[R-390] IF Subchassis

[KA9EGW]

Figure 106 is the chassis main harness breakouts.

TM11-856, Fig. 107, upper left corner--S501 shorts out the crystal in
position 3/4/5/6 [e.g. 2/4/8/16kc BW] although the way the schematic is
drawn it's not as apparent as it could be.  It's much clearer drawn on p.3-9
of Y2KR3.

Getting two different bandwidths [.1 and 1] out of the same single-crystal
filter is accomplished by q-spoiling resistors R502 or 503 as detailed in
Y2KR3 Section 3.2.9.2:
"When the BANDWIDTH switch is turned to .1, the crystal circuit is loaded by
C503 in series with the combination of R502 in parallel with the series
combination of C501 and R503. The exact value of R503 is chosen between 560
and 2,700 ohms, to provide a bandwidth of 0.1 kHz.
When the BANDWIDTH switch is turned to 1, C501 and R503 are removed from the
circuit, and the bandpass is increased to 1 kHz. The value of resistor R502
is selected between 33K and 68K to provide a bandwidth of 1 kHz. When the
BANDWIDTH switch is turned to 2, 4, 8, or 16, T208 is coupled directly to
the control grid of V501 through capacitor C501, thereby removing the
crystal from the circuit."

Now go take your Aricept.  I'm done with this.

73, Brian KA9EGW



-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net]
On Behalf Of rbethman
Sent: Sunday, February 27, 2011 12:52 PM
To: R-390-List
Subject: [R-390] IF Subchassis

In spite of all assertions that Y-501 is SOLELY used as a crystal filter,
It would perhaps be of value to *carefully* look at TM 11-856, Figure 106.

Looking at the schematic, It is very obvious that Y-501 is ALWAYS tied to
the control grid of V501 without regard to the position of the S-501 wafer
of the bandwidth switch.

The other end of Y501 is tied from its other side to ALL the positions of
the S501 wafer for all filter positions from 2, 4, 8, and 16 positions.

I can indeed read the cited paragraphs!  I've done so.  They do NOT, however
"change" the circuitry and the feeding of the internals of Z501 to V501
whatsoever.

Exactly "how" two positions of S501 result in .1Kc and 1KC filtering, I do
not know.  I won't dispute the "written" function in the cited paragraphs.

However, it CANNOT be dismissed NOR ignored that Y501 is in "play" and feeds
V501 ALL the time.

Bob - N0DGN
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