FET Turns On or Off Very Slowly:
Generally causes problems at high frequency, 2 ohms.
If both slopes are equally slow, check the slope capacitors:
Positive,
C30
(035)
and Negative, C29 (034).
If only one slope is slow, check the resistors and buffer transistors:
Positive step:
R78,
79,
Dl5,
Q30-31
(R201,
202,
D40,
Q75-76)
Negative step:
R83,
84,
Dl7,
Q32-33
(R206,
207,
D42,
Q77-78).
Severe Step Oscillation.
Generally observed at low frequency, low impedance, right at threshold.
Positive step: check hysteresis resistor
R66
(RI 89)
Negative step: check hysteresis resistor
R69
(RI 92) and capacitor Cl 87 (Cl 93)
Troubleshooting DC Fault Shutdown
NORMAL BEHAVIOR OF THE CIRCUIT.
Any amplifier fault which causes a non-symmetrical output, such as premature clipping of one polarity, a
missing step,
etc,
may trigger DC fault shutdown. This indicates normal operation of the circuit
TRACING THE CAUSE OF FALSE TRIGGERS.
If amplifier is shutting down for no apparent cause, the source of the false signal must be found. Be sure the
output is checked with a DC coupled scope in order to confirm absence of an actual DC offset. The circuit
will trip on DC offsets exceeding about
4V.
The optocoupler’s input can be safely disabled by shorted
Ul5,
pins I-2 together. This will indicate if false
triggering is before or after
Ul5.
The output of
UlO:l,
pin 1 should be monitored. If it goes low during DC shutdown, it is sending the false
signal.
SHUTDOWN OCCURS AS SOON AS SWITCHING STARTS.
Disable
Ul5
as noted above, determine if there is a DC fault condition. CAUTION: use
50-ohm
resistor in
series with AC line to limit fault current in case of shorted outputs.
If amplifier output looks OK, check UlO:l output. If low, check voltage on pins 2 and 3
UlO:l,
pin 2: should be zero (no signal)
UlO:l,
pin
31
should be about
2V!
set by
R243,
244, 245.
Check R348 at Ul.5.
SHUTDOWN OCCURS ABOVE ABOUT 4V OUTPUT:
Q87,
C7,
R240
or D48 missing.
Confirm D48 is pulled low
(-13V),
holding Q87 on. If not, check RI 17, 118, Q42
NOTE: this control voltage responds to the Br Mono switch, pole
7.
Check R348 at
Ul5.
Bad connection at step diodes
(D21,
D22,
D46,
D47)
21
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