[ICOM] IC761 Power supply

[Gary Fiber]

Alan,

When you operate CW or place the 761 into RTTY mode you are able to 
see a 100 watt output. I would have said it was the current limiting 
resistor again as the 761 has the same supply as the 765 and nearly 
the same as the PS-35.
In this case it would seem RFI is getting into the rig. Are you using 
the rear 8 pin connector for the PSK connection?
That input if high impedance. Any RFI that gets impressed onto the 
audio or other leads get almost a direct shot inside past some of the 
filtering and low impedance loads of the mic input.
You might connect up a know good dummy load and see if the trouble 
goes away, if so then RFI is most likely.

But it sounds like the power supply has quit entirely now?
If you still have the supply apart have a look at that current shunt 
resistor. I wrote a description last week about that for another 
here. Never hurts to remove and replace it.

One other thing there is a 2 ohm 3 watt resistor that bypasses the 
relay. That is in the power supply start up circuit, current inrush 
limiter I think. Sometimes that resistor opens and the supply will not start.

There is an IC that likes to go bad too, it looks like a little round 
tube as I remember.  Thats about as far as I can go with the supply, 
its been 5 years and there used to be a PS-35 write up on Icom' 
America's web site, common troubles found but they re organized the 
faq section and its gone.

Gary K8IZ






At 07:43 PM 4/24/2006, you wrote:
>While operating PSK (approx 20-25 watts out) the overload cut in and 
>closed the rig down. After 2 to 3 minutes I switched back on and it 
>powered up ok, however as soon as I went to transmit it shut down again.
>I repeated this a couple of times with same result but then tried 
>the rig on CW and SSB where it was able to run full power, went back to PSK
>and it shut down again.
>Now on trying to switch on again the overload relay clicks in about 
>3 times and shuts down straight away.
>A visual inspection of the Power Supply shows no signs of Capacitor 
>leakage or overheating on the PC Board.
>I wonder if anyone has any suggestions as to the most likely place 
>to start looking for the problem please. I appreciate any help.
>Alan ZL3KR
>
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Gary Fiber K8IZ
GROL PG-19-6691
Washington State Resident