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

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

Allison wrote:

First pull the jumper.
Second pick up the rf at the last "live" stage and bring it to a jack.
I assume the transverter will permit seperate TX and RX paths, the
better ones do. If the pickoff point is more than you need still the
attenuator will not have to be big or high power.

As to just pulling the plug and hoping fo rf leak through, try it it
may work, my bet is no.

The band pass filter for the operating freqency is before the driver
and final so it's clean there. The transverter will also clean up the
input usually. So the level of excess harmonics should not be a
problem.

Allison
KB1gmX

Allison has touched all the important bases - his suggestion is the best
way to do it.

The idea of just pulling the plug will not work - you'll turn all the
power devices into passive diode limiters, and if anything comes out at
all, it will be filthy. Likewise the huge reduction in audio drive will
not work, because the SSB/CW signal will drop down into the circuit
noise, and probably well below the level of the not-quite-suppressed
carrier.

For a complete solution, install a little RF relay to switch the drive
signal to an output socket on the rear of the transceiver. As Allison
says, do this *after* the filters at the operating frequency, just
before the signal enters the driver/PA boards. Also install a DC power
relay to remove the supply to those RF power stages. Wire the two relays
so that when they are not energized, the transceiver works as normal.

The deluxe - and safe - way to control those relays is to supply 12V DC
down the coax cable between the new output socket and the transverter
(using RF chokes and blocking capacitors to separate the DC from the RF
at each end). When the transverter is powered-on, it commands the
transceiver to send it low power. When the transverter is off, the
transceiver simply goes back to normal.

You don't know exactly what power level the driver stages will provide.
If it's too high, attenuate it. If it's too low, use an MMIC amp. If -
as seems very likely - the transverter comes from Down East Microwave,
the facilities you need will probably be on-board.

As Allison says, use a separate route for RX input. This will require
yet another small RF relay (powered the same way as the others) but it
will make sure you can never empty the full transceiver power into the
RX output of the transverter.

This system with DC control down the low-level TX coax will take a
little time to install, but it is highly reliable and very safe. I've
used it on a number of rigs here over the past 20+ years, and it has
never let me down.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

Allison wrote:

First pull the jumper.
Second pick up the rf at the last "live" stage and bring it to a jack.
I assume the transverter will permit seperate TX and RX paths, the
better ones do. If the pickoff point is more than you need still the
attenuator will not have to be big or high power.

As to just pulling the plug and hoping fo rf leak through, try it it
may work, my bet is no.

The band pass filter for the operating freqency is before the driver
and final so it's clean there. The transverter will also clean up the
input usually. So the level of excess harmonics should not be a
problem.

Allison
KB1gmX

Allison has touched all the important bases - his suggestion is the best
way to do it.

The idea of just pulling the plug will not work - you'll turn all the
power devices into passive diode limiters, and if anything comes out at
all, it will be filthy. Likewise the huge reduction in audio drive will
not work, because the SSB/CW signal will drop down into the circuit
noise, and probably well below the level of the not-quite-suppressed
carrier.

For a complete solution, install a little RF relay to switch the drive
signal to an output socket on the rear of the transceiver. As Allison
says, do this *after* the filters at the operating frequency, just
before the signal enters the driver/PA boards. Also install a DC power
relay to remove the supply to those RF power stages. Wire the two relays
so that when they are not energized, the transceiver works as normal.

The deluxe - and safe - way to control those relays is to supply 12V DC
down the coax cable between the new output socket and the transverter
(using RF chokes and blocking capacitors to separate the DC from the RF
at each end). When the transverter is powered-on, it commands the
transceiver to send it low power. When the transverter is off, the
transceiver simply goes back to normal.

You don't know exactly what power level the driver stages will provide.
If it's too high, attenuate it. If it's too low, use an MMIC amp. If -
as seems very likely - the transverter comes from Down East Microwave,
the facilities you need will probably be on-board.

As Allison says, use a separate route for RX input. This will require
yet another small RF relay (powered the same way as the others) but it
will make sure you can never empty the full transceiver power into the
RX output of the transverter.

This system with DC control down the low-level TX coax will take a
little time to install, but it is highly reliable and very safe. I've
used it on a number of rigs here over the past 20+ years, and it has
never let me down.


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

Ian,
Thanks for the pointers! I could delete the relays since I am going to
dedicate the IC-251A to low power service on a permanent basis. I
bought the rig specifically to be used with transverters. I have a much
newer rig that covers 2M (Yaesu FT-857) and it puts out 50 Watts instead
of only 10 Watts like the IC-251A.

Now it's just a matter of where to "pick off" that low level RF. I have
it narrowed down to Q30 (pre-driver), but there is so much circuitry
(coils and caps) between it's collector and the next stage's base lead.
Do you think I could just remove the driver stage (Q31) and solder the
center lead of a piece of coax, say RG-174, into the now empty base lead
hole and bring it out the back of the rig? Is any sort of impedance
matching required?

The plan is to use this to drive the Down East Microwave series of
transverters, so harmonics may not be a real big issue since I believe
they have pretty tight filtering at their design frequencies that would
greatly attenuate the next "spur" 144 MHz above the desired output (i.e.
1440 MHz for the 1296 transverter)...

Any final thoughts???

Scott
N0EDV




Ian White, G3SEK wrote:
Allison wrote:


First pull the jumper.
Second pick up the rf at the last "live" stage and bring it to a jack.
I assume the transverter will permit seperate TX and RX paths, the
better ones do. If the pickoff point is more than you need still the
attenuator will not have to be big or high power.

As to just pulling the plug and hoping fo rf leak through, try it it
may work, my bet is no.

The band pass filter for the operating freqency is before the driver
and final so it's clean there. The transverter will also clean up the
input usually. So the level of excess harmonics should not be a problem.

Allison
KB1gmX


Allison has touched all the important bases - his suggestion is the best
way to do it.

The idea of just pulling the plug will not work - you'll turn all the
power devices into passive diode limiters, and if anything comes out at
all, it will be filthy. Likewise the huge reduction in audio drive will
not work, because the SSB/CW signal will drop down into the circuit
noise, and probably well below the level of the not-quite-suppressed
carrier.

For a complete solution, install a little RF relay to switch the drive
signal to an output socket on the rear of the transceiver. As Allison
says, do this *after* the filters at the operating frequency, just
before the signal enters the driver/PA boards. Also install a DC power
relay to remove the supply to those RF power stages. Wire the two relays
so that when they are not energized, the transceiver works as normal.

The deluxe - and safe - way to control those relays is to supply 12V DC
down the coax cable between the new output socket and the transverter
(using RF chokes and blocking capacitors to separate the DC from the RF
at each end). When the transverter is powered-on, it commands the
transceiver to send it low power. When the transverter is off, the
transceiver simply goes back to normal.

You don't know exactly what power level the driver stages will provide.
If it's too high, attenuate it. If it's too low, use an MMIC amp. If -
as seems very likely - the transverter comes from Down East Microwave,
the facilities you need will probably be on-board.

As Allison says, use a separate route for RX input. This will require
yet another small RF relay (powered the same way as the others) but it
will make sure you can never empty the full transceiver power into the
RX output of the transverter.

This system with DC control down the low-level TX coax will take a
little time to install, but it is highly reliable and very safe. I've
used it on a number of rigs here over the past 20+ years, and it has
never let me down.

Ian,
Thanks for the pointers! I could delete the relays since I am going to
dedicate the IC-251A to low power service on a permanent basis. I
bought the rig specifically to be used with transverters. I have a much
newer rig that covers 2M (Yaesu FT-857) and it puts out 50 Watts instead
of only 10 Watts like the IC-251A.

Now it's just a matter of where to "pick off" that low level RF. I have
it narrowed down to Q30 (pre-driver), but there is so much circuitry
(coils and caps) between it's collector and the next stage's base lead.
Do you think I could just remove the driver stage (Q31) and solder the
center lead of a piece of coax, say RG-174, into the now empty base lead
hole and bring it out the back of the rig? Is any sort of impedance
matching required?

The plan is to use this to drive the Down East Microwave series of
transverters, so harmonics may not be a real big issue since I believe
they have pretty tight filtering at their design frequencies that would
greatly attenuate the next "spur" 144 MHz above the desired output (i.e.
1440 MHz for the 1296 transverter)...

Any final thoughts???

Scott
N0EDV




Ian White, G3SEK wrote:
Allison wrote:


First pull the jumper.
Second pick up the rf at the last "live" stage and bring it to a jack.
I assume the transverter will permit seperate TX and RX paths, the
better ones do. If the pickoff point is more than you need still the
attenuator will not have to be big or high power.

As to just pulling the plug and hoping fo rf leak through, try it it
may work, my bet is no.

The band pass filter for the operating freqency is before the driver
and final so it's clean there. The transverter will also clean up the
input usually. So the level of excess harmonics should not be a problem.

Allison
KB1gmX


Allison has touched all the important bases - his suggestion is the best
way to do it.

The idea of just pulling the plug will not work - you'll turn all the
power devices into passive diode limiters, and if anything comes out at
all, it will be filthy. Likewise the huge reduction in audio drive will
not work, because the SSB/CW signal will drop down into the circuit
noise, and probably well below the level of the not-quite-suppressed
carrier.

For a complete solution, install a little RF relay to switch the drive
signal to an output socket on the rear of the transceiver. As Allison
says, do this *after* the filters at the operating frequency, just
before the signal enters the driver/PA boards. Also install a DC power
relay to remove the supply to those RF power stages. Wire the two relays
so that when they are not energized, the transceiver works as normal.

The deluxe - and safe - way to control those relays is to supply 12V DC
down the coax cable between the new output socket and the transverter
(using RF chokes and blocking capacitors to separate the DC from the RF
at each end). When the transverter is powered-on, it commands the
transceiver to send it low power. When the transverter is off, the
transceiver simply goes back to normal.

You don't know exactly what power level the driver stages will provide.
If it's too high, attenuate it. If it's too low, use an MMIC amp. If -
as seems very likely - the transverter comes from Down East Microwave,
the facilities you need will probably be on-board.

As Allison says, use a separate route for RX input. This will require
yet another small RF relay (powered the same way as the others) but it
will make sure you can never empty the full transceiver power into the
RX output of the transverter.

This system with DC control down the low-level TX coax will take a
little time to install, but it is highly reliable and very safe. I've
used it on a number of rigs here over the past 20+ years, and it has
never let me down.

acepilot wrote:
Ian,
Thanks for the pointers! I could delete the relays since I am going to
dedicate the IC-251A to low power service on a permanent basis. I
bought the rig specifically to be used with transverters. I have a
much newer rig that covers 2M (Yaesu FT-857) and it puts out 50 Watts
instead of only 10 Watts like the IC-251A.

OK, that's fine. You can easily make the modifications reversible in
case you might want to sell the rig.

Now it's just a matter of where to "pick off" that low level RF. I
have it narrowed down to Q30 (pre-driver), but there is so much
circuitry (coils and caps) between it's collector and the next stage's
base lead. Do you think I could just remove the driver stage (Q31) and
solder the center lead of a piece of coax, say RG-174, into the now
empty base lead hole and bring it out the back of the rig? Is any sort
of impedance matching required?

There you've lost me... I don't know the circuit details of that rig,
but at these very low levels it might well work without re-matching.

The plan is to use this to drive the Down East Microwave series of
transverters, so harmonics may not be a real big issue since I believe
they have pretty tight filtering at their design frequencies that would
greatly attenuate the next "spur" 144 MHz above the desired output
(i.e. 1440 MHz for the 1296 transverter)...

That's correct. The level of the 1440MHz spur is mostly determined by
how hard you drive the mixer with the main 144MHz signal, and very
little by that signal's own harmonic content (unless of course it's
gross, which there is no reason to expect - the big harmonic generators
are the high-level stages that you won't be using).



--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek