Or modify your HM-152 to ground itself out. The gray lead that is connected to the mike hook knob can be connected to pin 2 (counting from the left to right) on the PCB where the leads are soldered from the mike cable. Voila. Your mike is grounded and your radio will scan.
DO NOT solder the leads together on the radio itself! It will work to force on-hook, but you will not be able to read or write the radio (communication uses the #8 'CLI' hook lead).
Pinout:
1: +8VDC for condenser mic bias and DTMF buttons
2: [CLO] Clone data out
3: AFO out (not speaker line level by default, but can be changed in some radios)
4: PTT (low to TX)
5: [MICE] Mic audio ground (Mic audio is balanced for noise reduction)
6: [MIC] Mic audio in
7: [GND] Chassis ground
8: [CLO] Clone data input when programming, AND low to put radio in mic-on-hook state.
NOTE: Icom's numbering, which may be backwards depending on how you look at the connection. Easy test to identify pin 1 is by checking for +8VDC when the radio is on.
Unlike Icom's newer radios ignoring the hook state is not a software option. There is a solder "bean" marked "CP1" on the F1721/F1821 front face board for the exact purpose of telling the radio to ignore the on/off hook state. A tiny drop of solder will make the radio think the mic is grounded no matter what, plus it will not interfere with programming.
Or you can ground the mic internally. Same effect, but from experience mics can be a pain to take apart while the front board solder bean is right out there and easy to get to without disconnecting the ribbon cable..
Or you can get a HM148G which has a split hanger button on the back. As long as the front half and the back half are connected by the hanger the radio is 'on-hook". A must for plastic dashes where grounding the hanger is not an option.