{Collins} 51S-1 PTO Issues

Working on bringing SN 131 back to life. Found PTO not PTO-ing.........
External to the PTO someone at one time replaced R62 with a 100 ohm
resistor, schematic calls for 1k. So far this is the only area of rework I
have found.
Resistance measurements on V15 pins 5 and 6 were not making sense. Pulled
PTO out and found R504 was fried and R503 measured 77k - should be closer to
Measuring between pins 1 and 4 of T503 shows 33m - basically open. Caps C507
and C508 are not shorted.
Bottom line, I can replace all of the parts to bring things back in spec
BUT, what about T501? Anyone know of a source for these?

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