Output Stage; Power Supply; Turn On/Off Muting Circuit; Troubleshooting Tips - Bose Panaray 502A Service Manual

6. Output stage

NOTE: The components mentioned in this paragraph are for the high frequency output only.
The output stage is compatible with balanced signal transmission systems. Inverting op-amp
stage U9 (pins 1, 2, and 3) provides the output signal. Diodes D9, D10, D29 and D30 provide
output clamping to protect against overvoltage and ESD. Relays K1 and K2 clamp the outputs
to ground to protect against power on/off transients. When the relay is de-energized, as when
the AC power is off, the outputs are clamped. About 1.2 seconds after the power is turned on,
the relay is energized by transistor Q1 and allows audio to pass through the controller. When
the power is turned off, a fast acting detector de-energizes the relays to protect against power
off transients.
NOTE: In earlier production units (before August 1993), the output stage was differential. Not
only was the positive (+) output pin driven, but the negative (-) output pin was also driven (out of
phase). The output stage was changed so that only the positive (+) output pin is driven, while
the negative (-) output pin is connected to signal ground through a series impedance identical to
that of the positive (+) output pin. This alternative output stage design has improved characteris-
tics into a variety of real world installation conditions. Both versions produce the same output
level when received by a balanced input.

7. Power Supply

The 502C uses full wave bridge rectifier Z1 and regulators U13 and U14 to develop the
+/-15 volt supply needed by the op-amps.

8. Turn On/Off Muting Circuit

Relays K1 and K2 clamp the outputs and are controlled by three sections of quad comparator
U15. U15 (pins 8, 9 and 14) drives the relay control transistor Q1 by pulling the relay-coil termi-
nal to nearly 15 volts. This energizes the relay for normal audio operation. At power on, U15
(pins 8, 9 and 14) is prevented from energizing the relay until capacitor C152 can be charged up
sufficiently. This takes about 1.2 seconds, and mutes any turn-on transients. At power off, U15
(pins 2, 4, and 5) quickly detects the loss of AC power and causes Q1 to snap quickly off. This
de-energizes the relay in about 100 ms, before any transients can reach the output.

9. Troubleshooting Tips

If a differential output test generator isn't available, connect the generator ground to the 502C
ground terminal, and the generator hot lead to the negative (-) input terminal. Leave the positive
(+) terminal unconnected.
If you need to connect the 502C outputs to a single-ended (not differential) instrument, connect
the 502C output ground to the instrument ground, and the 502C positive (+) output terminal to
the instrument hot pin. Leave the negative (-) output pin unconnected.
Since the 502C is a two channel device, you can troubleshoot any problem occurring in only one
channel more easily by applying the identical signal to both Channel 1 and Channel 2 inputs and
signal tracing stage by stage, comparing the signal as it passes through the various stages. In
general, the corresponding Channel 1 and Channel 2 stages share the left and right hand sides
of the respective op-amps. This makes it easier to compare the stages.
THEORY OF OPERATION
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