Icom IC-781 Installation Notes, ALC mod, Fan mod

I bought one of the first in 1986 and have bought another one since. It is a fantastic radio, perhaps the best analog transceiver ever made. Operation of this rig is straightforward, except for the microphone. The 781 has much lower gain in its mike pre-amp circuit than does Kenwood or Yaesu rigs. This is because the 781 assumes the use of an electret condenser microphone while the other rigs assume a dynamic microphone. While there are some good engineering reasons why the rig is designed this way, many hams still insist on trying to mate incompatible dynamic elements to this rig. Now that there are inexpensive condenser headsets on the market for computer use, there an easy way out of this dilemma. While many of these are too bassy for communications use, two models I've used work well. They are the Plantronics 60 and the Heil IC models. These mikes will provide the proper ALC and compressor functioning to achieve good audio. 
If a dynamic microphone, like a Heil HC4 or HC5, is used with this rig, it is advisable to install a pre-amp between the rig and the mike. In my case, I simply modified an Icom SM-8 desk microphone to accept another microphone input. This was accomplished by removing the cable for the second rig and installing a normally closed mini phone jack in the hole normally occupied by the second rig cable. When a dynamic mike is plugged in, it disconnects the SM-8 mike element and inserts the dynamic mike element (put a small .01uf capacitor in series with the element to block DC). The SM-8 has a built in rig powered pre-amplifier, which is capable of providing adequate output to the 781 when driven by a dynamic mike element (the ARRL Handbook has a suitable circuit, if you choose homebrew). If you'd prefer a desk mike to a headset, the easiest thing to do is to remove the stock SM-8 element and replace it. I've done this with a Heil #5 dynamic element and it works great. When the 781 is modulated by a properly matched microphone, the ALC meter is easily kicked up with a relatively low microphone gain setting (around 10 o'clock), without the compression engaged. The bottom line is this: matching the microphone output to the optimum input level of the radio will greatly improve the audio performance.
The only other annoyance with the 781 has been the large spike generated on the leading edge of the output. Up to now, I have controlled this spike by keeping the drive level low, but my Alpha 87A often faults due to this high power spike. Moreover, amps with auto tune, like the 87A and the Acom 2000, never settle down because the spikes make the amp think it needs to retune.  Recently, I heard of people actually blowing their finals and decided that it was time to correct the problem. Thanks to the fine work of Vince, K1VF, the solution to the problem has been documented. Vince discovered several capacitors whose values were switched, some incorrect resistor values, and a missing diode. It's a mystery why Icom produced 781's that don't match their documentation, and then denied knowing how to fix a problem that's correctable simply by putting a few components back where the schematic says they are supposed to be. Here's a summary of the fix:
On IF board: C47 = 2.2uf ; C48 = .47uf (swap C47 and C48); R108 = 100 ohms (was 4.7k); R112 = 4.7k ohms (was 1k); D15 = 1N5711 or 1N914 (was missing).
On PI board: R8 = 4.7k ohms 1/2 watt (was 5.6k) - This change raises the voltage to the ALC circuit from 7.5v to the specified 9v. 
I have performed this modification on my 781 and the spike is gone. The amplifier runs much better and no longer faults or strays into the red zone. My contest scores will go up and my tubes will certainly last much longer! If you don't trust your tech skills, get a qualified technician to perform the mod. Don't get in over your head.
Rob Sherwood, NC0B, of Sherwood Engineering (famous for their Drake C-line enhancements), has a replacement cover for the internal power supply that adds a welcome cooling fan to what is one of the hottest parts of the rig - a circuit that is not cooled by the internal squirrel cage fan. I easily installed one on mine (don't crimp the power wire under the cover!) and it's sure to lengthen the life of this circuit. http://www.sherweng.com/IC781Fan.htm

1 comment:

  1. New user group forum to replace the old yahoo group for the IC-781. Need your support - https://groups.io/g/IC-781