The AM Forum

I just aligned my 5100 VFO. Per manual it states to to tune VFO frequency at 3.6 MHz and then adjust C3 trimmer until zero beat with standard frequency (i.e. RF Sig Gen). All went well however when checking frequency at high end of dial, it is off by a few kc's. Anything I'm missing here as the procedure it pretty straight forward. 

There is an adjustable coil in there that normally does not need to be adjusted.  However if the VFO doesn't make the end points, then you will have to do the complete alignment. 

Set the coil at the low frequency and the cap on the high end.  Tune back and forth until the end points are correct.  Normally that VFO tracks real well so you should not be off across the dial.  I forget the fundamental frequency of that VFO but it is from, maybe 1.75 up to 2. something. 

If you have a frequency counter, unplug the RCA connector from the VFO that goes into the buffer and couple the output into your counter with a cap then you can set the VFO very easy.  Whichever method works for you.

Thanks,

Looking at my 5100 schematic, there is no adjustable coil in the VFO. Not sure if you are referencing a 5100B.

If you will look at L101 you will see it has an arrow through it.  I forget exactly where the access hole is located but it is adjustable.   See the attached information and description.

One other thing while I am thinking about it.  Those dial scales usually are yellow and may not be attached very well.  Take the VFO unit out and use a straight blade razor to carefully remove that scale.  Go to some copy place and have them run off a couple of copies on a high quality copy machine.  Have them use 110 or 120 lb. paper which is about as thick as that scale.  Be sure it is as white as possible and you will find that the machine will produce a new, nice white document that you can trim and glue back on the VFO. 

Jim:
Your VFO schematic and info is for the 5100B. Attached is the VFO schematic for the 5100.

You are right Pete, I picked up the wrong diagram and the alignment is a bit different.  Attached is what the manual says.  So to do the alignment be sure you have the VFO on the proper frequency like one does with a Collins receiver.  Tune the VFO frequency to mid band of the 80 meter band (say 3.6 Mc) then set the dial to that frequency to be sure you are at the right point.   Then set the VFO dial to the low end of the band and move the pointer to the new lower frequency.  Change the dial to the high end and set the variable cap. 

Do that several times to get the end points as close as possible then you are done.  All this will work provided no one has fooled with the VFO.  I had one some years back that a CBer opened and fooled with the frequency  and that took some work to find the extra mica cap to remove.  If no turns have been changed, you should be able to get it pretty close.