y) Heathkit “S” Meter Circuit
Refer to the following diagram for the “S” meter circuitry. R104 and R105 provide a
small positive reference voltage for the “S” meter based on the cathode current of V3.
As the AVC voltage goes more negative in response to a received signal, the negative
bias on the grid increases reducing the current flow (gain) through V3, the 1
st
IF amp. As
this happens, the screen and cathode current go down causing the screen voltage to go
up while the cathode voltage goes down.
The screen current is reduced and the voltage at the junction of R106 and R107
increases while the voltage at the junction of R104 and R105 decreases causing the
meter to read higher (the “S” meter is measuring the difference between the screen
voltage and cathode voltage of V3).
Figure 7. "S" Meter Circuit
When the rig is first turned on (or if V3 is removed), V3 is drawing no current, the screen
voltage is at its maximum and the meter should deflect to full scale (S9+60). If this is not
the case, then the values of R106, R107, R110, and R105 should be checked and any
that are off by more than 10% should be replaced. Carbon composition resistors will
often drift higher in value as they age.
R106 (*) determines the screen voltage seen by V3. In early models, this resistor was
22k, but was later increased to 33k to reduce the failure rate of V3 due to excessive
screen dissipation. If a 22k resistor is present, it should be replaced with a 30k or 33k
2w resistor.
R107 (**) determines the ‘sensitivity’ of the “S” meter. Ideally, it would be chosen to
provide an S9 reading with a 50v input signal. Because the AVC voltage controlling V3
is a function of the gain of all of the RF and IF stages, there will be considerable
variation from one radio to another so the value of R107 is somewhat a compromise.
There was a (factory?) mod to reduce the value of R107 to 82k to increase the
sensitivity of the “S” meter. This will often cure the case where the “S” meter readings
seem abnormally low. This can be accomplished by replacing R107 or adding a 470k
resistor in parallel with the existing R107.
Note: Is the above paragraph actually correct? Need to verify that the screen voltage
does indeed vary with the AVC voltage and that R107 affects the sensitivity.
Heathkit “S” Meter Circuit Page 61
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