[NLRS] Icom IC-251A Low Power Mod?

[John P. Toscano]

Scott (N0EDV) wrote:
> 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???

Hi, Scott.

The IC-251a that I am currently using with my DEMI 902 and 1296
transverters was modified to produce a very low drive level for
them by Ron, KA0RYT, back when he owned it for the 1296 transverter
(which I also bought from him).  He also split the Tx and Rx lines
because the 1296 transverter was set up for split IF.  I'm not
sure how low the drive output is, because I don't have any meters
that read that low.  I would say it is no more than 100 mW, but
probably closer to the 1-10 mW range.

Ron, do you remember the details?  The only notes you made on the
schematic weren't very explicit -- you simply circled the ID of
Q30, a 2SC2053.  Did you run a jumper from there to the antenna
port, or something else?  (May be tough to remember by now, and I
never opened it up to look at what you physically did to get it
converted.)

At 144 MHz, it shouldn't be too hard to build a Pi or T network
attenuator that would dissipate 10 watts and produce whatever
output level you want.  Of course, if you go the route of the
attenuator, you will most likely want to be set up for split IF,
otherwise yuo will attenuate the signal received by the transverter
at the downconverted 144 MHz IF frequency also.

Although the IC-251a works well enough in the application, I plan
to convert my 902 and 1296 transverters to 10 watts of drive and
run them from an Icom IC-706 MkII, then I won't have to worry about
over-driving them and still get the benefits of a more modern IF
radio to operate them with.  My 902 came with parts for various
drive levels including 10 watts, and I still have the leftovers
floating around here somewhere.  My 1296 is an old model, but
DEMI has an adapter that converts a 10 watt (max) common IF to
the low-drive split IF that the transverter needs.  It has a
pair of BNC connectors on one end that are spaced just right to
slip right onto the IF BNC jacks of the transverter.  It is not
shown in their catalog, but when I asked them about converting
my old unit to 10 watt common IF, they said "here's what you need
all ready to go" -- part # "1296-HIF".  It basically has a relay
that splits the Common IF into separate TxIF and RxIF lines, and
a 10 Watt 50 ohm resistor from the TXIF position to ground, which
shunts most of the drive to ground but lets "enough" get past to
the transverter.

Hope that helps.
73 de W0JT