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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