Ok, my Apple II's silver power supply has a blow fuse. At first I thought it was the switch until I drilled out the rivets and the switch was fine, but the fuse is blown.
I will take it apart and check the bridge rectifier tomorrow. I think I have to unsolder the switch and power entry module to get to the bottom of the board.
Is the fuse some sort of soldered in fuse, I can't seem to get it out. It looks like its in a holder....
Also it looks like it says 1A 250v, but I can't be sure because I can't turn it to see. It might say 2.5 and the 1 may be something else I can't tell till I get it out.
Also can the fuse just be blown from something external or does a blown fuse anyways mean the bridge rectifier is bad.
Thanks,
Corey
Hello Corey,
a blown fuse must not indicate a busted rectifier, it can - but not must....
in fact the fuse just indicates that something went wrong within the primary part of the switchvoltage part
it may also be for example a dried electrolytic capacitor from the 2 large ones that has a shortcut or one of the diodes or the powertransistor as result to damaged capacitor....
you´ll also have to examine the parts behind the capacitor.....
it´s very common damage that the big foil-capacitor directly behind the rectifier gets killed and it´s rather hard to get exact replacement...
sincerely
speedyG
Thanks. I'll try to check those when I get the board out later.
The other question is an I right the fuse is soldered in and not a normal fuse?
Also what is the rating it should be for the fuse. The only schematics I found on the web indicate it should be a 2.5amp. But I can't tell if its a slow or fast blow.
Thanks,
Corey
One more question.
How do I test the caps. Usually a meter doesn't work well on caps actually in place. Is there a way to check without desoldering them.
Thanks
Hello Corey,
well first of all - there is nothing special if the fuse is soldered or not... it was just cheaper to solder them because a case for them would have cost more.... second : in general you should use slow fuses, because in the very moment the power is switched on the caps pull much current ( thats called "load-pull-current" ) and few milliseconds later the regular pull of power stabilizes to less current ...
the strength of the fuse depends to the switchsupplymodel.... is it a "international" that can be switched from 110 to 220 Volt or is it US-version only ( that only has 110 ) ? Normally at the fuse on one side of the metalcaps the strength is etched in like T1,25/110 which would translate to T=slow / 1,25 A / 110 Volt so its general speed/strength in A / voltage - so a value F0,6/220 would translate to F=Fast 0,6 A / 220 V.
If you only have a normal multimeter it´s quite difficult to measure caps - you have to at least unsolder them....
if you then make a measurement for resistance the value will be very low and thereafter rises up more and more during the time the capacitor loads up and finally the resistance gets very high....
for the measurement you must keep the "hot end" of the meter at the plus-pol of the capacitor ( red cable from meter ) and the "common"rail of the meter to the minus-pol of the capacitor.... if the cap has shortcut the value of resistance keeps low and won´t rise very much....
>TIP: if you have "the Apple II Circuit description" from Winston D. Gaylor... at the annexes there is a circuitplan of the silver powersupply.... very last page Figure 23 C23......
sincerely speedyG
Well it's a US only supply. The fuse was indeed soldered in. I'm going to put clips on it so I can change it out again.
I have a really good meter.
According to the diode setting everything is good with the bridge meter. The capacitor setting seems to indicate everything is ok with the caps
The fuse is only 1amp 250V so maybe it actually just blew from a bad card or something. The schematics I have shows a fast blow 2.5amp. So I'm wondering if this was a problem and apple changed from a 1amp slow to a 2.5 amp fast. I have both but no clips so I'll have to take a trip to the store later to buy clips. I guess since it will be easy to change the fuse with clips I should just try it.
I would expect it to immediately blow if there is a problem on the high side of the supply with no load and if put a load resistor on the low side and it would blow if the problem is there. Does that sound right?
If it doesn't blow then the issue is just a blown fuse that may have been spec'd wrong by apple and changed over the course of production.
This little side project needs to be solved soon its keeping me from my Datanetics keyboard project
Cheers,
Corey
I expect it not to blow at all - this is a really unusual failure mode. Apple put that fuse behind rivets and soldered it in. They didn't expect it to blow either. I'd just pop one in there and see how it goes...
Hello Corey,
i´d also guess the mistake issued by wrong fuse... but for a test to operate correct i´d recommend to put a load to the output.... i use for that normally an old "out of use" 3,5 IDE Harddisk and have an cableadapter for the +12 Volt and + 5 Volt with small plugs that fit in the powersupplyplug of the Applesupply. By that you also will be able to measure the correct voltages, because if there is no load you will get wrong values.... both votages should then show up with some value between -0,3 Volt and +0,5 volt of specified value - i.e. between 4,7 Volts up to 5,5 Volts and 11,7 Volts to 12,5 Volts.
sincerely speedyG
To quote the great Dr Frankenstein who brought life to the dead....
It's ALIVE!!!
Well I learned two things here... (I had to do some research with the blue and red books)
#1. The Non-Aztek powersupplies continually click like they are shorted when there is no load. The Aztek supplies click once then hum. After replacing the fuse, I turned on my supply to check it and compare with my gold supply(Aztek) for noise and they had different behavior. Thinking my supply had a short somewhere I kept looking for a schematic online with values on it since the only one I found for the early supply didn't have any cap values to compare against. When reading the books they mention the restarting cycle of a short under load, then the fact the supply shorts intentionally when not under load. So I tried the supply and it works...
#2. Reading the red book it actually mentions current draw. Well a 48k machine with all slots loaded should take 1.5 amps for the +5 alone. Well that explains why the later schematics has a 2.5 FI fuse and not a 1 amp like mine. I'm speculating here but I bet why we see a lot of Apple II (not plus) with replacement gold supplies was that the 1 amp fuse would "blow". It's soldered in. I guess I will do a little experimenting before I seal the supply up and rivet it closed, but I think a 2.5 amp fast blow will be what I finally put in.
Just for the record, when I got my Apple II, it was a 16k model upgraded to 48k. The only two cards were a Disk II and a RamEx 16 (language card that did not require a jumper wire because it did its own refresh). I will check current draw with and without cards (and disk drives) later when I get back from visiting a customer at work.
Cheers,
Corey
Sometimes fuses blow because of metal fatigue or repeated physical shock or vibration.
In-rush current can also flex the internal element over time.
I've seen failures from transformers where windings short once they've become hotter and metal expands.
It's very hard to say.
I'd put it under a decent load and leave it on for a while to check.