[R-390] Carrier balance pot adjustment
David Wise
[email protected]
Mon, 3 Jun 2002 14:11:49 -0700
> There was a discussion a few months ago about making a few changes to
> the carrier pot adjustment to make it less critical and easier to set.
> One of the changes consisted of a resistor change and a wiring change.
> Can anyone provide me a copy of the messages?
The mod you're probably thinking of is mine.
I posted instructions, but I couldn't find
them in a recent search of the archives.
I reconstitute it here, except I can't
remember the wire colors.
Get the schematic in front of you.
The original connection is, V504 cathode
via R524 to meter, wiper, R537, and CW end.
CCW end to ground. Other end of R537
to ground.
My connection is, V504 cathode via R524
to wiper. CCW end to ground. CW end to
meter and R537. Other end of R537 to
ground. Change R537 from 22 ohms to
15 ohms.
+ Smooth, almost linear adjustment range,
with the operating point near midrange.
- Lower V504 gain (trivial, less than
R524's 10% tolerance).
- More gain variation as you work R523 from
end to end (trivial, *less* variation than
before when near the setpoint).
- Smaller reading on MGC overload.
(Unimportant, overload is yes/no, not
calibrated.)
~ Smaller full-scale reading.
(Intentional; mine read high before.
To change it, tweak R537. Smaller
value gives higher FS. 5.6 ohms is
approximately original but puts
the setpoint all the way CW.
Still smooth though!)
Everybody seems to to have an opinion as to
how this should be fixed, and I'm no exception.
My engineer's intuition says that this is
the "sweet spot". It's how they should
have done it.
I don't know how well the R-390's carrier pot
works. I expect it is fairly linear, but
it might have a coarse granularity. If so, you
could change it to 100-ohm carbon and apply my
mod, and you'd get the same good result.
73,
Dave Wise