Many thanks to those who offered valuable advice, most especially K8CC, who
has apparently subconsciously taught me how to troubleshoot problems
systematically, and to VE3ZI, who confirmed my conclusion by describing how
the same thing happened to him several years ago.
The radio and amplifier oscillated due to operator error.
The only thing that has changed in my shack in the past several months is a
modification to my antenna-switching matrix (SO2R optimized) which allows me
to use the RX ANT port on the back of radio 1 (with the 160m amp) as radio
2. In other words, I can use one antenna (160m vertical, in this case) as
the TX antenna for radio 1, and the other antennas in the arsenal as RX
antennas on the same radio, utilizing the ANT 2 controls. This is all
accomplished rather elegantly through the use of a few additional A/B
switches.
Enter: my naivety ...
The problem is that, while radio 1 is transmitting, whichever receive
antenna is selected is feeding directly into the RX ANT port on the back of
it. It had not occurred to me that I should check the IC-765 schematic to
ensure that this port is isolated when in TX, which indeed it is not. Due to
the proximity of the TX and RX antennas, helped along with 500w, some power
was coupled into the RX antenna. This RF feedback eventually damaged the
transceiver to the point that the oscillation occurred.
Interestingly, and perhaps ironically, enough, the amplifier is fine. I
performed a few cursory checks, hooked it up to radio 2, and away it went. I
proceeded to make many more QSOs in the contest, without a glitch.
Now to fix the flawed RX antenna logic and send radio 1 to Bellevue hospital
...
Thanks again for the helpful suggestions.
73,
Don Chisholm
K8BB
Pontiac, MI
----- Original Message -----
From: <k8bb@comcast.net>
To: <amps@contesting.com>
Sent: Saturday, January 29, 2005 03 02
Subject: [Amps] please help with weird problem
> Oh amplifier gurus ... this is rather long ... I have searched the
archives,
> but I don't know what I am looking for ...
>
> I am using an AL-80 and an IC-765 - 50w drive for about 500w out. There is
> an ICE 419 bandpass network between the radio and the amp.
>
> While operating the CQ 160 CW contest (just a little while ago), all had
> been fine for many QSOs. Suddenly, something occurred:
>
> When I hit the F-key to send my call, I heard a "groan" in my headphones.
I
> looked up to see the radio and the amplifier - all needles all the way to
> the right - putting out what seemed to be a carrier, but waaaaay too much
> power!
>
> I quickly turned off the power, perplexed, and waited. I turned it back on
> again. The radio came back on and I heard signals again. After waiting a
> moment, to check for any *obvious* signs of a problem (smoke, smell,
etc.),
> I hit it again and it did the same thing.
>
> I determined that, in CW, I did not actually have to transmit power from
the
> radio, I only had to throw the TX switch on the radio for this occurrence
to
> happen. When I put the amp into standby, the radio transmitted fine -
> standby made no power and CW made power. I actually worked a few guys
> barefoot to check it out. Also, I shorted the relay cable and switched the
> amplifier into TX without the radio (radio was off) and nothing odd
> happened. It just sat there with appropriate idling current.
>
> Also, all indications pointed that any repeating of this would damage
> something worse than it already apparently was, but I had no idea what was
> wrong - no smoke, no flames, no arcing. All in all, this probably
occurred
> 4 short times (all approximately in a row) before I quit. (Yes, I know it
> was not the smartest method of troubleshooting ... )
>
> What gives?
>
> Did something in the amp "go" causing voltage(s) to enter the radio
through
> the antenna connector and freak out the transmitter? Did something in the
> transmitter "go" causing some weird voltage(s) to enter the amp through
the
> input and cause some feedback?
>
> Because the radio actually put out a gazillion watts (probably 150w-200w,
> according to what the RF power meter on the radio was doing when this
> happened and the associated "groans" and dimming lights), how did the
> radio's ALC not react? And why was the radio fine for many subsequent
> barefoot QSOs?
>
> I opened up the amp and looked around. I did not see anything amiss, but
> that is not saying much.
>
> Furthermore, I have determined that the radio's antenna tuner might have
> suffered some damage in the process, though it was not using it at the
time.
> At first it would not tune. (The IC-765 uses an "intelligent" SWR circuit
to
> automatically tune anytime bad SWR is detected.) I cycled the power off
and
> on and the tuner sprang to life. However, after several more QSOs with the
> internal tuner in-line, another problem developed. As soon as the radio
> transmits, the power shuts off, like some protection circuit kicks in.
> Cycling the power brings everything back to life. If I turn the internal
> tuner OFF, every thing is fine, and I can make more QSOs.
>
> I guess this is my big dilemma: How and Where do I start troubleshooting?
I
> have other radios I can use to check the amp and other amps to check the
> radio, but I am afraid I might destroy something ELSE in the process of
> determining the culprit.
>
> Feel free to reply off the list, as I am sure not everybody wants to see
> THIS thread ...
>
> Thanks and 73,
> Don Chisholm
> K8BB
> Pontiac, MI
>
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