Quick hack CW transceiver by ancient FM handy rig

Details

The basic idea for receiver section modification is quite simple. The original FM transceiver, not limited to IC-2N consisting double conversion with IF 10.7 MHz and 455 kHz. The local signal for the first mixer is generated by PLL. In deed if we make it from scratch, even now this signal generation is a headache but we can simply utlize in the modified CW receiver. After the 1st mixer, frequency is again converted to 455 kHz. This conversion and FM detection (F-V conversion) is mostly performed by super best seller IC, Motorola MC3357, 3359, or 3361. For second IF signal generation, 10.245 MHz oscillator is prepared by the crystal element. Here if change this frequency to , for example, 10.6985 MHz, the second mixer will work as product detector, and directly demodulated signal will be obtained. This way is, indeed, not new and in 80's 90's DIY receiver utilized for its simplicity, but I could not find "modification" in the any website....

Project Logs

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Next challenge: Standard C-111 144MHz FM
kodera2t • 07/15/2024 at 07:56 • 0 comments

It seems, no one on hackaday.com has interest on this type of hack, so I will close this project by this log.
FM detection circuit of C-111 consists of TK10420L, which is quite similar to MC3357.
By removing 10.245 MHz crystal and 455 kHz filter and bypassing FM detection block will make C-111 into SSB/CW receiver.
How to make IC-2N into CW transceiver?
kodera2t • 07/11/2024 at 10:24 • 0 comments

The modification is so simple. Just

(1) de-activate FM detector by MC3357

(2) change local oscillator crystal to 10.6985 or around

(3) change crystal filter from FM type to SSB/CW narrower bandwidth filter

That's all!

First, just we need to make path just after pin #3 of MC3357 (output of 2nd mixer) to the detector output. I did not pull out any component, so if I am tired or have interest on FM communication, just remove jumper and recover the cut patter will make it FM operation again.

Fortunately I have a small size 10.7 MHz crystal filter (from KENWOOD rig) and simply inserted instead of wide band FM crystal filter.

That's all. I confirmed -136 dBm CW signal detection by this modification.

The fault point is, the PLL has 10kHz step, meaning for example it can't receive 144.075 MHz but 144.080 will be ok.
So, So, so far, I've touched "receiver" not transmitter. How we make TX?
In this era, the FM modulation is realized by direct modulation to VCO in PLL loop. The FM signal without input is, yes, continuous wave with single frequency component. So I just cut a purple line connecting microphone amplifier to VCO.
After this modification, PTT switch turns into Morse code keyer and microphone will not work anymore!