Circuit Operation; Pa Overload Led Indicators - Shure Pro Master 700 Service Manual

No DB PEAK/frequen-
1. M a k e c e r t a i n FEEDBACK
cy band LEDindicators
flashing (console
output normal)
No console output;
1. Check PATCH BLOCKTM rear
no DB PEAK LED
indicatorsflashing
2. Check for improper connec-
3. Check settings of
Loud noise or 1. SIMPLEX 24V switch is on
clicks when certain
microphones or
2. Unbalanced cable used when
cables are used
3. Check for defective micro-
No monitor output 1. Check
(program output
normal)
2. M a k e sure MONITOR and
3. Monitor speaker volume con-
Sound quality poor 1. Excessive equalization o n
(weak or thin)
2. Defective input or patching
Two simple methods of localizing problems without
opening the console are as follows. These methods can
be used to find the abnormal console section with or with-
out test equipment. (The methods are not needed if the
problem is obvious: SHUTDOWN, no POWER indication,
etc.)
With test equipment: Set all console controls to full
clockwise, REVERB RETURN control to full counterclock-
wise, and PAN (700) and EQ controls to center. Do not
connect a speaker or headphone load. Apply a 0.5 mV,
1 kHz test signal across pins 2 and 3 of one of the BAL LO
IMP input connectors. Normal voltmeter readings taken
at each speaker, rear-panel PATCH BLOCKTM and front-
panel HEADPHONES connector are given in Table 1.
The top row of figures indicates the voltage i n each stage
from input to output. Similarly, a 5 mV signal inserted in a
HI IMP input will check these same voltages. The other
listed inputs give voltage indications only for those circuits
following the inserted signal. Ac voltages may vary t 3 0 %
from the values shown.
Without test equipment: Using only a microphone and
speaker, it is often possible to isolate problem areas using
the PATCH BLOCK rear panel. The ability to bypass cer-
tain circuits and interchange channel signals provides a
FINDER switch is set to A, B,
ON or OFF (not between posi-
tions).
panel for improper patching.
tions to EQ or PA INPUT.
channel
VOLUME and MASTER volume
controls.
(when not needed).
SIMPLEX 24V switch is on.
phone cables.
MONITOR OUTPUT
connection to EQ or PA INPUT,
or external amplifier.
MONITOR MASTER controls
are turned up.
trol (if present) may be turned
down.
graphic equalizers.
cables.
simple but powerful tool for troubleshooting. For example,
in a 700 console a suspected channel A equalizer can be
checked by patching between the A MIX OUTPUT and
A PA INPUT jacks. The signal bypasses the equalizer, and
a talk test will immediately indicate whether the problem
has been eliminated.
Similarly, a potentially defective channel B mix ampli-
fier can be verified by patching the A MIX OUTPUT to the
B MIX OUTPUT. The presence of a "clean" signal from
channel A to the B speaker output indicates that the prob-
lem precedes the B MIX OUTPUT jack. Further patching
between the EFFECTS OUTPUT and B MIX OUTPUT jacks
will isolate the problem to the B MIX amplifier.
TABLE 1. NOMINAL TEST VOLTAGES
I 7 -
I I
INPUT
M o n i t o r Effects M i x
2.7V 0.54V
Bal Lo Imp 0.5 mV 3.8V
2.7V
Hi Imp 5 mV 3.8V 0.54V
-
-
Aux (700) 0.15V 0.54V
-
-
AUX (706) 64 mV 0.54V
- -
-
Eq 0.54V
- -
-
PA 0.54V
sleeve.
'Tip or ring to
The above methods are extremely useful for saving time
in localizing problem areas. In addition, they may allow use
of the console in an emergency until repairs can be made.
Internal servicing should only be performed by qualified
service personnel.

CIRCUIT OPERATION

Most of the PRO MASTERTM circuitry uses conventional
operational amplifier technology, and can be serviced
using standard voltage-measurement procedures. How-
ever, certain circuits are unusual and may not be familiar
to many servicemen. This section describes these circuits
as an aid to servicing (see Figures 5-38).

PA OVERLOAD LED INDICATORS

The PA OVERLOAD LEOS (01109) are designed to
respond to error signals in their respective differential
amplifiers. The overload circuit is comprised of transistor
Q607 and associated resistors with the LED connected
across the collector and emitter of Q607.
The power amplifier assembly input is the differential
amplifier Q605-Q606, which amplifies the difference of the
signals at the bases (the input signal to Q605 and the nega-
tive feedback signal to Q606). With small error signals
(normal power amplifier operation), the dc voltage drop
across R611 is approximately 1.6V, sufficient to keep Q607
turned on. Q607's collector-emitter voltage drop is very
low (approximately O.IV), and as long as Q607 is on, the
associated overload LED is turned off.
When the differential amplifier input signals differ
greatly (such as when clipping or V-l protection takes
place), the ac signal across R611 increases rapidly. At
about 1% distortion, the large ac signal through R611
causes Q607 to turn off. With Q607 turned off, the current
flow is through the associated LED and R616 to ground,
lighting the LED.
I
OUTPUTS
Head-
Aux M i c Speaker phones*
Eq
0.54V 7.0mV 0.54V 18.OV 11.5V
11.5V
0.54V 7.0mV 0.54V 18.OV
0.54V 7.0mV 11.5V
0.54V 18.OV
0.54V 7.0mV 0 54V 18.OV 11.5V
- - 11.5V
0.54V 18.0V
- -
-
11.5V
1 8 0 V