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ICOM IC735 Blowing Fuse.

Posted: Tue Sep 13, 2011 3:17 pm
I am presently repairing an ICOM IC735 which has not been used for about 7 years.The circuit boards appeared in good condition when I examined them,but
when the power was switched on the 3 amp fuse was blown.I tested the finals,predriver and driver transistors,diodes and electrolytic capacitors in the power
amp circuits and they all tested good.Can anyone offer any help to solve this problem?

Re: ICOM IC735 Blowing Fuse.

Posted: Wed Sep 14, 2011 12:07 pm
by fineshot1
What 3 amp fuse are you referring to?

Where is this fuse located and do you have a schematic diagram?

Could be an internally shorted capacitor.

Go to IC10 on the PL Unit and check pin 8 to ground. If there is a short it could be C18 or C247.

Same procedure for all of the boards - check the 13.8V line to all boards and check all caps to
ground and see if there is a short in any of them. You may have to remove them individually
to confirm they are or are not shorted.

Re: ICOM IC735 Blowing Fuse.

Posted: Thu Sep 15, 2011 4:28 pm
The fuse is located in the PA UNIT, and I have a service manual for the radio.

Re: ICOM IC735 Blowing Fuse.

Posted: Fri Sep 16, 2011 10:46 am
by fineshot1
Referencing page 6-4 in the service manual of the PA schematic diagram.

P5 HV line is actually a 13.8V source.

P3 also has a 13.8V line clearly marked.

Unplug them and isolate the circuits and check each circuit for a short to ground with a ohm meter.

If either of them have a short to ground you will have to trace the line to all capacitors and active
devices(transistors) and check for shorts.

Not too easy but that's all i can suggest.

Re: ICOM IC735 Blowing Fuse.

Posted: Fri Sep 16, 2011 1:02 pm
Thank you very much.