Apple II Plus No Video, No Beep, but Power Light On

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Apple II Plus No Video, No Beep, but Power Light On

I recently bought a working Apple II Plus from eBay and it worked great. However, a few hours later I pulled the top off while it was on, and the video signal cut out. I turned it off and back on again and now only the power light comes on. No beep on boot or video output. Several Apple repair manuals I have found give a list of a few chips that may cause this exact issue. These are the 74LS00, 74LS174, 74S86, 74LS02, 74LS153, and 74LS195 chips at locations A2, B1, B2, B13, C1, and C2. There's a slight problem in that at positon B1 my board has a 74LS175 chip. The 174 is a hex d flip-flop while the 175 is a quad d flip-flop. I have been testing the chips to see which ones are bad, however, so far I have shown that the 74LS00, 74LS175, 74S86, and 74LS02 are fully functional. I tested the 74LS153, but the results were patchy. I have yet to test the 74LS195. One thing I did notice, the power supply is giving some strange readings on my multi-meter. There are slight variances in the voltage, around 0.1 V on the 5 V and -5 V lines and up to 0.5 V on the 12 V and -12 V lines. More importantly, the current on the different lines was vastly different than the ones listed on the power supply. All of the lines except for one had 0.1 A of current, while the 5 V had more than 10 A (It's supposed to have 2.5 A according to the sticker). Could the power supply be the root of my issue? Or is it more likely that the issue is in those last two chips? I think it may have been handled a bit roughly during shipping, the speaker was loose and the language card was also out of its socket. Any help is really appreciated! Thanks!

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Re: Apple II Plus No Video, No Beep, but Power Light On

So it's been about a couple weeks and I've figured a few things out. It is not the power supply, I supplied it with good power and still no boot beep or video signal. What I did find however, is that the +12v and +5v pins where the power supply connect, have only about 800 Ohms and 5 Ohms respectively of resistance between them and ground. I believe there is a short somewhere on the board. I measured the resistance across the remaining two chips that the Apple repair manuals said would cause my issue, and they both measured in with several mega ohms of resistance. Also to note, the -12v and -5v to ground resistances were several mega ohms, so 800 and 5 are tiny resistances in comparison. My best guess is that a capacitor somewhere on the board shorted out. Any suggestions on where to go from here? I could just start replacing the caps, but no guarantee of success. Again, any help with this issue is appreciated!

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Re: Apple II Plus No Video, No Beep, but Power Light On

Hi,
If something is shorted out, the PSU will protect itself and shuts off.
You are probably on the right track. But there is so many capacitors on the 5V rail, it will take you some time to identify the culprit(s).
A suggestion though : have you made the measurements without any IC plugged in ?

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Re: Apple II Plus No Video, No Beep, but Power Light On

Thanks for the reply! No, I have not tested with no ICs plugged in. Do you mean the resistance measurement? I have an IC puller coming in a couple days so that I can start pulling them without worrying as much about bending pins. I will try pulling them all and checking the resistance once I get it, that’s an excellent idea. Also, the power supply does not shut down, at least not as far as I can tell. Is that an indication of power supply issues as well?

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Re: Apple II Plus No Video, No Beep, but Power Light On

I have now tested the board with absolutely no ICs plugged in. The resistance from the +5v pin to ground is now around 240 ohms (it went up right after I disconnected the keyboard) and the +12v pin to ground is around 380 ohms. The only change was the slight jump with the keyboard detached. I'm wondering if anyone with a working Apple II Plus could measure the resistances on their power pins so that I know this is not normal? I feel like that is a very low resistance for the entire computer, but maybe I am wrong. Also, while the board is bare and completely removed from the case, what should I do to clean it and the case?

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