MRF 454 power Amplifier: Motorola AN762
 

Hi all,

During testing something went wrong: result no output. At first it operated
flawless: a good 140 W out with 4 W in (FT817). It must have gone into
oscillation because the resistors R1 and R2 were smoked. I used a transfer
relay without further precautions (I know, never ever again: I will make the
bias circuit switchable). Now the funny part: bias is easily adjustable as
ever. Remove the load and it oscillates with heavy harmonics at about 2 MHz,
taking about 3 amps. Iquiet about 200 milliamps. Both bases: +/-0.75 V. My
guess: one of the finals is blown (base bonding evaporated). So I have to
exchange them. Before disassembling this unit I'd like to get some advice
and comments. Is it normal if one transistor fails that all output is gone?
Or did something else fail (as well)? Then: if I need to buy new
transistors: do I need to buy MRF 454 or MRF 458. The latter are in my
actual amp (and seem to be cheaper) and said to similar but I don't have a
data sheet.
Please advise,
Harke

On Sat, 19 Jul 2003 15:36:51 +0200, "Yrrah"
wrote:

Hi all,

During testing something went wrong: result no output. At first it operated
flawless: a good 140 W out with 4 W in (FT817). It must have gone into
oscillation because the resistors R1 and R2 were smoked. I used a transfer
relay without further precautions (I know, never ever again: I will make the
bias circuit switchable). Now the funny part: bias is easily adjustable as
ever. Remove the load and it oscillates with heavy harmonics at about 2 MHz,
taking about 3 amps. Iquiet about 200 milliamps. Both bases: +/-0.75 V. My
guess: one of the finals is blown (base bonding evaporated). So I have to
exchange them. Before disassembling this unit I'd like to get some advice
and comments. Is it normal if one transistor fails that all output is gone?
Or did something else fail (as well)? Then: if I need to buy new
transistors: do I need to buy MRF 454 or MRF 458. The latter are in my
actual amp (and seem to be cheaper) and said to similar but I don't have a
data sheet.

What sort of frequency do you plan to your PA at?

On Sat, 19 Jul 2003 15:36:51 +0200, "Yrrah"
wrote:

Hi all,

During testing something went wrong: result no output. At first it operated
flawless: a good 140 W out with 4 W in (FT817). It must have gone into
oscillation because the resistors R1 and R2 were smoked. I used a transfer
relay without further precautions (I know, never ever again: I will make the
bias circuit switchable). Now the funny part: bias is easily adjustable as
ever. Remove the load and it oscillates with heavy harmonics at about 2 MHz,
taking about 3 amps. Iquiet about 200 milliamps. Both bases: +/-0.75 V. My
guess: one of the finals is blown (base bonding evaporated). So I have to
exchange them. Before disassembling this unit I'd like to get some advice
and comments. Is it normal if one transistor fails that all output is gone?
Or did something else fail (as well)? Then: if I need to buy new
transistors: do I need to buy MRF 454 or MRF 458. The latter are in my
actual amp (and seem to be cheaper) and said to similar but I don't have a
data sheet.

What sort of frequency do you plan to your PA at?

What is was ment for: 2-30 MHz. I've made a conventional 30 MHz low pass
filter at the output.
Paul Burridge schreef in berichtnieuws
...

What is was ment for: 2-30 MHz. I've made a conventional 30 MHz low pass
filter at the output.
Paul Burridge schreef in berichtnieuws
...

Eh? You probably should have switchable (band specific) output filters
to reduce harmonics to legal levels. With just one low pass filter set
around 30 MHz, when you run on 80 meters, you would have sizeable output
on 40M, 20M, 10M, etc.

Just some ideas....

Scott
N0EDV


Yrrah wrote:

What is was ment for: 2-30 MHz. I've made a conventional 30 MHz low pass
filter at the output.
Paul Burridge schreef in berichtnieuws
...


--

Scott
http://corbenflyer.tripod.com
http://groups.yahoo.com/group/ParasolAirplanes
Building RV-4
Gotta Fly or Gonna Die!

Eh? You probably should have switchable (band specific) output filters
to reduce harmonics to legal levels. With just one low pass filter set
around 30 MHz, when you run on 80 meters, you would have sizeable output
on 40M, 20M, 10M, etc.

Just some ideas....

Scott
N0EDV


Yrrah wrote:

What is was ment for: 2-30 MHz. I've made a conventional 30 MHz low pass
filter at the output.
Paul Burridge schreef in berichtnieuws
...


--

Scott
http://corbenflyer.tripod.com
http://groups.yahoo.com/group/ParasolAirplanes
Building RV-4
Gotta Fly or Gonna Die!