Hello everyone,
I used to have a perfectly working appleIIc, cleaned, retrobrighted, etc.
Today I wanted to use it again and it doesn't power up, making an horrible ticking noise, leds trying to light up and then coming off, like a short circuit of some sort
Sure enough, when powered with a lab PSU on 12V it triggers the maximum current protection.
What's weird is that this happens even with the green power switch OFF.
At first I thought, this must be that large capacitor next to the DIN power plug. But.... Removing the DC-DC convertor removes the problem !
Is it possible on this model (IIc from 1983) to have a short circuit with the green power switch OFF ? It makes no sense to me... It would mean this is wired in some way to the dc-dc converter even with the switch off ? I must be doing something wrong.
I've opened the can of the dc-dc converter and see nothing special, I know it doesnt mean anything but before a blind recapping I'd be curious if anyone can confirm that the power goes into this converter even with the switch off ?
thanks for any insight
The schematic shows that the DC-DC converter (the internal power supply module) is connected to +12VIN at all times, and the switch merely controls an enable signal to the converter. This means that power to the machine does not pass thru the switch.
Thanks !
At least I'm not crazy
Is there a schematic for the dcdc converter ? Couldnt find it
According to the schematic that robespierre linked to, if you 're having a dead short with the switch turned off, the first thing I'd check is that L1 isn't shorted to ground.
That's the first component after the connector (which might also have a short).
The problem only exists when the dc converter is connected
so to me it's not a problem with the switch or the connector
I tried applying a current limited 200mA and took thermal images and the two first caps ( close to the board edge ) in the converter seem to slightly heat up ...
Still for from a diagnostic but it's a start
interesting case to have a short with power off
It looks like you could run the DC-DC converter outside the Apple, by wiring a switch between FFF and ground. FFF is most likely an input to a p-channel MOSFET or PNP transistor that switches on the chopper transistor.
Exactly , thanks for confirming the schematic - because for our 2024 minds it's quite unusual
So in a few words, the DC-DC converter receives whatever you put on the DIN power jack even with the power switch set to off (I've confirmed that with a multimeter), and the "OFF" position also sends the 12VDC to pin 6 (depends on how you count th pins) of the converter, making all pins 6 to 11 receiving the +12V (or whatever you feed thru the psu and the DIN) EVEN WITH THE POWER SWICTH SET TO OFF
Also the ground seems to be always on as well ( ground set to chassis and ground of the dc converter )
What switches the dcdc converter, and the Apple IIc, on is passing pin 6 from 12V to ground, which is the only thing the power switch does.
This is all very unusual. It also means you probably should never leave an appleIIc on (now that they are vintage and fragile) with the power cord connected, you should disconnect it if you don't use it.
As you mentionned, I'll try powering the dcdc converter out of the Apple IIc, this should reproduce the problem, and make it easier to solve