Has anybody successfully modified the Icom IC-251A 2M all mode rig so
that it will provide only 1 mW at the antenna terminal to drive a
transverter? On the schematic, it looks like there is a plug that
supplies the 13.8 Volts to the PA and driver transistors. If I removed
this plug, effectively killing these stages, might there be enough power
leaking through to provide the 1 mW at the antenna jack? If so, how bad
do you suppose the harmonics would be at the output? Any other ideas
appreciated (I could control the power using the mic gain control down
to some lower power such as 1 Watt or so and then use a 30 dB pad. Down
side is if I ever adjust the mic gain to 10 Watts out, I'd pump 100mW
into the tranverter until the 30 dB pad finally blew)...What say???

Scott
N0EDV

On Mon, 15 Mar 2004 02:22:38 +0000, acepilot hath writ:
Has anybody successfully modified the Icom IC-251A 2M all mode rig so
that it will provide only 1 mW at the antenna terminal to drive a
transverter? On the schematic, it looks like there is a plug that
supplies the 13.8 Volts to the PA and driver transistors. If I removed
this plug, effectively killing these stages, might there be enough power
leaking through to provide the 1 mW at the antenna jack? If so, how bad
do you suppose the harmonics would be at the output?

That would be my concern: Just how much other "crap" would be
"leaking through" at or around the 1mW level.

I do have the IC-251A, but I've not had the need to do what you
are attempting ------ yet.

73 es gl
Jonesy
--
| Marvin L Jones | jonz | W3DHJ | OS/2
| Gunnison, Colorado | @ | Jonesy | linux __
| 7,703' -- 2,345m | config.com | DM68mn SK

In my humble opinion,
Oops, sorry
Anyway...
I thought of Plan B today while sitting on the crapper (or is that the
"Engineering Bench" ???). I'm thinking of adding a large resistor in
series with the top side of the MIC GAIN pot, something like 470K to
10M, determined experimentally, so that the 10K pot, when at fully
clockwise position would only provide enough audio to the modulator to
make 1 mW out to the antenna jack (simple voltage divider). I don't
want to fry a $300 transverter by forgetting to turn the Mic Gain down
=:O (which I KNOW I would do at some point -- like portable operation in
the USA's rarest grid square, where I'll be for the June VHFQSO Contest)

Scott
N0EDV

Allodoxaphobia wrote:
On Mon, 15 Mar 2004 02:22:38 +0000, acepilot hath writ:

Has anybody successfully modified the Icom IC-251A 2M all mode rig so
that it will provide only 1 mW at the antenna terminal to drive a
transverter? On the schematic, it looks like there is a plug that
supplies the 13.8 Volts to the PA and driver transistors. If I removed
this plug, effectively killing these stages, might there be enough power
leaking through to provide the 1 mW at the antenna jack? If so, how bad
do you suppose the harmonics would be at the output?


That would be my concern: Just how much other "crap" would be
"leaking through" at or around the 1mW level.

I do have the IC-251A, but I've not had the need to do what you
are attempting ------ yet.

73 es gl
Jonesy

In my humble opinion,
Oops, sorry
Anyway...
I thought of Plan B today while sitting on the crapper (or is that the
"Engineering Bench" ???). I'm thinking of adding a large resistor in
series with the top side of the MIC GAIN pot, something like 470K to
10M, determined experimentally, so that the 10K pot, when at fully
clockwise position would only provide enough audio to the modulator to
make 1 mW out to the antenna jack (simple voltage divider). I don't
want to fry a $300 transverter by forgetting to turn the Mic Gain down
=:O (which I KNOW I would do at some point -- like portable operation in
the USA's rarest grid square, where I'll be for the June VHFQSO Contest)

Scott
N0EDV

Allodoxaphobia wrote:
On Mon, 15 Mar 2004 02:22:38 +0000, acepilot hath writ:

Has anybody successfully modified the Icom IC-251A 2M all mode rig so
that it will provide only 1 mW at the antenna terminal to drive a
transverter? On the schematic, it looks like there is a plug that
supplies the 13.8 Volts to the PA and driver transistors. If I removed
this plug, effectively killing these stages, might there be enough power
leaking through to provide the 1 mW at the antenna jack? If so, how bad
do you suppose the harmonics would be at the output?


That would be my concern: Just how much other "crap" would be
"leaking through" at or around the 1mW level.

I do have the IC-251A, but I've not had the need to do what you
are attempting ------ yet.

73 es gl
Jonesy

On Mon, 15 Mar 2004 02:22:38 +0000, acepilot hath writ:
Has anybody successfully modified the Icom IC-251A 2M all mode rig so
that it will provide only 1 mW at the antenna terminal to drive a
transverter? On the schematic, it looks like there is a plug that
supplies the 13.8 Volts to the PA and driver transistors. If I removed
this plug, effectively killing these stages, might there be enough power
leaking through to provide the 1 mW at the antenna jack? If so, how bad
do you suppose the harmonics would be at the output?

That would be my concern: Just how much other "crap" would be
"leaking through" at or around the 1mW level.

I do have the IC-251A, but I've not had the need to do what you
are attempting ------ yet.

73 es gl
Jonesy
--
| Marvin L Jones | jonz | W3DHJ | OS/2
| Gunnison, Colorado | @ | Jonesy | linux __
| 7,703' -- 2,345m | config.com | DM68mn SK