[R-390] Carrier Level meter R-390 A

[David Wise]

Since the DC operating point, voltage gain, and
plate resistance of V506A are changed, I'd expect
some change in the LONG AGC action (MEDIUM too
if you are using my AGC mod), but I believe
you would not notice.

Reducing R524 will preserve the original IF gain.

Since the zero-point meter voltage is in the
hundreds of millivolts instead of the tens,
getting rid of R537 eliminates the problem with
the stock carrier pot where the whole adjustment
range is squished into the last 5% of the pot,
so my carrier pot mod is obviated.  (For its
original purpose, at least; see below.)

Because the pot is a larger percentage of the
6AK6's cathode bias resistor, IF gain is more
sensitive to the carrier pot setting than before.
This could be ameliorated by rewiring the pot
as a pot instead of a rheostat.

The meter's scale factor (as opposed to its
zero point) is as sensitive to the carrier pot
setting as ever.  This effect can be reduced
by wiring the pot as a backwards-facing pot
(the essence of my mod), but this would make IF
gain even more variable.  This problem and the IF
gain problem could be licked by installing a
dual pot in place of the original.  Most likely
it's not irritating enough to make you do it.

The main objection, in my opinion, is that your
deck is now incompatible with the stock meter.
Individuals will very wildly in the importance
they place on this, depending on personality and
the number of receivers and spare parts they have.
If I had exactly one radio, no spares, and no
intention of acquiring more, I'd do it.  It's
very much worth a try, and anyone who does it,
please post your findings.

Dave Wise (whose radio also has original meters)

PS - It occurs to me that the basic mod could be
wired through a switch hidden under the chassis,
so you could switch it back to stock before
mating it to a 17-ohm meter.

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Gary Gitzen
> Sent: Wednesday, May 24, 2006 1:05 AM
> To: r-390 at mailman.qth.net
> Subject: [R-390] Carrier Level meter R-390 A
> 
> 
> Hi R-390,
> 
> Pete from Oz wrote of problems finding original 17 ohm
> carrier level meters, and notes that available 100 ohm
> meters give at best grudging needle deflection.
> 
> This prompted a look at the schematic of the carrier
> meter circuit for the 390A. It's essentially a bridge,
> balancing currents between the cathodes of V504,
> the 6AK6 final IF, and V506A, the AGC time constant
> tube. The resistors in the bridge allow a voltage to
> be developed across them, based on the currents.
> Any voltage difference flows through the meter,
> moving it.
> 
> Why not, I asked myself, change resistors R537 & R548
> to larger values to allow larger deflection using
> 100 ohm meters? A quick guesstimate is that
> removing R537, the 22 ohm 1W across the carrier adj
> pot, and increasing R548 from 22 ohms to approx 160
> ohms should give reasonable results. It might
> even be appropriate to add a small resistance
> (22 ohms???) in series with R523, carrier adj,
> to get adequate adjustment range.
> Possibly also reduce R524, the 680 ohm 6AK6
> cathode resistor, to 600-620 ohms or so. Shunting
> R524 with 5.1K or 7.5K would give that result.
> 
> Can anyone shoot reasonable holes in this idea?
> Caveat #1: I haven't tried it.
> Caveat #2: If you do this, _please_ document it
> on both the chasis and the IF module.
> 
> Gary, glad both his R-390As have original meters
> 
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