RadioBanter - View Single Post - Repairing Icom 735

Gary P. Fiber · October 4th 06, 02:05 PM posted to rec.radio.amateur.equipment

On 2 Oct 2006 22:07:17 -0700, wrote:

Gary P. Fiber wrote:
On 1 Oct 2006 14:26:54 -0700, wrote:

I would like to repair an Icom 735 that is not able to receive from 4
to 11 MHz. Two diodes (D11 & D13) in the receive bandpass filter
section are blown (due to operation near another high power
transmitter/antenna).

These diodes are on the main board which can be accessed by removing
the bottom cover. However they can only be replaced by
unsoldering/soldering on the bottom of the main board. I have several
questions for anyone who has made repairs to this rig:

1. It is easy to remove the screws holding down the board but there is
extensive wiring connected to this board. Does the board have to be
totally removed in order to get at this area? How difficult is this
job? Any tips or tricks you might recommend?

2. The diodes are 1ss53 which is a Japanese switching diode. They are
hard to find. Has anyone found a direct replacement for this part that
gives equal or better performance? (I do not want to change any of the
bias resistors).

Thanks - Roger

The board is not that hard. YES there will be cables you will need to
disconnect. Have your digital camera handy or just a paper and pencil
and make a quick sketch of the board I used to do that before the
camera says and sometimes its still easier than photos. When
disconnecting multi pin connectors, pick on end and mark the wire
color so you will get the connector reinstalled correctly.

I think one coax gets disconnected that goes through the from of the
735. There is an other or so too that you will see get tight as you
try to life the board so disconnect it too. Just don't get then
confused, maybe take a moment and mark them with masking tape.

Basically you will unscrew to board. Now with the front of the radio
towards you gently lift the front of the circuit board. " walk " the
board towards the front lifting it above the front of the radio.

You are working towards getting the rear connectors out of the back
panel and clear of the rear lip. PAY attention to any cables that need
removing and do so as you go.

Once the rear connectors are clear of the rear chassis lip you can
then flip the back edge of the circuit board up and over to get to the
diode solder joints.

Its really easier than this write up sounds. I have talked several
through it over the telephone as you need to do the same thing to
install the CW filter.

Over the years many have tried substituting those diode, even to
installing pin diodes. While I was at Icom tried them at Icom and
found no performance enhancement. We had one person remove and replace
his band pass filter diodes with pin type and we could not fix that
rig. We even installed original diodes back into it. It was an IC-751.

Call Icom's parts department for the diodes. Another source of
Japanese parts used to be MCM audio

Gary K8IZ

Thanks Gary for taking the time to write a detailed response. It sure
helps when someone who has done it before offers tips and
encouragement. Good to hear that Icom still has the parts.

Interesting about the PIN diodes. I was reading some of the old posts
about this subject and there was some debate about whether it worked
but no one had done any tests to confirm it one way or another.

73 - Roger

I see I had a few misspellings. I meant camera days not says...
I remember one coaxial cable that runs from top to bottom through the
space between the chassis and front of the radio. I seem to remember
you will need to disconnect that cable.

I am not exactly sure if Icom still has those diodes but they were
used in quite a number of radios. I am sure if they don't someone
there will have a replacement recommendation.

You are correct the results were very mixed replacing the PN junction
diodes with pin diodes even though the pin diodes are better for RF
switching.

Some swear by the pin diodes and others don't. We could not see a
difference and the pin diodes are quite expensive. test gear did not
show any real difference, on the air testing was questionable with two
radio side by side on the same antenna.

Just give your self a couple of hours to do the job. It likely will
not take that long but you will not be rushed.

Gary K8IZ