I’ve had a lot of problems with my Dual Field units not tuning at preset Delay. There will be a strange, smooth overtone “hum” instead of the usual sharp crackle and no targets will respond to the coil. With this fault the machine needs to be turned up to almost one quarter before the normal t/h is heard. This is not the board or coil, but a simple moisture problem. The solution was taught to me by Bill Lahr–the SMPI circuit’s developer. Bill suggests setting the opened unit in front of a medium heat hair dryer for 10 minutes on, 10 minutes off. As you complete a few cycles you will see the proper t/h coming in lower–closer to the preset. To open the unit, sit in on the ground and step on the batteries with your dominate side foot. Then crimp the plastic spring with pliers–being careful not to pinch the board. Then get a flat screwdriver in the gap at the end and pop the cover panel off. Do the same thing with the actual board–being careful not to break the corner off. Then stand the board up and gently slide it into the end of the compartment–so that it sets in an “L” configuration. The unit can now be stood on it’s side. (It’s best to remove the cells so you don’t short anything with the side clips). Stand the unit about a foot back–testing to make sure there is not too much heat even once the drier has heated up fully. If this doesn’t work, make a cardboard partition to deflect the heat away from the board and focus the heat on just the pots array. If you have some Deoxit give them s very short spritz. Cheap contact cleaner is not a good idea as it will not dry completely. This is how to restore a Dual Field with this frequent moisture related fault. This same process is the way to dry out a flooded unit. It’s also the first thing to try with any Dual Field that’s acting up. (After removing any salt water residue).
Good Luck Detecting,