Ken, here you go:
From: <weingaertner@nac.net>
To: <tentec@contesting.com>
Subject: [TenTec] [Fwd: RF Gain Control For the Scout]
Date: Saturday, April 10, 1999 3:45 PM
To: tentec@contesting.com
From: weingaertner@nac.net on Sat, Apr 10, 1999 2:45 PM
Subject: [TenTec] [Fwd: RF Gain Control For the Scout]
ADDING AN R.F. GAIN CONTROL TO THE SCOUT
This is a particularly useful modification if you have the R37 full QSK
modification or if you operate on bands that have a lot of QRN, like
160, 80, and 40 meters.
No drilling is required. The mike gain potentiometer is used for R.F.
gain and a new mike gain pot will be installed on the rear panel in
place of the FWD/SWR switch.
The following instructions are not quite up to "Heathkit spec", so
study
the circuit board layouts, schematics, and these instructions before you
begin. It will be very
helpful to sketch the schematic of the mod before you proceed.
1. Remove top and bottom covers.
2. Remove all front panel knobs.
3. Remove the 4 screws holding the front panel to the chassis.
4. Using long-nose pliers, remove the nuts and washers on the 2 control
shafts.
5. Carefully slide the front panel forward so that it just clears the
front edge of the speaker mounting plate and the I.F./Audio board.
Do not try to remove the front panel completely; there are many wires
attached to the panel.
6. With the speaker facing up, remove the 4 screws that hold down the
speaker mounting plate.
These are the screws that are not recessed below the plate. Carefully
lift out
the plate. CAUTION: The digital display and display board are also
mounted on this plate.
Be careful not to damage them. Swing the plate out of the way.
It may be helpful to unplug the harness from the digital display board.
7. Looking down into the chassis, you will see the component side of the
I.F./Audio board.
Study the locations of connectors 26 and 27 (2 of each), connector 9,
and transistor Q5.
8. Remove and save the 2 harness jumpers marked 26 and 27 from the
I.F./Audio board.
9. Remove the wires from one connector marked 26 and one marked 27.
These connectors will be
used in steps 10 and 11.
10. Take connector number 26 and attach a small shielded cable to it. I
used RG-174/U miniature
coaxial cable. When you hold the connector with the number facing you,
the cable shield
should go on the left pin. I put a ferrite bead on the center conductor
to keep stray R.F.
out of the microphone circuit, but this may not be necessary. Plug this
connector into the
board at the number 26 location that is on the back center of the
board, near
connectors 25 and 9. When plugged into the board, the shield will be
closest to the PTO.
Use an ohmmeter to verify ground if you are not sure.
11. Take connector 27 and attach a second length of shielded cable to
it. This connector
plugs into location 27 near the center of the board, next to trimmer
pot R76.
When plugged into the board, the cable shield will be closest to the
PTO assembly.
Put a ferrite bead on the center conductor.
12. Note: Connectors 26 and 27 in the extreme corner of the board
between the Mic Gain/RIT pot
and the PTO will be left open.
13. While you have the Scout open, perform the following modification.
Locate the blue S-meter lamp wires, cut one and install a 56 ohm
resistor in series with it.
Use sleeving to insulate the resistor leads. Temporarily unplug the
lamp from the meter
housing, if necessary. This change will greatly reduce night time glare
from the meter
and will extend the life of the lamp.
14. Turn the transceiver over and unplug connector 41 from the Low Level
Driver board.
Remove the FWD/SWR switch from the rear panel. Remove the switch leads
from connector 41
and install a jumper on the connector to select the SWR meter function.
Plug connector 41
back into the Low Level Driver board.
Note: The new mic gain control will be installed in place of the
FWD/SWR switch. Use a
miniature 10 k ohm potentiometer. If you want to retain this switch,
consider mounting a miniature trim pot for mic gain inside the Scout. I
find that the mic gain seldom needs to be adjusted (never if you only
run CW!), so mounting the control internally, with or without an access
hole in the bottom cover is a reasonable alternative.
15. After choosing a location for the new mic gain control, route the 2
shielded cables installed in
steps 10 and 11 to the chosen location.
16. Reinstall the speaker/display mounting plate.
17. Connect the shield wires to the low side of the new control. Connect
the center conductor of the
cable from connector 26 to the wiper and connect the center conductor
from connector 27 to the
high side of the pot. I put ferrite beads on the center conductors
where they solder to the
pot, but again, this may not be necessary. Mount the pot in the chosen
location.
18. Turn the Scout over to expose the foil side of the I.F./Audio board.
19. Locate terminals 26 and 27 in the notched-out corner of the
I.F./Audio board, adjacent to the
Tune/NB switch. With an ohmmeter, determine which terminals connect to
the front panel mic gain
control, which will become the new R.F. gain control.
Note: Use sleeving on the diode and resistor leads in the following
steps. You will be working
on the foil side of the board. It is not necessary to remove the board.
20. Connect a 3300 ohm resistor from the low side of the R.F. gain pot
to the ground foil near the
circuit board mounting screw. Connect an 820 ohm resistor from the high
side of the pot to the
+13 V "R" (Receive) voltage at connector 9, at the opposite edge of the
board. Connector 9
has the "R" and "T" voltages (see schematic). The "R" voltage is the
thinner circuit trace.
21. Connect the pot wiper to the anode of a 1N914 or 1N4148 diode.
Connect the cathode of the
diode to Q5-emitter.
22. Check all wiring before powering up.
23. Power up the Scout, pick a noisy or busy band and check R.F. gain
control operation.
24. Using a dummy load (of course) set the mic gain using the new
control on the rear panel
(or wherever you mounted it).
25 Reassemble the Scout.
CHARACTERISTICS
Because a 10 k pot is being used, the useful range will be compressed
into the lower third of the
control rotation from fully CCW to about 11 o'clock. There may be a way
to spread out the
range using parallel resistors, but I haven't experimented with this. An
interesting side effect is that
as the gain is decreased, the AGC time constant increases somewhat,
further reducing the blast effects
of QRN and QRM.
"Keepin' my Scout" 73, Bob WB2VUF
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