64
Display PCB (continued)
The “-20dB”, “-12db”, and “-6dB” LEDs are driven by circuitry that samples the power amplifier
output. Due to the function of the output transformer, power amplifier output level of 25V corre-
sponds to rated output power. Signal is taken from output side of R646 via fuse resistor R650
and fed to the signal processor assembly (J601p2 Power Amp Assy to J601p2 Signal Processor
Assy) where R427, R428 attenuate the signal to levels compatible with op amp inputs and R427,
C586 form a low pass network rejecting RF noise. D426 rectifies the signal which is fed to the
display assembly (J700p15 Signal Processor Assy to P700p15 Display Assy) where C701 filters
the rectified signal and R709 sets filter discharge time constant.
U700B, U702A and U702B [G2/3] are used as comparators driving LEDs D711, D712 and D716
[C4]. Comparison voltages are set by divider network R710, R711, R714, R713, R712 and R715
[F2/3/4]. With no signal input, C701 [F3] is discharged and U702p5 and U702p3 are close to 0V
while the divider network sets U702p6 at 0.32V, U702p2 and U700p5 at 1.64V, U700p6 at 0.16V.
Under these conditions, outputs U702p1 and U702p7 approach the -15V rail and output U700p7
approaches the 15V rail. LEDs D712 and D716 are connected back to back between U702p1
and U702p7. Since these outputs are at the same potential, neither LED lights. U700p7 sources
current through R716 which is shunted via D720 and U702p7 to -15V: LED D711 is reversed
biased and does not light. When output signal level rises above 2.5V (20dB below 25V output
level), C701 voltage applied to U702p5 exceeds the 0.32V at U702p6 and output U702p7 is
driven to saturation at 15V. Current flows from U702p7 through R717 and lights “-20dB” LED
D716. D712 is reverse biased, and conduction through D720 [G2] stops as U702p7 reaches the
same potential as U700p7. As output signal level rises above 6.3V (12dB below 25V output level),
C701 voltage applied to U702p3 exceeds the 1.64V at U702p2 and output U702p1 is driven to
saturation at 15V. This re-biases of the divider network, sourcing current into the network from
15V via output U702p1, D719 and R715 [G3]. Resulting comparison voltages are 4.06V at
U702p6, 1.69V at U702p2 and U700p5, and 4.35V at U700p6. It’s input conditions virtually un-
changed, output U702p1 does not change. The reference voltage at U702p6 rises to 4.06V from
0.32V and output U702p7 is driven to -15V. Current flows from U702p1 to light “-12dB” LED D712
and returns via R717 to U702p7; LED D716 is reverse biased and turns off. Voltage at U700p6
rises to 4.35V while U700p5 increases to 1.69V and output U700p7 is driven to -15V. Outputs
U702p7 and U700p7 are at the same potential, so neither D720 nor LED D711 conducts. When
output signal level rises above 12.5V (6dB below 25V output level), C701 voltage applied to
U702p5 exceeds the 4.06V at U702p6 and output U702p7 is driven to 15V. Current flows from
U702p7 lighting “-6dB” LED D711 and returns via R716 to U700p7, while LED D712 turns off.
Further increases in output signal level do not change the state of LEDs D711, D712 or D716. As
output signal level decreases, the sequence of events is reversed: “-6dB” LED D711 turns off as
“-12dB” LED D712 turns on, “-12dB” LED D712 turns off as “-20dB” LED D716 turns on, and
finally “-20dB” LED D716 turns off.
“Clip” LED D715 lights when the peak output voltage nears power amplifier rail voltages. The
LED is driven from the Signal Processor assembly (J700p14 Signal Processor PCB to P700p14
Display PCB) by clip detector and limiting circuitry as noted above. D715 turns off as amplifier
output signal level decreases.
“Ready”, “Protect”, and “Thermal” LEDs are driven from open-collector comparator outputs in
control circuitry on the Signal Processor assembly as noted above. At power amplifier turn-on,
comparator output U425p14 [Signal Processor sheet2, F2] sinks current through limiting resistor
R445 lighting “Ready” LED D710 [Display PCB, D2] (J700p17 Signal Processor PCB to
P700p17 Display PCB).
THEORY OF OPERATION
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