[Amps] please help with weird problem

[Ian White, G3SEK]

We Never Close - your morning shift has started in Europe!

Steve Thompson wrote:
>k8bb at comcast.net wrote:
>> 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!
>>From what you describe, it sounds like there was oscillation involving the
>output stage of the rig, maybe some combination of rig and amplifier. If it
>was due to rf feedback, the feedback got in after the point where ALC is
>applied in the rig. Any amplifier (in this case the rig's output stages)
>will have a combination of frequency and load impedance where it's more
>susceptible to feedback and/or oscillation. If it's related to the impedance
>the rig is feeding into, then putting an attenuator of 2-3dB in line
>directly on the output socket will make a lot of difference.
>

Groaning sounds, dimming lights and crazy meter readings - all happening 
together - are classic signs of RF feedback, possibly leading to the 
oscillation that Steve describes.

It sounds very much like something has suddenly become un-grounded. Go 
find that ONE thing, and fix it, and everything will be OK again.

DO NOT TEAR YOUR WHOLE STATION APART! That is the worst thing you can 
do! Remember that *almost* every part of your station is still perfectly 
OK... so your aim should be to disturb as little as possible.

Leave everything as it is, and then very selectively check ONE thing at 
a time. If it's OK, put it back EXACTLY as you found it.

Don't automatically blame your amplifier. Sometimes it's simply the 
higher power that triggers a fault somewhere else, and there may 
actually be nothing wrong with the amp itself.

Prime suspects are all those PL-259 jumper cables. Unless you've been 
very good about soldering the shields, every jumper contains two 
un-grounding faults, just waiting to happen.


-- 
73 from Ian G3SEK         'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.co.uk/g3sek