[R-390] Continuing on the R-390A .........

[David Wise]

> From: pete wokoun, sr. [mailto:pwokoun@hotmail.com]
>=20
> Mouser sells a 10 ohm pot that works perfectly.
>=20
> Xicon/Alpha 24mm Wirewound Potentiometer
> 24MM WIREWOUND 10 ohm
>=20
> >From: Bob Camp <ham@cq.nu>
> >
> >Odd thing about the 10 turn pot modification. You really=20
> need a different
> >value pot. Strange but true ....

Rather than changing the pot (the brute-force
approach),  I changed the wiring slightly, and
got a linear adjustment range with the original pot.

If you compare the R-390 with the R-390A, you will
find that the former has a 15-ohm WW, while the
latter has the 100-ohm carbon || (in parallel with)
a 22-ohm fixed.  A comment in the Cost Reduction
Report says this was a response to the discrete
steps of the old WW pot.  Retrofitting a WW,
even a 10-turn, will bring back the discrete
steps unless it's some kind of metal film instead
of wound wire.

The R-390A arrangement is continuous, but it's
also very nonlinear.  According to Y2K Figure 5-6,
V506A idles with about 100mV at the cathode.
V504 cathode is 9V which puts 13mA through
the R524/R523/R537 network.  To get 100mV
(i.e. meter zero), R523||R537 must be 13 ohms,
i.e., R523 must be 32 ohms or about 1/3 rotation.

For smaller offsets things deteriorate rapidly.
My V506A idles at 60mV instead of 100 .
This requires 4.6 ohms total or R523 =3D 5.8 ,
which is practically against the stop.

Original circuit:

V604 <--^^^---------+-------+-----------> meter
cath   R524         |       |
        680         |       |
                    v  cw   |
         gnd-------^^^------+-^^^--gnd
                  R523       R537
                   100         22

V604 sees 680 to 700 ohms.  The meter
sees 0 to 18 ohms, with a nominal value
of 13 and in my case 5.

What I did:

V604 <--^^^---------|       |------------> meter
cath   R524         |       |
        680         |       |
                    v  cw   |=20
         gnd-------^^^------+-^^^--gnd
                  R523       R537
                   100         10

With this arrangement, V604 sees between
680 ohms and 710 ohms, and the meter sees
9 ohms.  Full CCW is 0mV.  1/4-rotation
is 30mV.  1/2-rotation is 60mV.  3/4-rotation
is 90mV.  Full CW is 120mV.  You can experiment
with various values for R537.  Larger values
will increase the adjustment range and decrease
the reading on strong signals.  My meter was
overindicating with the original circuit so 10
was about right.

Due to the way they used the bridged terminals,
the change requires that you move wires from
one to the other.  I don't remember the details,
but with the manual it's easy to figure out.

Why didn't they go the way I did?  My
humble guess is that this relatively
unimportant part of the job fell through
the cracks.  They probably had a junior
engineer calculate the 22 ohm resistor
and let it go at that.

This mod can be applied to the R-390
as well as the R-390A.

73,
Dave Wise