The early (Very Early) no S & S HL tanks were on 27 & 40. They had a switchable A-B-C setting to be synced with the tank. Many worked only on one of the ABC settings. Later when the smoke and sound tanks came out, they went to the crystals HL# 1 to 4 on 27MHz. Six more are available in the aftermarket. FYI, these are not the same as Traxas crystals. Good luck.