[Amps] please help with weird problem - culprit found

[R.Measures]

On Jan 30, 2005, at 4:57 PM, <k8bb at comcast.net> wrote:

> 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.

Hello, Don -- I do not believe that a stable amplifier can oscillate.
>
> 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 at comcast.net>
> To: <amps at 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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>

Richard L. Measures, AG6K, 805.386.3734.  www.somis.org