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.

Chris

You need to decrease the feedback resistor from collector
to base; too much low frequency gain.

Rick N6RK

"Chris" wrote in message
ink.net...
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.

Chris

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.

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.

Chris-

I agree with Allison that you should look at the power bus. You often find
designs with two or three parallel filter capacitors with values spread over
the range of 100 pf to 10 uf, all with shortest possible leads including PCB
tracks. As I recall, this was part of the cure for oscillations in the
Heathkit 10W Walkie-Talkie amplifier.

Then there is the effect of load impedance (or input impedance on the previous
stage) and its relationship to a VSWR protection circuit. With a resistive
load, even out-of-band signals are matched, so there is no drive reduction.
Therefore you have full power which seems to prevent the other-frequency
oscillation from occurring. However, a real-world antenna may be matched at
the intended frequency, but not at out-of-band frequencies. The VSWR
protection circuit reduces drive, which allows the spurious oscillation to
grow, causing more out-of-band signal, reducing drive even further. The result
is a complex waveform consisting of intermodulation products of the intended
signal and the spurious signal(s). This effect would be more likely to occur
where drive reduction is accomplished before the stage that is oscillating.

Although not a cure, you might try defeating any VSWR protection circuit in
either the driver or the amplifier, and see if the problem is reduced.

I found an almost identical problem in the old Regency HR-6 and BTL-301
tranceivers. The only thing that actually seemed to cure the problem was to
short the emitter lead of the final multiplier stage to ground. I noticed that
later versions of the BTL-301 moved the VSWR protection drive reduction from an
early stage to a later stage. This made significant improvement, but wasn't a
perfect cure.

73, Fred, K4DII

The driving radio is an RCI2950. The radio output goes directly into the
input transformer. There is a VSWR of 1.2:1 between the radio and the amp. I
also tried adding various caps on the power lines to no avail. As mentioned
earlier, I have another amp with MRF455's with no problems. The only big
difference I see in the circuits is thatr the good amp has an RC network
between the input transformer and the bases.

Chris
"Fred McKenzie" wrote in message
...
| 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.
|
| Chris-
|
| I agree with Allison that you should look at the power bus. You often
find
| designs with two or three parallel filter capacitors with values spread
over
| the range of 100 pf to 10 uf, all with shortest possible leads including
PCB
| tracks. As I recall, this was part of the cure for oscillations in the
| Heathkit 10W Walkie-Talkie amplifier.
|
| Then there is the effect of load impedance (or input impedance on the
previous
| stage) and its relationship to a VSWR protection circuit. With a
resistive
| load, even out-of-band signals are matched, so there is no drive
reduction.
| Therefore you have full power which seems to prevent the other-frequency
| oscillation from occurring. However, a real-world antenna may be matched
at
| the intended frequency, but not at out-of-band frequencies. The VSWR
| protection circuit reduces drive, which allows the spurious oscillation to
| grow, causing more out-of-band signal, reducing drive even further. The
result
| is a complex waveform consisting of intermodulation products of the
intended
| signal and the spurious signal(s). This effect would be more likely to
occur
| where drive reduction is accomplished before the stage that is
oscillating.
|
| Although not a cure, you might try defeating any VSWR protection circuit
in
| either the driver or the amplifier, and see if the problem is reduced.
|
| I found an almost identical problem in the old Regency HR-6 and BTL-301
| tranceivers. The only thing that actually seemed to cure the problem was
to
| short the emitter lead of the final multiplier stage to ground. I noticed
that
| later versions of the BTL-301 moved the VSWR protection drive reduction
from an
| early stage to a later stage. This made significant improvement, but
wasn't a
| perfect cure.
|
| 73, Fred, K4DII
|

"Fred McKenzie" wrote in message
...
...
I agree with Allison that you should look at the power bus. You often
find
designs with two or three parallel filter capacitors with values spread
over
the range of 100 pf to 10 uf, all with shortest possible leads including
PCB
tracks.

This can be an issue as well. When a cap is above its parallel resonant
freq., it looks inductive and can resonate with one of the other caps. A
parallel resonance has a higher impedance than the cap. While this
resonance may be of poor quality, the extremely low impedance of the
transistor collector makes an ohm or two rather significant. Our standard
practice was to place a small bead between the caps. They were about 1/8
inch in dia and had about 50 ohms real Z above a few Mhz. This helped to
destroy any higher freq resonances while preserving the low freq impedance
of the parallel caps.

In earlier days, on VHF, we would sometimes put a "banana" from base to
collector. This was our slang for a series orange drop (I think mylar) cap
and two (one on each end) low valued resistors. Don't remember all the
values, but the idea was to kill the low freq gain. The cap blocked the
collector voltage and had enough inductance and in combo with the two (I
think 33 ohm) resistors was high relative to the base and collector
impedances. I think the cap may have been .01, but memory faded... The low
freq collector load in this situation is the supply bypass method...or the
power supply leads, in lew thereof....

Heck. I remember running a Development Engineernig prototype 100W Micor on
30 Mhz for many months, wondering why I seemed to have battery problems,
when I found large (size -wattage) and quite low valued resistors directly
from base to collector of the finals (which were always tied to the
battery). Guess someone was trying a flight fix and didn't think any DC
blocking was necessary! Transmitter worked fine. I just had a high static
drain.

--
Steve N, K,9;d, c. i My email has no u's.