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ICOM IC-735 as a Receiver for WSPR on 630 meters – By Rick, W7RNB / WI2XJQ

– Posted in: 630 Meter Instructional Topics, 630 Meters

Below, Rick, W7RNB / WI2XJQ, explores some of the challenges and resolutions he had while using an older amateur receiver on 630-meters with modes requiring high accuracy and precision with respect to frequency.


At the beginning of my journey on 630 meters I did a lot of reading about different topics about the 630 meter band. There is a lot of information on the web for those who are considering applying for a special license and equipment being used. One piece of equipment mentioned is the ICOM Ic-735 transceiver both as a receiver and as a transmitter.

In keeping with my experimentation to build a station that was simple to use and to maintain two Ic-735 radios were purchesed. One for transmit and one for receive. These radios albeit excellent HF radios of their time are now older and certainly out of any service warranty possibilities.

So, troubleshooting and repair can be pretty omnious if one should suffer a failure. But their reasonably priced and plentiful, so another can be had off the web.

One radio having been received was put through its paces in my radio shop and it faired up quite well as a 474Khz receiver. This radio, in some regards, is well suited for MF because it is a triple conversion receiver that is pretty quiet and seems to handle outside noise influences well. Installed into the station and set up for 474Khz it was noticed that the digital dial was about 100Hz low when receiving WSPR. Although this error wasn’t seen as a big deal it was certainly not desirable.

Additionally it was discovered is that there is no 1Hz digit, nor is the RIT frequency displayed. Reading the maunal tell us that only the 10Hz digit is displayed. This makes things a bit more complicated for tuning to the required 474.2Khz of WSPR because one to two Hz does make a difference on how signals are received and reported.

It was decided to take the radio off line and try to find the 100Hz error in tuning. A factory alignment was done, using the procedures provided by ICOM in the service manual. Although somewhat complicated and time consuming it was discovered that for normal everyday use the radio was in good alignment. But for WSPR it was multiple parts of a 100Hz off. The good news is that alignment, with the right test equipment was a success. Paying attention to alignment measurements yielded a well tuned receiver.

Something worth mentioning before going on, is that being that these radios have been around for sometime. People have, most likely, been inside these radios with the “golden screwdriver”. So don’t be surprised that some measurements may very well be outside of the published specifications. That is what was found here. Both radios have been “tweaked” to some degree and realignment was definitely necessary.

At the completion of the alignment, the radio was placed back in service and set to 474.2Khz. Now stations began to come through on WSPR and decodes were reported up to 50Hz high. This is due to not having the capability to “fine tune” the receiver to exactly 474.2Khz using the main dial. So, the RIT had to be employed. As part of the alignment procedure the RIT circuits are tuned. Close attention to this detail is well worth the effort. And in the end yielded the ability to tune as close to the desired frequency as possible.

Now that the radios is working correctly and reporting properly, only one more issue remains. When the temperature of the radio and the room are constant the reports are quite accurate, but as the temperature drops in the room, over a long period of time that reports drift down 7 to 8Hz. Make sure that covers are installed and all the screws are in place so as to keep the internal temperature of the radio as constant as possible. A slight realignment of the RIT maybe necessary from time to time but overall the radio performs quite well and it worth a look.