The IC-245 may be Icom's first 2 meter all-mode radio. When it went on sale (in the 1970's), it was sold as a 2 meter FM mobile rig. An add-on SSB/CW unit was available as an option - note the included picture.
I purchased my IC-245 (used) in November 2001, with one plan in mind - working satellites! This radio would be used as the uplink radio on some satellites, downlink radio on others. It has the optional ssb/cw unit installed, but I didn't realize at the time that many satellites use inverting transponders, and that not having Lower Sideband (LSB) capability would make some satellite operation tough (or impossible)!
An inverting transponder is used on many satellites to minimize the doppler effect. Signals that are uplinked at the bottom end of the passband are downlinked at the upper end of the passband. Another characteristic of inverting transponders is that SSB signals are inverted (from LSB to USB, and vice versa). The convention for using linear transponders (inverting or not) is to uplink so that the resulting downlink uses Upper Sideband (USB).
While assembling my satellite station, I learned about inverting transponders, and realized that operation on birds utilizing mode J (145 MHz LSB up, 435 MHz USB down) would be restricted to CW only - I could not uplink LSB! This presented a problem to SSB operation on FO-20 and FO-29! My only other hope would be attempting to find someone willing to work me 'backwards' (USB uplink, resulting in a LSB downlink).
I didn't have this problem on all satellites, RS-13 used mode T (15 meters up, 2 meters down) and did not use an inverting transponder (USB on both). Sadly, RS-12/13 perished in August 2002. It had quickly become my favorite satellite, having ample room for 'ragchew' QSOs, a welcome change from my FM satellite roots!
With RS-12/13 gone, and only CW capability on the Fujisat birds (FO-20 & FO-29), I decided to try AO-40 - full story here , and the IC-245 would serve as the IF radio for my 2.4 GHz downlink. I do not need LSB for AO-40 receive, so the lack of LSB capability was temporarily off my mind.
I had scoured the internet looking for any plans to allow LSB operation, with no success! A friend promised to send me the details as soon as he could find them, and 2-3 days later, they arrived!Finally, LSB/USB capability!
Modification using TS/NOR switch
I am listing two different mods - this one uses the TS/NOR switch to select USB/LSB. Disclaimer - This is an untested mod, use at your own risk! I believe that this came direct from Icom, but cannot be positive! I am not responsible for damage, etc!
Modification Steps
With the IC-245 disconnected from power and after carefully removing the top and bottom covers, and unsoldering the speaker leads and disconnecting the key leads, the following steps are taken to actually convert the unit to an LSB/USB switchable unit:
- Remove the pink & green wires from the TS/NOR switch. Solder these both to pin 1 of the NB switch. (Used as tie point)
- Remove black wire connected between the TS/NOR switch and the CW-T switch.
- Very carefully remove C64. (located behind XTAL which is behind CW-T switch)
- Install a mini 10pf trim cap in C64's holes.
- Solder a 15pf cap across the 10pf trimmer on the bottom of the board.
- Remove D11 and replace with the following in figure 1:
Figure 1
- Connect wire from junction of 470 ohm resistor and diode to pin 6 of TS switch.
- Install a wire from the cathode of D13 (located on the other side of wire loom from D11) to terminal #5 on TS switch.
- Add a 1K trimpot as shown in figure 2
Figure 2
- Move red wire from X to pin 2 of TS switch.
- Add 1K pot, and wires to TS switch as shown.
- With the TS switch depressed, align the 10pf cap from 10.7015 MHz at CP1 or to half mesh.
- Adjust the 1K trimpot (added) so that the LSB signal is on the same frequency as the USB signal.
Note: Figure 2 notes are difficult to read, even on original copy. The descriptions are: (counter-clockwise from left)
Currently installed in unit
Bottom View
TS sw
To Diode D-13 Cathode
To Diode & 470 Ohm resistor at D-11 position
Red Wire Previously connected at "X"
Theory of operation
The circuit which normally offsets the USB crystal enough to put it in the crystal filter band pass for CW, is used to move the USB crystal all the way across the filter passband to the lower sideband BFO position. In addition the CW feature is still retained.
Removed was the capacitor C-64 which used to offset the crystal for CW and in it's place was put a variable capacitor and a padder so that the frequency of the crystal would go up when diode D-1 was back-biased. The variable capacitor allows us to put this on exactly 10.7015 MHz as needed. The voltage used to activate this circuit comes from what used to be the TS (tuning speed) switch.
In the CW mode the 470 ohm resistor and the 1N914 diode still offset the crystal into the passband of the filter by turning on the offset circuit only partially. No problems have been found with excessive drift with this technique.
The 1K pot is used to offset the VCO frequency in LSB so that no frequency change will be noted when switching between sidebands. Without this resistor, approximately 3000 hz shift is noted when switching between sidebands. This 1K pot is adjusted for zero beat on LSB that coincides with a zero beat at the same frequency on USB.
LSB is selected by pressing the TS switch in. USB is selected with the TS switch out.
Use of the TS switch for sideband selection has not deleted your capability to fast tune (5 KHz steps) in the sideband mode as described in the manual. You can accomplish the same thing by merely switching back to the FM mode and tuning to the desired frequency. You then switch back to SSB for 100 hz tuning.
Modification using NB switch
I am listing two different mods - this one uses the NB switch to select USB/LSB. Disclaimer - This is an untested mod, use at your own risk! I believe that this came direct from Icom, but cannot be positive! I am not responsible for damage, etc!
Modification Steps
With the IC-245 disconnected from power and after carefully removing the top and bottom covers, and unsoldering the speaker leads and disconnecting the key leads, the following steps are taken to actually convert the unit to an LSB/USB switchable unit:
- Remove the 2 violet wires from top and bottom of NB switch. Splice and tape these together.
- Remove the 3 brown wires from top and bottom of NB switch. Splice and tape these together.
- Very carefully remove C64. (located behind XTAL which is behind CW-T switch)
- Install a mini 10pf trim cap in C64's holes.
- Solder a 15pf cap across the 10pf trimmer on the bottom of the board.
- Cut the anode end of D11. (located approx. 1 inch behind XTAL) Leaving 1/4 inch of lead from D11 for another connection.
- Install a wire from the anode of D11 to terminal #6 of NB switch.
- Install a wire from the cathode of D13 (located on other side of wire loom from D11) to terminal #5 on NB switch.
- Install a diode (1N 914 etc.) cathode to terminal #6 on NB switch.
- Install 470 ohm resistor from anode of this diode to terminal #6 of CW-T switch.
- With NB switch depressed, align new 10pf trim cap for 10.701 MHz at CP1 or if set at approx. half mesh should work.
Theory of operation
The wires removed from the noise blanker switch are connected so that the blanker runs continously while in the SSB mode. This has not been a problem for those who have done this mod. However, if you wish to turn the noise blanker off a small spst switch can be mounted near the key jack on the rear panel.
The circuit which normally offsets the USB crystal enough to put it in the crystal filter band pass for CW, is used to move the USB crystal all the way across the filter passband to the lower sideband BFO position. In addition the CW feature is still retained.
Removed was the capacitor C-64 which used to offset the crystal for CW and in it's place was put a variable capacitor and a padder so that the frequency of the crystal would go up when diode D-1 was back-biased. The variable capacitor allows us to put this on exactly 10.7015 MHz as needed. The voltage used to activate this circuit comes from what used to be the noise blanker switch.
In the CW mode the 470 ohm resistor and the 1N914 diode still offset the crystal into the passband of the filter by turning on the offset circuit only partially. No problems have been found with excessive drift with this technique.
|
