Hello Mark,
I had a 464 and mine had H.V. problems... so I know quite well this one.
Well... There is no test that will give you absolute certainty that T1501 is in working order, besides seeing the circuit itself working, but there are a few things you CAN do.
Be warned, however, that to diagnose / troubleshoot this circuit, you will need to remove the H.V. protection shields and you will be playing with a toy that can bite you big time.
Use the usual caution measures... Insulated shoes or floor mat, keep one hand on your pocket, and preferably, have someone nearby.
Past the histeric precautions
1. First of all, make sure you have +15V UNREG past the fuse F1487, reaching pin 5 of T1501 and leaving the other side of its primary, pin 6, and to the PCB tab that goest to the collector wire of Q1486.
2. Since Q1486 is a TO-3 transistor, and the collector terminal is fastened with a screw and terminal... checking only where the wire leaves the PCB won't tell you much if the screw is lose.
The only way to be sure here is unsolder the 3 wires from the PCB, unscrew the 2 bolts that hold the transistor cage and slide it out (from the 464's right side).
While you have the transistor cage and transistor in your hands, make sure the screws are well fastened, that the collector wire makes good contact to the transistor's case, and take the opportunity to test the transistor, if not with a transistor tester, at least a full diode test in all the junctions. Beware of old ohmmeters that uses 12V batteries when measuring the E-B junction in reverse, as this can irreversibly (and unpredictably) damage the transistor.
Take the opportunity as well to make sure that the transistor case is not making contact to the cage (in case the insulated washer gets crushed by too much tightening).
When all is right, put the transistor back in place and solder the wires.
Last but not least, check the part number of the transistor... if it's an original Tek Transistor (and if it's the right part number) or if it's an industry standard (e.g. 2N3055).
3. Take transistor Q1492 out (to rule-out that the oscillator is being "shut-down" by this protection transistor (It's just for a while... you won't actually run the scope without it).
4. Take transistor Q1484 out as well (this transistor adds juice to the oscillator, but even without it, the oscillator must be able to run on its own).
5. Hook a voltmeter to pin 3 of the transformer (where it's connected to the emitter of Q1484).
6. Turn on the scope, while monitoring the voltage on the voltmeter...
6.A.If it goes negative, it means the oscillator started. It would usually take no longer than a couple of seconds to start oscillating.
Turn the power down and try to insert both transistors back, turn the power on again and see if your problem is solved.
6.B. If it stays positive, the oscillator is stuck... That can have many causes, including an inadequate Q1484 or a faulty T1501 or many other things yet, (even a worn out C1487)
Let us know the outcome.
Rgrds,
Fabio
Fabio Trevisan