here is another broken Power Supply. A friend helped a little - he measured as far as he was able to - the results:
1.) I bought a replacement for the security filtercap on the left side (thanks to Speedy for the link in another thread!) - but the old one seems to be o.k. (The replacement is much bigger, so I would like to use the old as far as possible).
2.) I replaced the filtercapacitor on the left side of the first pic - the little white one is the new one.
After another measurement the situation is as follows:
- the primary side of the power supply (left side in the picture) seems to be o.k.
- the secondary side has absolutely no voltage.
As my friend is not able to measure and identify the problem - my question:
Which parts of the secondary side should I replace (I want to go step by step till it works again).
Thanks for any help.
(Also thanks for any hint towards a modern replacement for the power supply - but I do not want that! I want to keep as much as possible from the old parts!)
If in doubt, replace all the electrolytic (cylindrical) capacitors, starting from the right (bad) side, and if still nothing, then those on the left also. You can try and eyeball the ones that don't 'look' right, if they are bulging or anything like that, and replace those first.
You could probably be more deductive and figure out exactly what capacitors are bad with further extensive testing, but all the capacitors will start to go bad at some point - so if you have a spare few hours, replacing them all would be my recommendation.
The debate between replacing all capacitors as a matter of course, and only replacing those that need it, is currently taking place in the vintage radio community, where aesthetics and 'genuine' parts are extremely important to some and not at all to others. Whether this will one day become a consideration for Apple ][ power supplies, I'm not sure.
EDIT: In case you don't know, it's completely okay (and sometimes recommended) to replace the electrolytic capacitors with new ones rated to a higher voltage (but always the same capacitance). The original capacitors used in the power supply are just 'good enough' to make it economical, but by installing a capacitor with a higher voltage rating, the capacitor won't need to work as hard - meaning it will deliver cleaner DC voltage and will last a lot longer. When I recapped my power supply I also noticed that the physical size of new capacitors had shrunk compared to the older ones, so from an aesthetic standpoint, I bought capacitors of a much higher voltage rating to match the physical footprint of the ones I was replacing. Wins all round.
Are you saying that newer capacitors of higher voltage would be about the same size as the original, lower voltage ones?
Always test the bridge rectifier- it's the covered black part next to the white RIFA cap you replaced. I've had several of these die on me, but usually they short out and blow the fuse when they die. I guess they could also fail open...
Although if you have voltage on the primary, and nothing on the secondary (measured at the transformer itself) then the transformer winding could be bad.
That was my experience when I replaced my capacitors recently.
The technology in recent years has improved quite a bit so if you have an old cap from 30 years ago and you compare it to one from one made today, the newer cap will often be smaller.
So to use one that has a higher voltage rating will be larger in size which would be similar in physical size to the older cap.
Just remember that not all caps are the same.
Switching power supplies require capacitors which have low ESR, high ripple current and high temp 105c.
These are the ratings that are needed for capacitors used in high speed switching power supplies.
And always get a good brand name cap.
Not a cheap one from a company you've never heard of before.
Make sure that you read the datasheet of the cap you wish to get to verify that it has the specifications that you want.
There are other things to consider besides the electrical specifications.
For example the lead spacing and the height and diameter.
All of these measurements use the metric system (millimeters).
Thank you for the suggestion and information.
extended reply this evening ....
It seems that several members did not read the initial posting carefull....
thsis is not releted to the primary part but to the secondary part (!) check first posting (!)
here the extended answer:
Red Capital "C":
It seems that the transformator has seen temperature beyond limits....
There might be a risk that the isolation paint might have melted
If that happened there might be a shortcut between parts of the secondary
Violet capital "C":
The pack of windings at secondary side is one large coil with different
tappings that divide the large coil in various sections.
The only chance to detect damages is to disconnect the
secondary components and perform measurements without
voltage present !
First you must check integrity from outer connections for
conection to detect damage by disruption/ break of winding.
Then each other tapping must get measured from one tapping
to each other tapping to detect shortsuts from one winding packet
top any neighbor winding packet because of loose of isolation
by melted isolation paint..
The measurement must must be larger than zero Ohm and less than 2 Ohm.
That value is very small and most multimeters
are not able to deliver reliable results - you will need a
special meter that is a so called "Milliohm Meter".
For the measurements you might either desolder the Wires from
the joints at violet "C" or cut the traces at the Line red "A"
and red "B" ( feedbasck rail ).
At the circuitplan that part is marked by red A and
marked by violett highlight.
Next part of examination shall only be performed by experienced technicians !
Then the traces disrupted by the red "B" line at the feedback rail must be restored to
prevent a shutdown by that feedbackrail.
Because the shortcut protection of the powersupply is controlled
by parts of the +12 Volt rail the next point would be to only connect the +12 Volt rail and connect that rail to a +12 Volt load and short time switch power to that rail and just check if voltage is present at that rail.
next step would be to check the +5 volt rail also with load present and very short Go-No Go Test.
third step would be the reactivation of the -12 Volt rail
and finally the - 5 Volt rail.
Thanks a lot for all your answers and information!
The way with the capacitors, could be a solution - but, all of them look very good - no bulging at all ...
I forgot to mention, that my friend measured the bridge rectifier - it is all right (the fuse is also perfectly all right).
Looking forward to your answer Speedy.
Does anyone have a circuit diagram exactly for the shown power supply?
ok - one step forward:
My friend measured again:
1.) The security filtercap is all right
2.) According to the circuitplan, posted in this thread by Speedy ( http://www.applefritter.com/content/apple-ii-europlus-power-supply-chirping-and-gives-fluctuating-voltage-output ) the resistance R3 is broken - maybe here is the point where no voltage passes through.
thank you very much for your detailed explanation!
I don't know if my friend is able to do the described way of measuring.
So as an alternative way I think about replacing the transformator - in the plan you sent I can not recognize the exact data of it - can I use one like this:(?)
you can't use that or similar device !
That device from Conrad is not usable in a switching power supply !
That transformer is a custom device specially for switching power supply
for higher rectangular switching only !
Neither Apple nor Aztec ever published the data of such transformers used in that
Replace C7 and verify that resistor R3 is not open. Connect the +5, +12, and common to an old hard drive for a load and see if the power supply comes back to life.
East Syracuse, NY
Thanks a lot for all your answers! I will try that.
I have an Apple II sn 277 that I bought from Team Electronics in 78 (it was a demonstrator). I used it for years then (foolishly) stored it in an attic. I tried to fire it up the other day and no power. I set up a dummy load to test it with appropriate sized resistors. No outputs. I measure about 335 volts between the cathode of D2 and the anode of D1 so iguess it is working up to the FW rectifier but don't know enough beyond that. I am considering trying to fire up the computer with an old Sola Basic 83-83-3250 PS I have on hand. It has 12v .5A, 12v .5A, 5v 5A all voltages isolated. I'll use a regulator to get the -5 from the -12. The trouble is that the label on the PS says that the +12v needs to supply 18 watts which is 1.5A. What does the bare machine actually draw from the +12v line with no peripherals or cards? If needed I will buy the universal PS from ReactiveMicro. Has this thing established a reputation in the community? Also The top cover broke years ago. Is there any chance of finding one from a II? Would color matching be a problem?
Thanks a lot !!!
I replaced C7 and now: it's working perfectly all right!