Attenuator And 10 Db Range Extender; Potentiometer Adjustment Procedure; Distortion Adjust - Welch Allyn Grason-Stadler GSI 17 Series Service Manual

8.11 ATTENUATOR AND 10 dB RANGE EXTENDER

The attenuator (U301) is a voltage controlled amplifier.
Although it does have amplification gain, the majority of its
dynamic range is used as an attenuator.
attenuator is 112 dB which is adjustable using a variable dc
voltage applied to Pin 3 (EC) of the device.
operating mode, the attenuator is set to 5 dB resolution.
calibration mode, however,
made possible by an 8 bit digital-to-analog converter (DAC).
Earphone calibration is accomplished by storing an HL to SPL
value per frequency in a look-up table in EEPROM.
at 1 KHz and 80 dB HL the CPU will set the attenuator to a
position which will establish 87 dB SPL in either earphone.
Therefore, it can be stated, that the attenuator for a given
frequency is offset from the nominal indicated HL by a decibel
value equal to the Reference Threshold Value
resolution of
amplifier
The
switchable feedback resistor.
range extender circuit used during normal operation.
the stage is changed by switching Resistor R304 in parallel
R303 or grounding R304 at one end (+10 dB position).
8.12 POTENTIOMETER ADJUSTMENT PROCEDURE S
I
Set front panel controls to:
a.
Channel ON.
b. Set a reference on Sound Level Meter (SLM) with either left
or right earphone coupled to it.
Reduce HL dial to 60 HL and note reading on SLM.
C .
Adjust R404 for a -20 dB change from the reference level
d.
obtained in step "b".
Re-check 80 HL for 20 dB change, readjust R404 as necessary.
e .
II. DISTORTION ADJUST (R403)
NOTE: Tone may be locked "ON" in the Normal Mode by connecting
TP100 to the cathode of CR3.
a. Set front panel controls to:
activated), Tone Steady, Channel ON.
Connect a distortion analyzer to either left or right phone
b.
output, load with 60 ohms and select the appropriate output
using the front panel switch.
0.5 dB resolution is available and
dB.
stage (U300) following the attenuator has a
The total range of the
This stage serves as the
1 KHz, 80 HL, Tone Steady,
1 KHz, 90 HL (+10 dB
In the normal
In the
For example,
(RTV) to a
+10 dB
The gain of
with