Icom IC-751 All Band HF All Solid State Transceiver

As I said on the previous page, this rig had a label on it saying "the smoke has been let out". A quick insection of the insides showed no sign of burnt components or tracks so I did what I usually do when I am confronted with a repair like this, I joined the appropriate Web Forums. Once accepted for membership I asked the question, where may have the smoke come from. Within a week I got an answer that pointed me in the right direction.

May call for help read: G'day folks I have an IC-751 (not an A version) that was working well and then there was a cloud of smoke out of the PA module on the back.The only evidence I can see so far is R22 (accross a toroid in the collector circuit of output devices) has a crack down it and it looks like it has been very hot. As this resistor is just a shunt accross the inductor I cannot see how this will stop the output stage but there may be something else I am not seeing here. The resistor looks to me like a 5 Watt carbon device as it would have no self inductance. If anyone can shed some light on this it will be greatly appreciated.

The reply I received back from Michael read: I have actually repaired a friends 751 with this exact symptom. An output transistor shorted. I caused high current in the toroidal choke. This melted the solder and opened the choke. At that point all the current had to go through the resistor. It smoked. Good luck, it is not difficult to repair. In a later email he mentioned the PA transistors were still available on the internet but they were quite expensive.

My rely to Michael: G'day Michael thanks again, you were correct. I am sure we now know what happened. I have taken the board out and there was no burnt solder but one joint looked like it may have been fractured/dry. I lifted the Collector and the Emitter of each transistor and they looked fine. The only thing wrong I could find was R22 (4.7 Ohm) open circuit so hence the smoke. When I first got this rig on the bench I measured accross the choke on the top side of the board but not on the tracks under the board. with the solder joint to the BIG RF choke fractured the choke was open circuit and the only current to the PA transistors was through R22 and it was only there as a damping resistor accross the coil and soon let out the smoke. Interesting, these transistors have a built in protection diode from Collector to Emitter but it does not mention that on the Mitsubuish data sheets I have found on the web. Would not be unusual.

I am going to be dedicated to driving my 400 Watt linear amplifier which in turn will be connected to my Tet-Emtron three element, three band HF antenna mounted on an eight metre tower. Above this antenna there is a vertical polarised three element 2 Metre yagi, on the oposite side of the tower is a fourteen element 70 cMetre yagi and sticking out of the top of the tower at 10 Metre hight is a 70 cM J-Pole.

A later comment I received from Michael read: Glad you did not have to buy a final transistor. I guess the weight of that choke broke it loose from the board. Maybe it was used mobile. That would do it. That resistor cannot handle all the power to the transistors.

When I was looking at this radio with a scope I discovered how bad the signal looks as it comes out of the PA board. It is truly ugly. The filters clean it up nicely. When I was working on it and had just replaced the transistor and put everything back, I looked at the sine wave at 75 meters. I expected it to look OK. It was so bad I believed I had done something wrong. I then looked on the other side of the 75 meter filter and the sine wave looked fine. I also checked another 751A and found the same thing. These amplifier boards are a long way from linear

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Written by and Copyright, Phil. Storr © Last updated 17th Spetember 2013