[ICOM] Receive level dropping-rig or antenna system issues?

[George Jones]

Recently I had a similar problem on my IC-706MKII mobile. The problem was 
more noticeable on transmit rather on receive.  I have operated this mobile 
for some time without experiencing any problems on a Hustler antenna with 
separate coils for each band.  About a month ago I picked up a LGG tuner at 
a local hamfest and installed it in the mobile. I switched the antenna out 
to a 108-inch steel whip. The tuner worked with this combination and no 
apparent problem for several weeks except it seemed to take longer to tune 
on 20-meters. After many attempts to check into a net on 40-meters I began 
to suspect a problem. My first response was to think that the problem was 
with the tuner or new antenna combination.
After bypassing the tuner and switching the antenna back to the Hustler with 
40-meter coil, the problem persisted.

Out came the test equipment. First I connected a Bird watt meter and dummy 
load to the IC-706MKII, HF/6 antenna connecter. After keying up the radio in 
the FM mode and checking each of the HF bands, I noted that the TX power 
would barely register any output on the Bird's lowest setting.
Now my thoughts turned to a defective PA section on the IC-706MKII radio. My 
previous thought that there may have been a problem with the tuner and 
108-inch whip combination was now confirmed and that I had damaged the PA 
section with that antenna combination.

Further test....  Out came my other IC-706MKIIG radio. After connecting the 
second radio up the problem seem to be no better. In fact, the Bird watt 
meter indicated the same problem.  This had me really scratching my head!  I 
began to check the 12-volt line and noted that the voltage looked good 
around +13.8 volts at the connector to the radio.  I even keyed the 
IC-706MKIIG transmitter while monitoring the voltage at the connector. The 
voltage drop noted looked to be in the normal range with only about 0.3 
volts drop.

Now with both radios on the bench in the house with an Astron 50 amp supply 
connected, both radios produced rated power out on the Bird meter.  Back to 
the car to further check out power cable.  I connected a 12-volt, 60-watt 
camper style light bulb to the power cable an immediately observed the 
problem. The light bulb lit up very dimly. Using the light bulb for a test 
instrument, I began  testing each connection along the power cable. In my 
vehicle, an SUV, I have a #6 wire running from the battery to a distribution 
block near where the radios are mounted. The #6 wire runs through a 
cartridge fuse right at the battery tap. The light bulb indicated a problem 
with the fuse. I replaced the defective fuse and now both radios make rated 
power in the car.  Further test with the Bird meter indicated that the tuner 
and 108-inch whip combination was working as expected.

What we learned from this experience is that the original fuse was stressed 
in a manor that it would not fail until a heavy current was drawn through 
it. A simple continuity check with an ohm meter shows the fuse to be good. 
The voltage drop across the fuse with the bulb connected indicated several 
volts drop. The digital volt meter alone did not provide enough of a load to 
indicate the problem.

George - W4AQR

--------------------------------------------------
From: "Larry Young" <k4lxv at bellsouth.net>
Sent: Sunday, March 24, 2013 3:46 PM
To: "ICOM Reflector" <icom at mailman.qth.net>
Subject: Re: [ICOM] Receive level dropping-rig or antenna system issues?

> John: It should be easy to determine if it is internal or external to the 
> rig. When the  receive is normal, attach an external signal source to the 
> antenna input and note its level on the S meter. When the receive level 
> drops, reconnect the same signal source and again not the S meter level. 
> If it is the same as before when good, then obviously you have some 
> oxidation occurring somewhere in the antenna/feedline/balun path.
> Larry  K4LXV
>
>
> --- On Sun, 3/24/13, John G. <af5cc at fidmail.com> wrote:
>
> From: John G. <af5cc at fidmail.com>
> Subject: [ICOM] Receive level dropping-rig or antenna system issues?
> To: "ICOM Reflector" <icom at mailman.qth.net>
> Date: Sunday,
> March 24, 2013, 3:03 PM
>
> Here is a message I posted a few months ago about the receive level 
> dropping off at times in my Icom 706 original:
>
> "I have noticed for the past few months that the receive seems to drop
> at times on certain bands-it started on 12m and now occurs on 15m as well. 
> If you click the key or mic to quickly put it into transmit the receive 
> comes back. To me this sounds like a dirty relay or one that is going bad"
>
> Now here are some additional details, it seems to be occurring on other 
> bands as well. 17m at times, and was doing it on 30m last night. Someone 
> suggested to me that it could be a problem in the antenna system as well. 
> I did some checking this morning, and have determined that it isn't in the 
> low pass filter, the MFJ tuner, or any jumper coaxes in the shack. It 
> would have to be in the feedline going to the G5RV, or the balun there. I 
> haven't had time to change out the coax run
> yet.
>
> I have also noticed that on 12m and 15m the receive will come back up by 
> itself at times. Sometimes it is a quick drop down and come back several 
> times in succession. Would a relay problem do this?
>
> So, with this latest information, what is the better guess as to the 
> problem-in the rig or the antenna system?
>
> 73 John AF5CC
>
> ----
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> ----
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