On Tue, 07 Sep 2004 13:16:35 GMT, "Chris"
wrote:

I have 2 amplifiers with similar problems. One is an EB63 kit (
http://www.communication-concepts.com/eb63.htm ) with MRF455's. The other is
an old broadband "brick" with MRF450a's. They operate normally into a dummy
load. The problems start when connected to an antenna. At low output levels,
there are multiple frequencies present. It's not just harmonics. At 28 MHz,
there is a very clear .5 to 2 MHz component visible on the oscilloscope. On
the power/SWR meter, it looks like a high SWR and incorrectly high power
reading. When the output power of the amp gets close to max, the
oscillations disappear. The scope trace, power, and SWR readings are normal.
This happens on several mobile antennas and an inverted V so I can't blame
the antenna or cable. I also have 3 other amps that work perfectly in the
same setups. I'll post the scope traces to alt.binaries.pictures.radio if
someone wants to look at them. They are also posted at the bottom of
http://home.earthlink.net/~cjohnson1379/ . I have tried collector to base
feedback and different combinations of capacitors on the input and output
sides with little or no change. What about changing the number of turns on
the transformers? I hope someone can figure this out because I'm about to go
nuts. Thanks so much to anyone who can help.

It looks salvageable. The quick and cheap way to get operational would
just be to improve the post-PA filtering, but that won't prevent
parasitic oscillation of the PA itself if that's where the fault lies.
The problem may well be a few stages back. I'd try using the 'scope to
check the purity of the pre-PA stages back along the tx chain and see
where exactly the instability arises. Then you'll have a much better
idea how to fix the problem. If your lucky it might just be poor
decoupling at an earlier, low-power stage.
--

"What is now proved was once only imagin'd." - William Blake, 1793.