Icom IC-751 (IC 751 IC751) transceiver mods reviews software and diagrams

Posted on Feb 7, 2014

The 751 required turning the volume control almost to 12 o`clock before getting any hearable volume so I thought maybe something was wrong here. I replaced all the electrolytics associated with the audio path, i. e. after the detector stage and the audo amp. I couldn`t really hear any difference but at least I felt better. At this point, I dug out the scope and started

Icom IC-751 (IC 751 IC751) transceiver mods reviews software and diagrams
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looking at where the thumping was coming from. The 751 uses two electronic switches (IC7 C and D) to mute the audio, one for the actual audio and one for the monitor/sidetone circuit. The scope indicated that maybe these were causing the pulses generated when they were switched. (I should have realized at this point that this wasn`t the case because when switching to transmit a large positive pulse occured and when switching back to recieve a large negative pulse was generated. I figured out later what was happening. ) Anyway, I started cutting traces and adding jumpers and effectively bypassed the electronic switches. What I ended up doing was connecting the monitor and audio stages to the audio amp with no switching. This basically fixed the problem but I lost the use of squelch and the volume was reduced quite a bit. So I started looking elsewhere. A couple of things I learned was that the electronic switches were not needed for cw operation. The receiver mutes very well without them and the monitor/sidetone only works when the key is down. This is exactly what was needed. The electronic switches are really only needed for squelch operation but the circuit designers built the 751 so that the audio path is switched off when going to transmit and the monitor/sidetone audio path is switched on when in transmit. The 761 uses a couple of transistors to "short" the audio path to ground when in transmit. I built up a little...

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