Thanks for the replies.

Yes, the transmitters are (or were) 75MHZ when made, the box has a label mentioning the frequency of the crystals that originally came with the transmitter.

The receivers (R122JE model, bought used at eBay) I have no idea, but I have found some pages on the internet mentioning 75MHz and 27MHz versions of them. So I may or may not have the correct ones, and I would like to use my electronics gear to find out.

I'm also sure I'm using AM crystals, matched to TX and RX. The RX ones have a sticker that I use to easily pull them from the mounting on the receiver. The TX ones are bare crystals, no sticker.

Thanks for any help you can give me!

Eduardo