Heathkit SB-10 Series Troubleshooting Manual page 91

Figure 15. Modulator Circuit Board Modifications
Additional rolloff of the high frequency response will remove the raspy nature of the
receive audio, and this is accomplished by changing the value of capacitor C119 on the
I.F. board from 500pf to .001 μfd (see Figure 14. IF Circuit Board Modifications on page
84). The low-end audio response of the audio amplifier increases slightly by replacing
coupling capacitor C306 on the audio board with a .01 μfd disk capacitor (see Figure 13.
Audio Circuit Board Modifications on page 82).
Any possibility of voltage regulator hash being generated by V18 is reduced by
connecting a .001 μfd 500V disk capacitor from V18 pin 1 to the ground foil on the audio
board.
Some of the 'birdies' and other internally-generated signals heard in the tuning range of
the receiver are reduced considerably by bypassing the filament string to ground with a .
01 μfd capacitor connected from the common filament point on the bandpass circuit
board to ground. This is the point where the four brown wires connect about an inch
below pin 1 of V19 (see Figure 16. Bandpass Board Modifications on page 87).
Modifications Page 91