U)Signal Level Measurements; U.1.Receive Signal Levels; U.1.1.Carrier And Heterodyne Oscillator Levels - Heathkit SB-10 Series Troubleshooting Manual

u) Signal Level Measurements
The purpose of these measurements is to determine the receiver gain through various
stages. This can help isolate problems due to insufficient gain in a particular stage in the
signal path. I have not found any published values, so the expected values are based
on the average measurements from a number of different rigs.
Measurements start at the audio output stage and progressively work back towards the
RF amp stage. By following this progression, each step builds upon results of previous
steps.
Differences between the expected and measured signal levels are normal due to
component tolerances, aging and vacuum tube condition. These differences should only
be of concern when the actual value differs significantly from the expected value.
The following figure shows the receive signal path and test points used by this
procedure.
Figure 4. Receive Signal Path Test Points
u.1. Receive Signal Levels
This section describes a procedure to measure the signal levels required to produce a
given output level on a stage-by-stage basis. It is broken into two parts which are
independent of one another. The reason for this is that the measurement procedure is
different for the audio stages versus the RF/IF stages. It also removes the AF gain
control as a variable factor.
u.1.1. Carrier and Heterodyne Oscillator Levels
Before making any signal level measurements, check for correct operation of the carrier
(BFO), LMO, and heterodyne (HFO) oscillator frequencies and levels.
Connect a scope or RF probe to the carrier null pot center lug test point (or pin 3 or 8 of
V16) and switch between sideband modes, checking for equality in level. If need be,
adjust the value of R6 or R7 to achieve equality. The level should be at least 1 volt
RMS, or 3 volts P-P.
Ensure that the heterodyne oscillator level is -2.0 VDC for all positions of the band
switch (see Preliminary Heterodyne Oscillator Alignment on page 31). Values less than
-1.5 VDC will result in reduced conversion gain through the 1 st RX Mixer.
Signal Level Measurements Page 44