[ICOM] 756ProII Reset on TX Fixed...maybe

[Frank N. Haas]

I finally had a chance to work on my 1.5 year old 756ProII Saturday night. 
I believe I have corrected the problem myself.

To review: Any attempt to transmit would cause the radio to instantly and 
completely shut down and turn back on. This occurred even if the RF power 
output was set to its lowest level. The symptom occurred in all modes and 
with any type of transmit key (CW keyer, SSB VOX and front panel Transmit 
button.)

Icom tech support recommended swapping the power supply (PS-125) and many 
of you suggested I look over the fuseholders.

The PS-125 powered other 100 watt radios without any problem so I concluded 
the problem was NOT the power supply.

There are NO external fuseholders connected with either the power supply or 
the rig. I didn't open the power supply but inside the 756ProII I did see a 
5A power supply in what I believe is a DC to DC converter circuit.

The Fix:  I looked over the 5 amp fuse and didn't see anything unusual. No 
corrosion and no deformation of the fuse element. The 12 volt power leads 
from the Molex connector on the rear panel of the rig are screwed to 
inverted U shaped metal pieces securely soldered to the circuit board in 
the center of the top inside of the radio. I unscrewed each of the power 
connections (noting that they were tight to begin with) and examined them. 
Nothing unusual seen so I screwed them back down securely. A second set of 
positive and negative 12 volt leads run to the aforementioned DC to DC 
converter from the vicinity of the center circuit board as the input leads. 
This second set of leads are also screwed down to inverted U shaped metal 
pieces that are securely soldered to the circuit board. I unscrewed them 
(again noting that they too were tight to begin with) and saw nothing 
unusual. I reconnected them securely.

After doing all of this, I powered the radio, connected a dummy load and 
attempted to transmit. To my joyous delight, the radio did NOT reset 
itself. Even with full output, the radio now seems to be working well again.

Conclusion: I didn't see any corrosion or even any dirty contacts anywhere. 
I suppose it is possible that one of the screwed down connections may have 
not been good. It is also possible that a stray bit of solder (from the 
Icom repair shop effort in January 2004) may have found its way into a 
circuit which didn't appreciate its presence. It is possible that the 
process of opening the radio may have dislodged the bit of solder ending 
whatever circuit anomaly it was causing or my re-tightening the hardware 
actually did some good. I don't know which.

The radio has returned to its dominant position on my operating desk where 
it seems to be working happily. We'll see if it can work for more than one 
week in a row.

73,

Frank N. Haas KB4T
Florida