The modulator wants to see a 75Ω load, not some random match through a crappy matching transformer. This is why you need several feet of RG6 or 59.
All the radio guys know antenna is everything. Connect some random antenna to just about any transmitter over a few watts and the final transistors will instantly short.
These modulators are fairly robust, but the hybrid package they use for a final is not feasibly replaced. If it fails best to buy a whole new mod
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