Tuesday, May 4, 2010

LSB for the IC-M700D

As I wrote before, the Icom IC-M700D, the German version of the famous IC-M700 marine radio, did not come with LSB. What I was reading on the internet lately, there is a French version, the IC-M700F, which also lacks LSB. There is hope, maybe something could be done...

Luckily, my IC-M700D came with a circuit diagram. The schematics of the IC-M700 can be found on the internet.
Studying both circuits, the following can be said (corrections made, due to an error in the Service Manual):
  1. the MAIN UNIT pcb of the IC-M700D is the same as the one of the IC-M700
  2. both sidebands are using the same oscillator
  3. two crystals are switched by means of 1SS53 switching diodes (a 1N4148 will probably do)
  4. the LSB crystal's traces are present on the D's pcb, so is the LSB control lead from the MATRIX (ending at Q334's base)
  5. the D's LSB crystal and surrounding bits'n pieces (a switching transistor(2SC3402), a varactor (FC51M), an inductor and some resistors) are not populated
  6. for LSB a 4.908MHz 4.920MHz crystal is required
  7. the mode switch needs to be wired up correctly, there may be a further transistor (Q1210) missing on the MATRIX pcb, further, there could be a jumper somewhere...
Have a look at the IC-M700D PCB

and the corresponding IC-M700 circuit diagram

 The question now would be, if there is any cheap option for the crystal. Well, I figure, we are lucky on that one... there is a cheap crystal for 4.9152MHz. I intend to open one up and pen it down to 4.908MHz grind it up to 4.920MHz. A drop of 7kHz  raise of 4.9kHz should easily be doable, although, this is a little harder than it sounds.

The plan now is to pen down grind up a crystal first and think of the rest of the modification later.

Remark: Never trust a Service Manual!
Reasons: The filter center frequency is 9.0113MHz. The (USB) beat frequency is a mix of 10.24MHz and 4.908MHz divided by four, i.e. 10.24-1.227=9.013.
To get to the other side of the filter, the resulting beat frequency should be higher, i.e. 10.24-(4.92/4)=10.24-1.23=9.010MHz.
I should have seen that before going into penning down crystals, which was a good exercise however.
Now, I need to do both, grinding up, and pen down, when ground away too much crystal material....