[Icom] IC756 Marker and Preamps

[Sean Nolan]

I occasionally use the marker signal in my IC-756 (Non Pro version) to check
the receiver sensitivity.  I do this wirh the rf gain full up and the ant
terminals terminated in computer type 50 ohm loads built into BNC plugs.  In
doing this to-day I noticed the following which had previously escaped my
attention. On 3.7MHz and 50.1MHz  the marker signal read as follows on the S
Meter

3.7MHz  Preamp off S9+10db  Preamp 1  S9+12db Preamp 2 S9+10db
50.!MHz Preamp off S7.5  Preamp 1 S5  Preamp 2 S 6.5

I checked in the workshop manual and found that the marker signal at 100KHz
is outputted from pin 68 of IC101 in the PLL unit and goes to the input of
the RF amplifiers via IC192 the mute switch and J851 on the PLL unit.  This
means the marker signal is fed into the rx circuit at the output of the
Preamps/input of the RF amps. I presume the readings I am getting are
explained by some change in loading of the input to the rf amps which takes
place as the preamps are switched from out to 1 to 2.

Using band noise the figures are as one would expect for example
3.7MHz Preamp off S5/6  Preamp 1 S 7/8  Preamp 2 S9-9.5
50.1MHz Preamp off S0  Preamp 1 S1-2  Preamp 2  S5

Obviously I am not going to lose any sleep over this but maybe someone would
like to comment or try the same thing with an IC-756 and see if the results
are the same.  Many thanks.  By the way in relation to all the
correspondence on the list about the keying relay I can guarantee you that
the one in the IC-756 and I expect the other versions are the same, is not
capable of carrying 2 amps.  I had to replace mine which was keying a Yaesu
FL-2100 amp. While I cant remember the exact current involved it was
certainly significantly less than 2 amps.  When I was replacing it I left
the wire legs a little long so that if ever it had to be replaced again the
legs could be snipped off and the stubs used to mount the replacement relay.
I never want to have to take it off the board again.  I now of course use an
external relay.

Sean EI7CD