[Icom] IC751A - FSK

Sat, 25 Jan 2003 12:38:05 -0500

Hello,

There are two slide switches in the IC-751A, located on the MAIN Unit (the 
top board in the 751A). One of these switches controls NORMAL/REV and the 
other selects between 170 and 850 shift. They are easy to find using the 
user's manual. When keyed in RTTY mode, the NORMAL/REV switch (S2) can be 
toggled and the 751A's output will switch between MARK and SPACE. This 
switch is handy during BFO alignment to verify the shift amounts without 
having to apply an external FSK ground to force SPACE. It would be quick for 
you to try this switch, and if the transmitted frequency of the radio goes 
between MARK and SPACE using it, then most likely the external keying input 
line is bad, if this test does not work OK then there is a problem with the 
BFO or BFO alignment. There are separate BFO alignment trimmers for CW/MARK, 
170 SPACE, and 850 SPACE. They are located together next to the RTTY slide 
switches. It is possible that the rig is aligned so that the MARK and SPACE 
frequencies are so close together that it appears not to change state under 
keying. I have seen a number of 751A's that are improperly aligned for FSK 
RTTY, the adjustments are somewhat tedious and have some interaction, I 
think many just don't bother with them during alignment.

Hope this is helpful, and good luck.

73, Frank W3UHF

>From: TL <[email protected]>
>Reply-To: [email protected]
>To: [email protected]
>Subject: [Icom] IC751A - FSK
>Date: Sat, 25 Jan 2003 17:19:57 +0200
>
>Hello all
>Just preparing for WPX RTTY and having trouble with FSK on my 751A.
>I've wired the acc. plug according to the manual (pin 9 for 5v , 8 gnd) but
>the rig will not make FSK.
>Same plug works great on my 720A (same socket, identical pins for FSK
>purpose etc') so I guess this is an internal issue.
>Any ideas?
>http://www.iarc.org/~4z4tl/
>-------------------------------------------
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>Your Moderator: Dick Flanagan W6OLD, [email protected]
>Icom Users Net: Sundays, 1700Z, 14.315 MHz
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