*** SOLVED ***
Dears
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After hours of wire tracing and elimination of possible defective components, I finally ended up at a..... stupid mod made by a previous owner for whatever reason.? One single resistor that screwed up the behaviour of various functions of the T-4XC.? @Gary: thanks for the tip!
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One other beautiful Drake rig preserved for the future :)
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73 de Rob PA9Z
Rob PA9Z