[Boatanchors] BA Tune-Up Without B+?

[Rob Atkinson]

First, do something like I do which is make a sign that says "Kill B+"
and put it next to the power switch with magnets.  My sign says VFO to
remind me to have a VFO connected before I try to transmit, but it is
the same idea, make a reminder that you can attach near where you will
be so you see it before you stick your hands into the rig.

It's also a good idea to have a shorting stick.  on a small rig I just
use a clip lead to chassis with the other end holding around 50 ohms
resistance and touch it to caps like low B+ supply can caps.  The
resistor depletes the charge without a bang.   For a big rig you want
a pole of pvc holding a lug with a 10 ohm resistor in series between
the lug and an insulated cable to a spring clamp you can clamp to
ground and safely touch the lug to swing link output coils, h.v.
terminals from power supplies etc.

Another trick that is popular now is to calculate the anode Z of the
final, you need to know your plate v. and current of course, but it
doesn't have to be exact.  Then take a resistor cluster that equals
that calculated final impedance and connect it between the plate and
ground.  Then if you have a swr analyzer  you put it on the antenna
jack and inject it into the output network and adjust it for minimum
vswr on your desired operating frequency.  You don't even have to
light up the filaments.   I have done this and it gets you in the
ballpark.   This works well with pi output network.   I don't know
what the 669 uses.

73

Rob
K5UJ

On Sat, Mar 26, 2016 at 4:54 PM, David Stinson <arc5 at ix.netcom.com> wrote:
> Soon I'm going to need to tune the PA of a BC-669 transmitter.  If you've
> ever seen one, you know
> this means opening covers, sticking your hand in where the B+ lives,
> fiddling with a bunch of slides and etc.,
> and trying again about a dozen times per fixed frequency.
> Each time you hope and pay you remembered to turn
> off the B+.   Well, I'm old now and I will most certainly
> forget sooner or later.  I've already played that game and
> have the scars from it.  Don't think my weak old ticker will take many more
> jolts.
>
> This is my idea- tell me if it will work:
> Remove all power from the transmitter.  Jam the PTT relay closed.  Terminate
> the antenna connector
> with a 50-ohm load.   (I know it's not designed for 50-ohms.  I'll be using
> an external cap to fix that).
> Feed a hefty on-freq signal to the PA grids.  Put a scope across the 50-ohm
> load on the output.  Fiddle for max voltage across the load.
> If I'm right, this will get the tuning pretty close and minimize fiddling
> with it and skip having an early estate sale.
> Should work even backwards- feed a 50-ohm terminated
> signal genny into the antenna and put the scope on the tube grids.
>