Editing IBM PCjr Troubleshooting Guide (section)

=== System will not power up ===

* Verify the wall outlet, the AC mains lead, and the external power brick output. The PCjr's brick supplies regulated DC at the system unit's DIN connector; check the pinout in the IBM PCjr Technical Reference.
* Verify there is no short on the brick output. A failed tantalum on the motherboard's +5 V line will pull the brick to fold-back protection and the system will not power up.
* Remove all sidecars and cartridges. Re-test. A faulty sidecar (especially a memory or modem sidecar with a failed tantalum) is a common cause of "PCjr will not start."