Your voltages are low.
This device is not a conventional regulator and is a transistor. It's still acting as a regulator however.
I measured mine and the center pin appears to be the supply voltage supplied to the radio from the power jack.
Mine reads 8.42 on the left leg.
11.91 on the center leg and 7.79 on the right leg.
11.91 would be correct for my power supply as it comes from a supply set right on 12.0 VDC.
Something is drawing your power supply down before it gets to the regulator being as your device is not even warm to touch.
Look at the large capacitor near the rear of the radio close to the power jack, does it look normal with no signs of bulging. You may also feel heat from this cap if it is partially shorted.
Somewhere near that cap is probably a reverse polarity protection diode that could have failed. I'd think it would be obvious though and they usually fail completely instead of partially. If the diode is shorted, it should be producing heat as well.
Try and feel components around the large filter capacitor and also around the area below the three legged device you measured.
There is another three legged device mounted on the board almost directly below the device you tested. It is also a regulator. The right leg should be near 9 VDC and the left leg should be B+ in. The ground is on the center tab and is soldered to the board for a heat sink. It's the same as the unused middle leg when looking down at it. Chances are you will also find the voltage on its left leg reads very low. It should be close to 12 VDC just like the center pin of the device you tested.
With the voltage so low, I'd think you should be able to detect heat somewhere on a component between the power jack and the device you tested or the one below that is soldered directly to the board. The device soldered to the board is marked Q2. As UBBE said, be very careful if you try and measure this device!
The large filter cap near the rear is just to the right of a small transformer. My transformer is wrapped in a red color and yours is likely the same.
I'd suspect that cap may be the problem. There is also a fairly smaller electrolytic cap mounted near the large one. Pay attention to it for heat or signs of bulging out the top as well.
I forgot to write down the number for the large filter cap but it was something like C168 maybe.
I can open mine back up easily if I can help take some readings for you. It's my only radio that I can easily access!
edit: I could not determine which wires are going to the power switch for what UBBE calls the SWB. Without that, it's just a guess if the switch is passing full power. If you can measure the voltage at your power connector with the radio on, that should tell you if the supply voltage is being drawn down before it even hits the radio or not.
The power wires to the on/off switch are in that 8 pin connector you mentioned. One in and one back out. The voltage difference between them should be minimal with the radio on if the switch is passing current correctly. That would be a point to check if you can figure out which pins the power in and out are on.
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