Take a look at the transistor that is used to key the rig. That's usually the last transistor in the keyer, and the line that would go to the 520. Usually across the emitter and collector circuits. Sounds like it's shorted. My guess is you tried to key the rig without a buffer stage between the keyer and the 520. For the most part, the keyers are great for a solid state rig, but for tube and some hybrids, the keying transistor won' handle the keying current. Buffers circuits are all over the web. The Heath manual might have a circuit in the in it for use with tube circuits, or describe their recommendations. I don't have a manual handy, so check in yours. Good luck.
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