Adding an RF Gain Control to the Scout
From the Ten-Tec Reflector April 10, 1999
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.
- Remove top and bottom covers.
- Remove all front panel knobs.
- Remove the 4 screws holding the front panel to the chassis.
- Using long-nose pliers, remove the nuts and washers on the 2 control shafts.
- 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.
- 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.
- 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.
- Remove and save the 2 harness jumpers marked 26 and 27 from the I.F./Audio board.
- Remove the wires from one connector marked 26 and one marked 27. These connectors will be used in steps 10 and 11.
- 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.
- 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.
- 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.
- 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.
- 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.
- 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.
- Reinstall the speaker/display mounting plate.
- 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.
- Turn the Scout over to expose the foil side of the I.F./Audio board.
- 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.
- 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.
- Connect the pot wiper to the anode of a 1N914 or 1N4148 diode. Connect the cathode of the diode to Q5-emitter.
- Check all wiring before powering up.
- Power up the Scout, pick a noisy or busy band and check R.F. gain control operation.
- Using a dummy load (of course) set the mic gain using the new control on the rear panel (or wherever you mounted it).
- 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
