A SCAF-1 user, W0VX, offered a circuit improvement that we recommend to all users, and we include now in all new units.
The loading pot and a loading resistor on the output are deleted, and a new resistor is inserted in series with the headphone jack. This change eliminates the white noise hiss heard when using headphones that the earlier circuit also addressed. However, the new circuit offers greater dynamic range, making the filter more useful in many situations.
You can tell whether your filter already has the upgrade by looking for trimmer pot R27.
If your circuit board has an empty spot for that pot, you already have the upgrade.
Additional parts needed 1 – 100 ohm 1/4 watt resistor.
Remove resistor R27, the 200 ohm pot.
Remove resistor R28, a 10 ohm resistor.
Remove the wire from Switch terminal 5 going to an output jack. Switch terminal #5 is the center pin on the switch.
Add the 100 ohm resistor between Jack J3 and Jack J4. Note that the audio signal wire from the front panel switch should go to the jack used for speaker output. The 100 ohm resistor goes from that connection to the headphone jack.
You will probably need to reset the audio gain pot, R17, to an appropriate level. This is done by receiving a signal from the receiver with the filter switched in to the “OFF” position, then switching the filter on and adjusting the pot level so that the filtered signal is either at the same level as in the “OFF” position or is greater, as desired.