Why do you want a PS-501 as opposed to a PS503 or PS503A? Except for the built-in meter, the PS503 is superior, having dual +/- 20 volt supplies with tracking capability. The PS503A can supply 1A on the +/- 20 volt supplies when used in a high-power slot in a TM504 or TM506. The vintageTEK Museum has a number of PS503's and PS503A's and we could set up a sale.
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Bob Haas
Bob Haas