I got an Apple IIc but it didn't boot. Both the drive led and power led blink when the computer turns on. There is no video output and no activity on the disk drive. It seemed to be a power supply issue, so I measured the VCC of a couple of 74LS374 and found that the voltage is close to zero and flutuating. I then checked the internal power converter and found that its D7 diode is bad: the resistence between its two pins is small in both ways. Besides that, I wondered if that was caused by a short on the motherboard so I also measured the resistence on the power rail between 12v and gnd, 5v and gnd. The resistence between 12v and gnd is 12kohm, but the resistence between 5v and gnd is only 135ohm. Is that normal?
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Ohm's Law (which is the definition of what is called "resistance") is V = I⋅R.
Using basic algebra, 5.0/135 = 0.037, or 37 milliamperes, which is a rather tiny current. So far, there is no indication of a short circuit here.
But this type of measurement is only valid when there is no power source applied. On the IIc, that requires that the DC-DC converter module be unplugged from the main board.
This measurement is also not actually applying 5 volts, and the "active" components, the transistors, don't have a fixed resistance. So such a measurement gives limited information.
The diode D7 is also not something that can necessarily be diagnosed while it is installed in-circuit. When you probe the two ends of the component using a DMM set to 'Resistance', or to 'Diode Test', the DMM sources a current through the circuit. When the component is isolated, laying loose on the bench, the only path for that test current is through it, so the DMM measures its parameters. For example, a loose working diode has a 'Diode Test' reading of around 0.6 V one way [anode to cathode], and "OL" or over-range the other way [cathode to anode]. But when installed in-circuit, there are multiple paths for current to flow, both through and around the component. So the reading shown cannot be a measurement of the component itself.
As an example of how this leads to incorrect diagnosis of a diode, consider a simple switching power supply with the following subcircuit:
(this is from the Astec AA11040, used in early Apple IIs):
False diode reading.png
If you probe the diode D2 in the forward direction, from anode to cathode, it conducts. But if you probe it in the reverse direction, from cathode to anode, the transistor Q1 to which it is connected conducts. The result is that D2 would be misdiagnosed as shorted.
Thank you for the information and tips. I measured the resistancce when the internal power converter was unplugged. It looks like you are correct as I can't really find a short. I checked the continuity for all the capacitors and resistors on the motherboard but can't find a deadshort. I also checked the resistance between the Vcc pin and gnd pin on some ICs but there is no short. As for the D7 diode, I also measured it after taking it out from the circuit. Interestingly now it doesn't conduct in the forward directtion but conduct in the reverse direction. I was surprised to see the result so I double checked my measurement to ensure I measured it in the correct direction. Maybe desoldering it from the circuit caused it to overheat too much?
c523251f-8874-4d7c-b6de-adce349cf50e~1.jpg
PXL_20260422_025338270.jpg
Need to look up the datasheet for this part (S5S4M) to confirm the meaning of the "+ –" marks.
Normally diodes are marked with a stripe on the cathode side. The cathode is the pin where "current goes out" and corresponds to the horizontal line on the schematic representation of a diode. When the ground probe is on the cathode, current is flowing in the direction from anode to cathode, which is called the forward direction.
You can check the datasheet for Shindengen S5S4M, a 40 V, 5 A Schottky rectifier:
The left pin, numbered ①, is the cathode on the schematic symbol. The right pin, numbered ③, is the anode, despite that pin ① is marked "+" and pin ③ is marked "–" on the case. This makes sense when you consider the function of a rectifier. In a full-bridge configuration, the "+" pins of the high-side diodes will connect to the positive output, and the "–" pins of the low-side diodes will connect to the negative output. In other words, when the + pin is at a higher voltage than the – pin, the diode blocks current in what is defined as the reverse direction, because the rectifier's function is to prevent current from going backwards from output to input.
The 0.170 V shown on the DMM is the forward voltage drop, VF. It is lower than a typical silicon diode because Schottky junctions have a lower bandgap than PN junctions, which is why they can be used in higher-power applications, or to reduce power losses.
I think you need to measure voltage at the power supply itself. Both in from the "brick on a leash" and then out to the motherboard.
That diode is fine.
Are you getting the required voltage from the brick?
Not only that, but does it fall apart under load?