blown fuse" belongs in a museum for the incredible like Ripley's Believe It Or Not. Minimum familiarity with measurement technique is clearly missing here.
I don't want to be too harsh, as some hobbyists for whatever reason skipped the stage of 100-in-1 Electronic Kits and do their learning on vintage computers. Maybe not the best curriculum but it is what it is.
I do agree that you should never bridge a fuse with wire (also seen: aluminum foil, bullet cases, nails, steel wool). People who do this appear to have never heard of electrical fires.
True and thx for elaborating/clearing that up ;]
You have a nice way with words m8, had me laughing a lot ;]
Tbh, i dont know all that much about electronics. It was in the 70ies and i studied chem and we also had to learn some electrical stuff for a year. Dont think i picked up much of that; basically how not to get eletrocuted. Oh and we learned how a generator worked. So last year we had a power out here and i found myself heavily debating on the phone with a guy from the elec company, my idea of how it worked and he said i was wrong. After some 15 minutes we decided we werent going to convince eachother, so parted. And later i googled it and it seemed they had changed the things i had learned in school, decades ago...
But i got some experimenting board also in the 70ies and managed to build a shitty radio. And some other stuff. Which got me ackqainted with other people who happened to be 'in electronics' so not long after i found myself building radio (fm) transmitters, semi-legally, in after school hours. And of course i got on the radio illegally. Man what a time that was. ;]
And not long after that i bought my first computer. Had already learned algol i think... in 75 in school and it was kinda magical back then; computers. So i got a 2nd hand Pet2001 in 1980 and had to discover everything myself. Boy that really consumed a lot of days and nights, but i managed to get really good at basic, fair in pascal, some other and eventually 6502 assembler and later on z80 aswell. At some point i was so good that people from all over the country called me with questions. And i wrote articles for a national magazine.
And i was repairing, mostly c64's on the side. Started a company too, but then made some stupid decisions carreer wise. Or bad luck, i dont know.
Anyway, i learned the most important thing to know is what i dont know. Thats pretty much the basis on which i operate still.
Some guy, who had an electronics shop and did all sorts of repairs himself, named 'Kees', he's prolly dead for a while already, otherwise he's got to be in his 90ies, but he always said "meten is weten als je weet wat je meet", which sounds great in dutch and translated literally to english, means
"measuring is knowing, if you know what you are measuring"
I found that good advice. Everybody can read a thermometer, but if you dont know what it means, it wont help you much. ;]