External Power Supplies:

• TI-99/4A USA: 

Internal Power Supplies:

Power supply from a 99/4 (NOTE: NOT COMPATIBLE WITH 4/A)

Power supply from a 99/4A

Power supply from a 99/4A QI

 

This is the connector on the back of the console that the ac adapter plugs into. Some of these connectors have 4 pins, while others have 3. The newer power supplies (QI) only have 2 pins. They way they were set up you can use any ac adapter with any of the consoles.
	The AC voltages in are: 1. Black 8 volt 2. White 16 volt 3. Red 5 volt 4. No connection

The power supply is a very standard setup, and could be replaced by any standard PC power supply.
	The DC voltages out are: Pin 1 = -5 volt Pin 2 = +12 volt Pin 3 = GND Pin 4 = +5 volt.

No AC Voltage? check the fuse!

TI was required by the UL to add a fuse to the external power supply. Their solution? add a pigtail to the power cord with the fuse in a little plastic box.

These fuses tend to blow after so many years of use, so the fuse needs to be replaced or just remove the pigtail if you are using a power strip with built in circuit breaker making the fuse unnecessary.  You will find it’s just stuck on the old plug with some adhesive and it should come apart with some effort.

Fuse type:  4/10A 250V 

 

Schematic for power supply (not QI)

Here is an interesting project replacing the console power supplies with a new board: