I think you have a short on the 5 V power rail that is causing the power supply to cycle in and out of over-current protection, effectively resetting the machine repeatedly. The power supply:
- Ramps up the output current for a “soft start”.
- Detects over-current and shuts down the power.
- Waits a short time.
- Goes to 1.
The beeper speaker (in the back-left corner) normally chirps when the machine is reset. You can hear it after hitting Ctrl-Alt-Del, for example. The fan runs because it’s connected to the 12 V rail, which is independent.
If you check the 5 V rail with a multimeter in volts mode, you may be able to see the voltage jumping up and down.
Because you’ve disconnected everything else, the short must be on the motherboard. On machines of this era, tantalum capacitors are the prime suspects for shorted power rails. The only tantalums in the PCjr are on the diskette drive’s board, however, and you’ve eliminated them as possibilities by disconnecting the diskette drive.
Here are some things to try:
- Examine the motherboard closely for any metallic debris that might have fallen in and caused a short. Look at components that stick up, making sure they haven’t been bent in a way that would cause their leads to touch.
- Look for any charring or things that smell burned. (Unlikely in this case, I think, because of the over-current protection.)
- Leave the machine running (though repeatedly resetting) for about 5 or 10 minutes, then start feeling the tops of the chips with your fingers. Shorted chips often run noticeably hotter than the others, though the over-current protection may make the difference less noticeable in this case. Prime suspects are the RAM chips (in a 2×4 array in the front-left corner), the character ROM (the fat chip immediately behind the IR receiver), and the BIOS and BASIC ROMs (the two fat chips behind the left cartridge slot), though any chip could be at fault.
- If the ROMs and CPU are in sockets on your motherboard (as they usually are), you can try gently prying them out of their sockets. The machine won’t boot without them, obviously, but shouldn’t reset repeatedly if one of them is the culprit.
One curious thing to note is that the program that eventually froze it was giving me an error along the lines of “not enough memory to run,” which I thought was odd because clearly it was used on this computer based on the floppy disks I found.
This might have been a coincidence. I see three sidecars in your video and bet that at least one of them is extra RAM. Most software won’t be able to use that RAM if you haven’t booted with something like JrConfig in your config.sys. Furthermore, a spontaneous RAM failure would cause your machine to lock up, corrupt the screen, behave erratically, or something like that, not just give you a message about not having enough memory.
I did purchase and try a couple replacement power supplies to test out the power supply theory. They were labeled as “new old supply” so allegedly were sitting in a warehouse for a few decades, so there’s a chance those were also no good haha. Seems unlikely that they would all fail in the same way, but who knows!
New-old-stock PSUs from a warehouse…hmm, sounds as if they came from
Computer Reset.
CR had (and still has) boatloads of NOS long-board PSUs, and of the 100+ we used in PCjr kits, I’m aware of only 2 that failed. They seem pretty solid.
