You are trying to operate the new RX with the old TX? Or the new RX with the new TX? or the old RX with the new TX?

If you don't even know which frequency the old TX is on... its probably not on the same frequency as the new RX. (same goes if operating old RX with new TX... gotta know what frequency the RX is on)

The crystals in the RX's will have a little tape flag on them telling what frequency.

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All Futaba PCM capable TX's I have seen have also been PPM capable... Check its settings. If its in PPM mode than it has a chance at operating the Tower RX if its on the same frequency.

Jim is correct. The Tower system 3000 series radios are "FM" (also called PPM)

A PCM signal might wiggle the controls erratically at best. If my 7UAP is on PCM mode the PPM RX has the servos jump a bit. (if the PPM RX is on the same frequency as my PCM TX... My other frequency RX's totally ignore the PCM signal.) When the PCM capable TX is in PPM mode the Tower System 3000 RX I have works fine.

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ASSUMING both RX's are PPM...

If the Trainer's radio syttem all works... you can pull the RX crystals and swap them. Now you can check to se f the old TX operates the new RX OK.... and if the new TX operates the old RX (don't fly that way... without confirming that there's no problems... this is just a ground check) If the new RX starts working with the crystal change and the old one stops working... you have a bad crystal.

If it passes or fails... swap the crystals back. If you proved you have a bad crystal... thats what you need to buy. (match the label on the crystal. do not put a different maker's crystal in.)

Do not swap crytals in the tX's. That is against FCC rules.