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PS/1 repair question - power board

alank2

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Aug 3, 2016
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I've got a 80286 PS/1 that a friend gave me that is having some odd issues.

First, the keyboard didn't work, but the computer did boot with a different ps/2 keyboard.

But, then it locked up and when I powered it off/on, it never came back again. The monitor turns on, but the system won't post or show anything.

I checked the monitor voltage out and it is 37 volts DC. I then opened up the main system and there is a power supply card that converts that to the voltages needed for the system. I can see a -11 something, +11 something, and a +5.1 volts on some of the various pins on this card, but I don't have the pinout so I don't know if all of the signals are correct or not. If I try to read the + side of some of the electrolytic capacitors on the mainboard, they are near 0V, so that must be why it isn't working. I've pulled the modem, floppy, and hard drive with no differences so I have to assume it is something in the power board that isn't working properly. Does anyone have the pinout for it?

I found a manual, the output of the power card is good. I found a different capacitor on the motherboard and it does have 5V, so perhaps the first cap I found is used for something different (its + side is ground and its - is -3V).

So the issue is no video, no beep, etc. It _was_ working. Any tips on what I should check next?
 
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Does anyone know how the reset functionality works on these? I looked at the reset* pin on the modem connector and it is low and stays low so I wonder if it is never coming out of reset.
 
I saw a YT video last night where the guy had the same symptoms - no video and he was having issues with the Dallas RTC, so maybe I have the same issues. Maybe it doesn't initialize the video until it talks to the RTC. I'm going to cut away the side and install a new battery tonight and see if this brings it back to life.
 
I made up a test AVR project to try to test out the DS1287. I did already drill holes in it and break the internal lithium coin connection and connected a new cr2032 battery. I think though that something else is off or odd about the chip. I can read its 64 bytes and they make sense. I can set a time in it and it will count time. If I cut its power and restart it, sometimes it will work fine and still have counted the seconds properly. Other times it will be filled with 0x66 values for the time. I've also seen a failure condition where it will not respond at all, resulting in 0xff's being returned for all values. A power cycle seems to bring it back. Since I saw the guy in the YT video have no video until he put a battery in, I think that the BIOS must wait on the DS1287 with no timeout and I am hoping that is why I have no video. Anyway, I ordered some DS1287's off eBay today so hopefully when they come I can drop one in and it maybe I'll get some video.
 
I put a logic analyzer on the parallel port, no activity.

I then decided to look at the A0 signal on one of the ROMs - turn the system on and it just goes high and stays high, no activity.

Any thoughts on what to look at next?
 
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