Check the transistor to ground with power off. Put your DMM lead on the metal tab of the transistor and the other lead on ground. Does it buzz out (show 0 ohms). If so, the transistor is bad again. If not, then the circuit that drives the transistor is turning the transistor on. There I'd suspect one of the IC chips.

Ground strap in the head....Or look on your board for a test point marked GND.

Yeah it's probably bad

Wait....That's a high power solenoid....Those aren't TIP transistors. They should be power Mosfets. The manual says STP20N10L replaceable by a IRL540N. You said you put in a 450N?

In anycase, you can't really test a Mosfet in circuit like you would a TIP type transistor. Sorry, I didn't realize what transistor you were checking when you gave your earlier info.

Try this....From Capcom's pinwiki, but same principle for the N channel Mosfet you have:

A locked on coil is often caused by a defective MOSFET driver on the driver board. With the power off, first test the resistance between pin 2 (middle) and 3 (right) of the MOSFET, disconnect the connector to the affected coil first so you only measure on the MOSFET. When the resistance is zero or close to zero the MOSFET is defective and needs to be replaced. Before powering on the machine after replacing a defective MOSFET, test the coil's resistance and check the tie back diode and it's connections because these are the main causes for a defective MOSFET. If the MOSFET seems to be good next to test is the input of the MOSFET on pin 1 (left) for a signal. When there's no pulse on pin 1 while testing the solenoid in diagnostics it could be a defective 74LS74 driving the MOSFET.

And more info:

Repeatedly broken MOSFET driver When a defective MOSFET is replaced but fails again after a short period of use, or even the first solenoid activation, test the coil's resistance and check the tie back diode and it's connections because these are the main causes for a defective MOSFET. This also occurs when a wrong type of MOSFET is used, a non logic level MOSFET (IRF540 for example) will not be driven correctly because the gate voltage will be insufficient to fully drive the MOSFET causing it to get too hot and fail.
STP20N10L MOSFET alternative In case of a defect STP20N10L MOSFET, this part is getting scarce but it can be replaced by the IRL540N which is readily available. Do NOT use the IRF540N instead (not logic level)! Please note that these MOSFETS are quite sensitive to static electricity.

So, I hope your 450N is just transposing and you actually have a 540N (and not a IRF version either).

Make sure the coil measures correctly (ohms), the diode measures good (just replace if in doubt), and that the diode is connected in the right orientation.

Next thing upstream is the octal D flip-flop U1.

Get a 20 pin socket while you're at it....Chip from here: http://www.greatplainselectronics.com/Category-60.asp?pg=2

Yes, that's correct and it is the chip right above that bank....

The problem is Stern's whitestar documents aren't the greatest, and there was never a repair guide done on Whitestar. So, I can only tell you what I see from Stern's limited docs:

You've already checked the transistor Q4...The only things left are the two caps, the resistor and the flip-flip IC....Unless the databus from the CPU is crapped, but I would expect other issues.

And yes, if you shorted that transistor to the extent you burnt traces you most definitely could have blown U1.

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