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

WHAT?!?! Sell a rig? NEVER! =:O

Oops, sorry. I was thinking you had a schematic for the IC-251A.

Anyhow, I will proceed with my plan and throw it on the old spectrum
analyzer and look for harmonics up to 26 GHz (the upper range of the
spectrum analyzer)

Scott
N0EDV

Ian White, G3SEK wrote:



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


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.



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).

WHAT?!?! Sell a rig? NEVER! =:O

Oops, sorry. I was thinking you had a schematic for the IC-251A.

Anyhow, I will proceed with my plan and throw it on the old spectrum
analyzer and look for harmonics up to 26 GHz (the upper range of the
spectrum analyzer)

Scott
N0EDV

Ian White, G3SEK wrote:



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


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.



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).