SB 401 No PTT in Tune mode

[Robert Chavers]

Season's Greetings All,

I'm trying to bring a SB301/401 pair back to life.  Got the 303..it had the
world's worst vfo backlash.  After reading the archives I got courage enough
to battle the LMO & got it going great after an alignment.

The SB 401 has a problem that I'm having trouble putting my finger on.  I
rebuilt the on/off switch, replaced a couple of electrolytics in the power
supply, and now have the xmtr powered up with good voltages.  The first
thing that I noticed was no PTT in tune mode.  CW mode will pull in both
relays, as will the mic in lsb or usb...but not in tune.  I can ground the
"key" while in tune mode and it will pull in the relays and hold until I
switch out of tune.  I could just use the cw key to tune, but this has
become a challenge!

I performed the mod by Stu (at work, don't remember his call...sri) to add a
zener in order to help the relay amp (V12) pull in RL-1(?)..but this didn't
help.  I would think that the relay amp is working OK since I have PTT in
cw,usb & lsb. BTW I do have about 60w out on 40m...haven't attempted
alignment or neutralization yet.

I notice also that the -bias supply ties to V12...there is a little ripple
on this voltage (gotta locate another eletrolytic), but I wouldn't think
that would be my problem.  I confirmed that the tune mode switch is applying
a ground to the generator board..which has a ptt lead and a tune lead.  I
tried following the ground applied by the tune switch thru the mode and
function switches on the schematic....but it's hard for a country boy like
me (there's more to it than is showed in the circuit description).

One last thing, when I place the rig in tune mode relay RL-1 (I think, it's
the one in the middle of the rig) will pull in ever so slightly, just not
enought to make the contacts ...but it works great in all the other modes.
I haven't changed V12 (don't have another) I have checked all of the
discrete components around V12...haven't changed D105.  I did adjust all of
the vox pots.

These radios are not in 'mint' condition, I bought'em for $20.00 w/ a heath
code osc (I just had to tell..grin) so they are in rough shape...but heck,
I'm gonna use 'em not sell 'em.

Any ideas or help with 'brain stormin' greatly appreciated.

73,
        Robert
------------------------------------------------
             Robert Chavers
  robertc at arbuckleonline.com
                KD5XW
          Ardmore, OK  73401

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